Introduction to Linux - A Hands on Guide

Table of Contents
Introduction

    1. Why this guide?
    2. Who should read this book?
    3. New versions and availability
    4. Revision History
    5. Contributions
    6. Feedback
    7. Copyright information
    8. What do you need?
    9. Conventions used in this document
    10. Organization of this document

1. What is Linux?

    1.1. History
    1.2. The user interface
    1.3. Does Linux have a future?
    1.4. Properties of Linux
    1.5. Linux Flavors
    1.6. Summary
    1.7. Exercises

2. Quickstart

    2.1. Logging in, activating the user interface and logging out
    2.2. Absolute basics
    2.3. Getting help
    2.4. Summary
    2.5. Exercises

3. About files and the file system

    3.1. General overview of the Linux file system
    3.2. Orientation in the file system
    3.3. Manipulating files
    3.4. File security
    3.5. Summary
    3.6. Exercises

4. Processes

    4.1. Processes inside out
    4.2. Boot process, Init and shutdown
    4.3. Managing processes
    4.4. Scheduling processes
    4.5. Summary
    4.6. Exercises

5. I/O redirection

    5.1. Simple redirections
    5.2. Advanced redirection features
    5.3. Filters
    5.4. Summary
    5.5. Exercises

6. Text editors

    6.1. Text editors
    6.2. Using the Vim editor
    6.3. Linux in the office
    6.4. Summary
    6.5. Exercises

7. Home sweet /home

    7.1. General good housekeeping
    7.2. Your text environment
    7.3. The graphical environment
    7.4. Region specific settings
    7.5. Installing new software
    7.6. Summary
    7.7. Exercises

8. Printers and printing

    8.1. Printing files
    8.2. The server side
    8.3. Print problems
    8.4. Summary
    8.5. Exercises

9. Fundamental Backup Techniques

    9.1. Introduction
    9.2. Moving your data to a backup device
    9.3. Using rsync
    9.4. Encryption
    9.5. Summary
    9.6. Exercises

10. Networking

    10.1. Networking Overview
    10.2. Network configuration and information
    10.3. Internet/Intranet applications
    10.4. Remote execution of applications
    10.5. Security
    10.6. Summary
    10.7. Exercises

11. Sound and Video

    11.1. Audio Basics
    11.2. Sound and video playing
    11.3. Video playing, streams and television watching
    11.4. Internet Telephony
    11.5. Summary
    11.6. Exercises

A. Where to go from here?

    A.1. Useful Books
    A.2. Useful sites

B. DOS versus Linux commands
C. Shell Features

    C.1. Common features
    C.2. Differing features

Glossary
Index

List of Tables
1. Typographic and usage conventions
2-1. Quickstart commands
2-2. Key combinations in Bash
2-3. New commands in chapter 2: Basics
3-1. File types in a long list
3-2. Subdirectories of the root directory
3-3. Most common configuration files
3-4. Common devices
3-5. Color-ls default color scheme
3-6. Default suffix scheme for ls
3-7. Access mode codes
3-8. User group codes
3-9. File protection with chmod
3-10. New commands in chapter 3: Files and the file system
3-11. File permissions
4-1. Controlling processes
4-2. Common signals
4-3. New commands in chapter 4: Processes
5-1. New commands in chapter 5: I/O redirection
7-1. Common environment variables
7-2. New commands in chapter 7: Making yourself at home
8-1. New commands in chapter 8: Printing
9-1. New commands in chapter 9: Backup
10-1. The simplified OSI Model
10-2. New commands in chapter 10: Networking
11-1. New commands in chapter 11: Audio
B-1. Overview of DOS/Linux commands
C-1. Common Shell Features
C-2. Differing Shell Features

List of Figures
1. Introduction to Linux front cover
1-1. OpenOffice MS-compatible Spreadsheet
2-1. Terminal window
2-2. Konqueror as help browser
3-1. Linux file system layout
3-2. Hard and soft link mechanism
4-1. Fork-and-exec mechanism
4-2. Can't you go faster?
4-3. Gnome System Monitor
8-1. Printer Status through web interface
9-1. Floppy formatter
10-1. Evolution mail and news reader
10-2. X-Chat
10-3. SSH X11 forwarding
11-1. XMMS mp3 player



Introduction

1. Why this guide?

Many people still believe that learning Linux is difficult, or that only
experts can understand how a Linux system works. Though there is a lot of
free documentation available, the documentation is widely scattered on the
Web, and often confusing, since it is usually oriented toward experienced
UNIX or Linux users. Today, thanks to the advancements in development, Linux
has grown in popularity both at home and at work. The goal of this guide is
to show people of all ages that Linux can be simple and fun, and used for all
kinds of purposes.



2. Who should read this book?

This guide was created as an overview of the Linux Operating System, geared
toward new users as an exploration tour and getting started guide, with
exercises at the end of each chapter. For more advanced trainees it can be a
desktop reference, and a collection of the base knowledge needed to proceed
with system and network administration. This book contains many real life
examples derived from the author's experience as a Linux system and network
administrator, trainer and consultant. We hope these examples will help you
to get a better understanding of the Linux system and that you feel
encouraged to try out things on your own.

Everybody who wants to get a "CLUE", a Command Line User Experience, with
Linux (and UNIX in general) will find this book useful.



3. New versions and availability

This document is published in the Guides section of the Linux Documentation
Project collection at [http://www.tldp.org/guides.html] http://www.tldp.org/
guides.html; you can also download PDF and PostScript formatted versions
here.

The most recent edition is available at[http://tille.garrels.be/training/tldp
/] http://tille.garrels.be/training/tldp/.

The second edition of this guide is available in print from [http://
fultus.com/] Fultus.com Books as paperback Print On Demand (POD) book. Fultus
[http://store.fultus.com/product_info.php?products_id=140] distributes this
document through Ingram and Baker & Taylor to many bookstores, including
[http://www.amazon.com/exec/obidos/ASIN/1596821124] Amazon.com, [http://
www.amazon.co.uk/exec/obidos/ASIN/1596821124] Amazon.co.uk, [http://
search.barnesandnoble.com/booksearch/isbninquiry.asp?ISBN=1596821124]
BarnesAndNoble.com and [http://froogle.google.com/froogle?q=1596821124]
Google's Froogle global shopping portal and [http://books.google.com/books?
isbn=1596821124] Google Book Search.

Figure 1. Introduction to Linux front cover

[itl]

The guide has been [http://www.geocities.com/linuxparichay/] translated into
Hindi by:

  • Alok Kumar

  • Dhananjay Sharma

  • Kapil

  • Puneet Goel

  • Ravikant Yuyutsu

Andrea Montagner [http://www.codex.altervista.org/introlinux.html] translated
the guide into Italian.



4. Revision History

 

Revision History
Revision 1.27            20080606           Revised by: MG
updates.
Revision 1.26            20070919           Revised by: MG
Comments from readers, license.
Revision 1.25            20070511           Revised by: MG
Comments from readers, minor updates, E-mail etiquette, updated info about
availability (thanks Oleg).
Revision 1.24            2006-11-01         Revised by: MG
added index terms, prepared for second printed edition, added gpg and proxy
info.
Revision 1.23            2006-07-25         Revised by: MG and FK
Updates and corrections, removed app5 again, adapted license to enable
inclusion in Debian docs.
Revision 1.22            2006-04-06         Revised by: MG
chap8 revised completely, chap10: clarified examples, added ifconfig and
cygwin info, revised network apps.
Revision 1.21            2006-03-14         Revised by: MG
Added exercises in chap11, corrected newline errors, command overview
completed for chapter 9, minor corrections in chap10.
Revision 1.20            2006-01-06         Revised by: MG
Split chap7: audio stuff is now in separate chapter, chap11.xml. Small
revisions, updates for commands like aptitude, more on USB storage, Internet
telephony, corrections from readers.
Revision 1.13            2004-04-27         Revised by: MG
Last read-through before sending everything to Fultus for printout. Added
Fultus referrence in New Versions section, updated Conventions and
Organization sections. Minor changes in chapters 4, 5, 6 and 8, added
rdesktop info in chapter 10, updated glossary, replaced references to
fileutils with coreutils, thankyou to Hindi translators.



5. Contributions

Many thanks to all the people who shared their experiences. And especially to
the Belgian Linux users for hearing me out every day and always being
generous in their comments.

Also a special thought for Tabatha Marshall for doing a really thorough
revision, spell check and styling, and to Eugene Crosser for spotting the
errors that we two overlooked.

And thanks to all the readers who notified me about missing topics and who
helped to pick out the last errors, unclear definitions and typos by going
through the trouble of mailing me all their remarks. These are also the
people who help me keep this guide up to date, like Filipus Klutiero who did
a complete review in 2005 and 2006 and helps me getting the guide into the
Debian docs collection, and Alexey Eremenko who sent me the foundation for
chapter 11.

In 2006, Suresh Rajashekara created a Debian package of this documentation.

Finally, a big thank you for the volunteers who are currently translating
this document in French, Swedish, German, Farsi, Hindi and more. It is a big
work that should not be underestimated; I admire your courage.



6. Feedback

Missing information, missing links, missing characters? Mail it to the
maintainer of this document:


                    <tille wants no spam _at_ garrels dot be>
                

Don't forget to check with the [http://tille.garrels.be/training/tldp/]
latest version first!



7. Copyright information

* Copyright (c) 2002-2007, Machtelt Garrels
* All rights reserved.
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
*     * Redistributions of source code must retain the above copyright
*       notice, this list of conditions and the following disclaimer.
*     * Redistributions in binary form must reproduce the above copyright
*       notice, this list of conditions and the following disclaimer in the
*       documentation and/or other materials provided with the distribution.
*     * Neither the name of the author, Machtelt Garrels, nor the
*       names of its contributors may be used to endorse or promote products
*       derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS "AS IS" AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE AUTHOR AND CONTRIBUTORS BE LIABLE FOR ANY
* DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
* ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

The logos, trademarks and symbols used in this book are the properties of
their respective owners.



8. What do you need?

You will require a computer and a medium containing a Linux distribution.
Most of this guide applies to all Linux distributions - and UNIX in general.
Apart from time, there are no further specific requirements.

The Installation HOWTO contains helpful information on how to obtain Linux
software and install it on your computer. Hardware requirements and
coexistence with other operating systems are also discussed.

CD images can be downloaded from [http://www.linux-iso.com/]  linux-iso.com
and many other locations, see Appendix A.

An interesting alternative for those who don't dare to take the step of an
actual Linux installation on their machine are the Linux distributions that
you can run from a CD, such as the [http://www.knoppix.com/] Knoppix
distribution.



9. Conventions used in this document

The following typographic and usage conventions occur in this text:

Table 1. Typographic and usage conventions


Text type        Meaning                                                  

"Quoted text"    Quotes from people, quoted computer output.              

terminal view    Literal computer input and output captured from the      
                 terminal, usually rendered with a light grey background. 

command          Name of a command that can be entered on the command     
                 line.                                                    

VARIABLE         Name of a variable or pointer to content of a variable,  
                 as in $VARNAME.                                          

option           Option to a command, as in "the -a option to the ls      
                 command".                                                

argument         Argument to a command, as in "read man ls ".             

prompt           User prompt, usually followed by a command that you type 
                 in a terminal window, like in hilda@home> ls -l          

command options  Command synopsis or general usage, on a separated line.  
arguments                                                                 

filename         Name of a file or directory, for example "Change to the /
                 usr/bin directory."                                      

Key              Keys to hit on the keyboard, such as "type Q to quit".   

Button           Graphical button to click, like the OK button.           

Menu->Choice     Choice to select from a graphical menu, for instance:    
                 "SelectHelp->About Mozilla in your browser."             

Terminology      Important term or concept: "The Linux kernel is the heart
                 of the system."                                          

                 The backslash in a terminal view or command synopsis     
\                indicates an unfinished line. In other words, if you see 
                 a long command that is cut into multiple lines, \ means  
                 "Don't press Enter yet!"                                 

See Chapter 1    link to related subject within this guide.               

[http://                                                                  
tille.garrels.be]Clickable link to an external web resource.              
The author                                                                


The following images are used:

Note This is a note
     It contains additional information or remarks.

Caution This is a caution
        It means be careful.

Warning This is a warning
        Be very careful.

Tip This is a tip
    Tips and tricks.



10. Organization of this document

This guide is part of the Linux Documentation Project and aims to be the
foundation for all other materials that you can get from the Project. As
such, it provides you with the fundamental knowledge needed by anyone who
wants to start working with a Linux system, while at the same time it tries
to consciously avoid re-inventing the hot water. Thus, you can expect this
book to be incomplete and full of links to sources of additional information
on your system, on the Internet and in your system documentation.

The first chapter is an introduction to the subject on Linux; the next two
discuss absolute basic commands. Chapters 4 and 5 discuss some more advanced
but still basic topics. Chapter 6 is needed for continuing with the rest,
since it discusses editing files, an ability you need to pass from Linux
newbie to Linux user. The following chapters discuss somewhat more advanced
topics that you will have to deal with in everyday Linux use.

All chapters come with exercises that will test your preparedness for the
next chapter.

  •  Chapter 1: What is Linux, how did it come into existence, advantages and
    disadvantages, what does the future hold for Linux, who should use it,
    installing your computer.

  •  Chapter 2: Getting started, connecting to the system, basic commands,
    where to find help.

  •  Chapter 3: The filesystem, important files and directories, managing
    files and directories, protecting your data.

  •  Chapter 4: Understanding and managing processes, boot and shutdown
    procedures, postponing tasks, repetitive tasks.

  •  Chapter 5: What are standard input, output and error and how are these
    features used from the command line.

  •  Chapter 6: Why you should learn to work with an editor, discussion of
    the most common editors.

  •  Chapter 7: Configuring your graphical, text and audio environment,
    settings for the non-native English speaking Linux user, tips for adding
    extra software.

  •  Chapter 8: Converting files to a printable format, getting them out of
    the printer, hints for solving print problems.

  •  Chapter 9: Preparing data to be backed up, discussion of various tools,
    remote backup.

  •  Chapter 10: Overview of Linux networking tools and user applications,
    with a short discussion of the underlying service daemon programs and
    secure networking.

  • Chapter 11: Sound and video, including Voice over IP and sound recording
    is discussed in this chapter.

  •  Appendix A: Which books to read and sites to visit when you have
    finished reading this one.

  •  Appendix B: A comparison.

  •  Appendix C: If you ever get stuck, these tables might be an outcome.
    Also a good argument when your boss insists that YOU should use HIS
    favorite shell.



Chapter 1. What is Linux?

    We will start with an overview of how Linux became the operating system
    it is today. We will discuss past and future development and take a
    closer look at the advantages and disadvantages of this system. We will
    talk about distributions, about Open Source in general and try to explain
    a little something about GNU.

    This chapter answers questions like:

     

      □ What is Linux?

      □ Where and how did Linux start?

      □ Isn't Linux that system where everything is done in text mode?

      □ Does Linux have a future or is it just hype?

      □ What are the advantages of using Linux?

      □ What are the disadvantages?

      □ What kinds of Linux are there and how do I choose the one that fits
        me?

      □ What are the Open Source and GNU movements?



1.1. History

1.1.1. UNIX

 In order to understand the popularity of Linux, we need to travel back in
time, about 30 years ago...

Imagine computers as big as houses, even stadiums. While the sizes of those
computers posed substantial problems, there was one thing that made this even
worse: every computer had a different operating system. Software was always
customized to serve a specific purpose, and software for one given system
didn't run on another system. Being able to work with one system didn't
automatically mean that you could work with another. It was difficult, both
for the users and the system administrators.

Computers were extremely expensive then, and sacrifices had to be made even
after the original purchase just to get the users to understand how they
worked. The total cost per unit of computing power was enormous.

Technologically the world was not quite that advanced, so they had to live
with the size for another decade. In 1969, a team of developers in the Bell
Labs laboratories started working on a solution for the software problem, to
address these compatibility issues. They developed a new operating system,
which was

 1. Simple and elegant.

 2. Written in the C programming language instead of in assembly code.

 3. Able to recycle code.

The Bell Labs developers named their project "UNIX."

 The code recycling features were very important. Until then, all
commercially available computer systems were written in a code specifically
developed for one system. UNIX on the other hand needed only a small piece of
that special code, which is now commonly named the kernel. This kernel is the
only piece of code that needs to be adapted for every specific system and
forms the base of the UNIX system. The operating system and all other
functions were built around this kernel and written in a higher programming
language, C. This language was especially developed for creating the UNIX
system. Using this new technique, it was much easier to develop an operating
system that could run on many different types of hardware.

The software vendors were quick to adapt, since they could sell ten times
more software almost effortlessly. Weird new situations came in existence:
imagine for instance computers from different vendors communicating in the
same network, or users working on different systems without the need for
extra education to use another computer. UNIX did a great deal to help users
become compatible with different systems.

Throughout the next couple of decades the development of UNIX continued. More
things became possible to do and more hardware and software vendors added
support for UNIX to their products.

UNIX was initially found only in very large environments with mainframes and
minicomputers (note that a PC is a "micro" computer). You had to work at a
university, for the government or for large financial corporations in order
to get your hands on a UNIX system.

But smaller computers were being developed, and by the end of the 80's, many
people had home computers. By that time, there were several versions of UNIX
available for the PC architecture, but none of them were truly free and more
important: they were all terribly slow, so most people ran MS DOS or Windows
3.1 on their home PCs.



1.1.2. Linus and Linux

By the beginning of the 90s home PCs were finally powerful enough to run a
full blown UNIX. Linus Torvalds, a young man studying computer science at the
university of Helsinki, thought it would be a good idea to have some sort of
freely available academic version of UNIX, and promptly started to code.

He started to ask questions, looking for answers and solutions that would
help him get UNIX on his PC. Below is one of his first posts in
comp.os.minix, dating from 1991:

From: torvalds@klaava.Helsinki.FI (Linus Benedict Torvalds)
Newsgroups: comp.os.minix
Subject: Gcc-1.40 and a posix-question
Message-ID: <1991Jul3.100050.9886@klaava.Helsinki.FI>
Date: 3 Jul 91 10:00:50 GMT
Hello netlanders,
Due to a project I'm working on (in minix), I'm interested in the posix
standard definition. Could somebody please point me to a (preferably)
machine-readable format of the latest posix rules? Ftp-sites would be
nice.

From the start, it was Linus' goal to have a free system that was completely
compliant with the original UNIX. That is why he asked for POSIX standards,
POSIX still being the standard for UNIX.

In those days plug-and-play wasn't invented yet, but so many people were
interested in having a UNIX system of their own, that this was only a small
obstacle. New drivers became available for all kinds of new hardware, at a
continuously rising speed. Almost as soon as a new piece of hardware became
available, someone bought it and submitted it to the Linux test, as the
system was gradually being called, releasing more free code for an ever wider
range of hardware. These coders didn't stop at their PC's; every piece of
hardware they could find was useful for Linux.

Back then, those people were called "nerds" or "freaks", but it didn't matter
to them, as long as the supported hardware list grew longer and longer.
Thanks to these people, Linux is now not only ideal to run on new PC's, but
is also the system of choice for old and exotic hardware that would be
useless if Linux didn't exist.

Two years after Linus' post, there were 12000 Linux users. The project,
popular with hobbyists, grew steadily, all the while staying within the
bounds of the POSIX standard. All the features of UNIX were added over the
next couple of years, resulting in the mature operating system Linux has
become today. Linux is a full UNIX clone, fit for use on workstations as well
as on middle-range and high-end servers. Today, a lot of the important
players on the hard- and software market each have their team of Linux
developers; at your local dealer's you can even buy pre-installed Linux
systems with official support - eventhough there is still a lot of hard- and
software that is not supported, too.



1.1.3. Current application of Linux systems

Today Linux has joined the desktop market. Linux developers concentrated on
networking and services in the beginning, and office applications have been
the last barrier to be taken down. We don't like to admit that Microsoft is
ruling this market, so plenty of alternatives have been started over the last
couple of years to make Linux an acceptable choice as a workstation,
providing an easy user interface and MS compatible office applications like
word processors, spreadsheets, presentations and the like.

On the server side, Linux is well-known as a stable and reliable platform,
providing database and trading services for companies like Amazon, the
well-known online bookshop, US Post Office, the German army and many others.
Especially Internet providers and Internet service providers have grown fond
of Linux as firewall, proxy- and web server, and you will find a Linux box
within reach of every UNIX system administrator who appreciates a comfortable
management station. Clusters of Linux machines are used in the creation of
movies such as "Titanic", "Shrek" and others. In post offices, they are the
nerve centers that route mail and in large search engine, clusters are used
to perform internet searches.These are only a few of the thousands of
heavy-duty jobs that Linux is performing day-to-day across the world.

It is also worth to note that modern Linux not only runs on workstations,
mid- and high-end servers, but also on "gadgets" like PDA's, mobiles, a
shipload of embedded applications and even on experimental wristwatches. This
makes Linux the only operating system in the world covering such a wide range
of hardware.



1.2. The user interface

1.2.1. Is Linux difficult?

Whether Linux is difficult to learn depends on the person you're asking.
Experienced UNIX users will say no, because Linux is an ideal operating
system for power-users and programmers, because it has been and is being
developed by such people.

Everything a good programmer can wish for is available: compilers, libraries,
development and debugging tools. These packages come with every standard
Linux distribution. The C-compiler is included for free - as opposed to many
UNIX distributions demanding licensing fees for this tool. All the
documentation and manuals are there, and examples are often included to help
you get started in no time. It feels like UNIX and switching between UNIX and
Linux is a natural thing.

In the early days of Linux, being an expert was kind of required to start
using the system. Those who mastered Linux felt better than the rest of the
"lusers" who hadn't seen the light yet. It was common practice to tell a
beginning user to "RTFM" (read the manuals). While the manuals were on every
system, it was difficult to find the documentation, and even if someone did,
explanations were in such technical terms that the new user became easily
discouraged from learning the system.

The Linux-using community started to realize that if Linux was ever to be an
important player on the operating system market, there had to be some serious
changes in the accessibility of the system.



1.2.2. Linux for non-experienced users

Companies such as RedHat, SuSE and Mandriva have sprung up, providing
packaged Linux distributions suitable for mass consumption. They integrated a
great deal of graphical user interfaces (GUIs), developed by the community,
in order to ease management of programs and services. As a Linux user today
you have all the means of getting to know your system inside out, but it is
no longer necessary to have that knowledge in order to make the system comply
to your requests.

Nowadays you can log in graphically and start all required applications
without even having to type a single character, while you still have the
ability to access the core of the system if needed. Because of its structure,
Linux allows a user to grow into the system: it equally fits new and
experienced users. New users are not forced to do difficult things, while
experienced users are not forced to work in the same way they did when they
first started learning Linux.

While development in the service area continues, great things are being done
for desktop users, generally considered as the group least likely to know how
a system works. Developers of desktop applications are making incredible
efforts to make the most beautiful desktops you've ever seen, or to make your
Linux machine look just like your former MS Windows or an Apple workstation.
The latest developments also include 3D acceleration support and support for
USB devices, single-click updates of system and packages, and so on. Linux
has these, and tries to present all available services in a logical form that
ordinary people can understand. Below is a short list containing some great
examples; these sites have a lot of screenshots that will give you a glimpse
of what Linux on the desktop can be like:

  • [http://www.gnome.org] http://www.gnome.org

  • [http://kde.org/screenshots/] http://kde.org/screenshots/

  • [http://www.openoffice.org] http://www.openoffice.org

  • [http://www.mozilla.org] http://www.mozilla.org



1.3. Does Linux have a future?

1.3.1. Open Source

The idea behind Open Source software is rather simple: when programmers can
read, distribute and change code, the code will mature. People can adapt it,
fix it, debug it, and they can do it at a speed that dwarfs the performance
of software developers at conventional companies. This software will be more
flexible and of a better quality than software that has been developed using
the conventional channels, because more people have tested it in more
different conditions than the closed software developer ever can.

The Open Source initiative started to make this clear to the commercial
world, and very slowly, commercial vendors are starting to see the point.
While lots of academics and technical people have already been convinced for
20 years now that this is the way to go, commercial vendors needed
applications like the Internet to make them realize they can profit from Open
Source. Now Linux has grown past the stage where it was almost exclusively an
academic system, useful only to a handful of people with a technical
background. Now Linux provides more than the operating system: there is an
entire infrastructure supporting the chain of effort of creating an operating
system, of making and testing programs for it, of bringing everything to the
users, of supplying maintenance, updates and support and customizations,
etcetera. Today, Linux is ready to accept the challenge of a fast-changing
world.



1.3.2. Ten years of experience at your service

While Linux is probably the most well-known Open Source initiative, there is
another project that contributed enormously to the popularity of the Linux
operating system. This project is called SAMBA, and its achievement is the
reverse engineering of the Server Message Block (SMB)/Common Internet File
System (CIFS) protocol used for file- and print-serving on PC-related
machines, natively supported by MS Windows NT and OS/2, and Linux. Packages
are now available for almost every system and provide interconnection
solutions in mixed environments using MS Windows protocols:
Windows-compatible (up to and includingWinXP) file- and print-servers.

Maybe even more successful than the SAMBA project is the Apache HTTP server
project. The server runs on UNIX, Windows NT and many other operating
systems. Originally known as "A PAtCHy server", based on existing code and a
series of "patch files", the name for the matured code deserves to be
connoted with the native American tribe of the Apache, well-known for their
superior skills in warfare strategy and inexhaustible endurance. Apache has
been shown to be substantially faster, more stable and more feature-full than
many other web servers. Apache is run on sites that get millions of visitors
per day, and while no official support is provided by the developers, the
Apache user community provides answers to all your questions. Commercial
support is now being provided by a number of third parties.

In the category of office applications, a choice of MS Office suite clones is
available, ranging from partial to full implementations of the applications
available on MS Windows workstations. These initiatives helped a great deal
to make Linux acceptable for the desktop market, because the users don't need
extra training to learn how to work with new systems. With the desktop comes
the praise of the common users, and not only their praise, but also their
specific requirements, which are growing more intricate and demanding by the
day.

The Open Source community, consisting largely of people who have been
contributing for over half a decade, assures Linux' position as an important
player on the desktop market as well as in general IT application. Paid
employees and volunteers alike are working diligently so that Linux can
maintain a position in the market. The more users, the more questions. The
Open Source community makes sure answers keep coming, and watches the quality
of the answers with a suspicious eye, resulting in ever more stability and
accessibility.

Listing all the available Linux software is beyond the scope of this guide,
as there are tens of thousands of packages. Throughout this course we will
present you with the most common packages, which are almost all freely
available. In order to take away some of the fear of the beginning user,
here's a screenshot of one of your most-wanted programs. You can see for
yourself that no effort has been spared to make users who are switching from
Windows feel at home:

Figure 1-1. OpenOffice MS-compatible Spreadsheet

[openoffice]



1.4. Properties of Linux

1.4.1. Linux Pros

A lot of the advantages of Linux are a consequence of Linux' origins, deeply
rooted in UNIX, except for the first advantage, of course:

  • Linux is free:

    As in free beer, they say. If you want to spend absolutely nothing, you
    don't even have to pay the price of a CD. Linux can be downloaded in its
    entirety from the Internet completely for free. No registration fees, no
    costs per user, free updates, and freely available source code in case
    you want to change the behavior of your system.

    Most of all, Linux is free as in free speech:

    The license commonly used is the GNU Public License (GPL). The license
    says that anybody who may want to do so, has the right to change Linux
    and eventually to redistribute a changed version, on the one condition
    that the code is still available after redistribution. In practice, you
    are free to grab a kernel image, for instance to add support for
    teletransportation machines or time travel and sell your new code, as
    long as your customers can still have a copy of that code.

  • Linux is portable to any hardware platform:

    A vendor who wants to sell a new type of computer and who doesn't know
    what kind of OS his new machine will run (say the CPU in your car or
    washing machine), can take a Linux kernel and make it work on his
    hardware, because documentation related to this activity is freely
    available.

  • Linux was made to keep on running:

    As with UNIX, a Linux system expects to run without rebooting all the
    time. That is why a lot of tasks are being executed at night or scheduled
    automatically for other calm moments, resulting in higher availability
    during busier periods and a more balanced use of the hardware. This
    property allows for Linux to be applicable also in environments where
    people don't have the time or the possibility to control their systems
    night and day.

  • Linux is secure and versatile:

    The security model used in Linux is based on the UNIX idea of security,
    which is known to be robust and of proven quality. But Linux is not only
    fit for use as a fort against enemy attacks from the Internet: it will
    adapt equally to other situations, utilizing the same high standards for
    security. Your development machine or control station will be as secure
    as your firewall.

  • Linux is scalable:

    From a Palmtop with 2 MB of memory to a petabyte storage cluster with
    hundreds of nodes: add or remove the appropriate packages and Linux fits
    all. You don't need a supercomputer anymore, because you can use Linux to
    do big things using the building blocks provided with the system. If you
    want to do little things, such as making an operating system for an
    embedded processor or just recycling your old 486, Linux will do that as
    well.

  • The Linux OS and most Linux applications have very short debug-times:

    Because Linux has been developed and tested by thousands of people, both
    errors and people to fix them are usually found rather quickly. It
    sometimes happens that there are only a couple of hours between discovery
    and fixing of a bug.



1.4.2. Linux Cons

  • There are far too many different distributions:

    "Quot capites, tot rationes", as the Romans already said: the more
    people, the more opinions. At first glance, the amount of Linux
    distributions can be frightening, or ridiculous, depending on your point
    of view. But it also means that everyone will find what he or she needs.
    You don't need to be an expert to find a suitable release.

    When asked, generally every Linux user will say that the best
    distribution is the specific version he is using. So which one should you
    choose? Don't worry too much about that: all releases contain more or
    less the same set of basic packages. On top of the basics, special third
    party software is added making, for example, TurboLinux more suitable for
    the small and medium enterprise, RedHat for servers and SuSE for
    workstations. However, the differences are likely to be very superficial.
    The best strategy is to test a couple of distributions; unfortunately not
    everybody has the time for this. Luckily, there is plenty of advice on
    the subject of choosing your Linux. A quick search on [http://
    www.google.com/linux] Google, using the keywords "choosing your
    distribution" brings up tens of links to good advise. The [http://
    www.tldp.org/HOWTO/Installation-HOWTO/]  Installation HOWTO also
    discusses choosing your distribution.

  • Linux is not very user friendly and confusing for beginners:

     It must be said that Linux, at least the core system, is less
    userfriendly to use than MS Windows and certainly more difficult than
    MacOS, but... In light of its popularity, considerable effort has been
    made to make Linux even easier to use, especially for new users. More
    information is being released daily, such as this guide, to help fill the
    gap for documentation available to users at all levels.

  • Is an Open Source product trustworthy?

    How can something that is free also be reliable? Linux users have the
    choice whether to use Linux or not, which gives them an enormous
    advantage compared to users of proprietary software, who don't have that
    kind of freedom. After long periods of testing, most Linux users come to
    the conclusion that Linux is not only as good, but in many cases better
    and faster that the traditional solutions. If Linux were not trustworthy,
    it would have been long gone, never knowing the popularity it has now,
    with millions of users. Now users can influence their systems and share
    their remarks with the community, so the system gets better and better
    every day. It is a project that is never finished, that is true, but in
    an ever changing environment, Linux is also a project that continues to
    strive for perfection.



1.5. Linux Flavors

1.5.1. Linux and GNU

 Although there are a large number of Linux implementations, you will find a
lot of similarities in the different distributions, if only because every
Linux machine is a box with building blocks that you may put together
following your own needs and views. Installing the system is only the
beginning of a longterm relationship. Just when you think you have a nice
running system, Linux will stimulate your imagination and creativeness, and
the more you realize what power the system can give you, the more you will
try to redefine its limits.

Linux may appear different depending on the distribution, your hardware and
personal taste, but the fundamentals on which all graphical and other
interfaces are built, remain the same. The Linux system is based on GNU tools
(Gnu's Not UNIX), which provide a set of standard ways to handle and use the
system. All GNU tools are open source, so they can be installed on any
system. Most distributions offer pre-compiled packages of most common tools,
such as RPM packages on RedHat and Debian packages (also called deb or dpkg)
on Debian, so you needn't be a programmer to install a package on your
system. However, if you are and like doing things yourself, you will enjoy
Linux all the better, since most distributions come with a complete set of
development tools, allowing installation of new software purely from source
code. This setup also allows you to install software even if it does not
exist in a pre-packaged form suitable for your system.

A list of common GNU software:

  • Bash: The GNU shell

  • GCC: The GNU C Compiler

  • GDB: The GNU Debugger

  • Coreutils: a set of basic UNIX-style utilities, such as ls, cat and chmod

  • Findutils: to search and find files

  • Fontutils: to convert fonts from one format to another or make new fonts

  • The Gimp: GNU Image Manipulation Program

  • Gnome: the GNU desktop environment

  • Emacs: a very powerful editor

  • Ghostscript and Ghostview: interpreter and graphical frontend for
    PostScript files.

  • GNU Photo: software for interaction with digital cameras

  • Octave: a programming language, primarily intended to perform numerical
    computations and image processing.

  • GNU SQL: relational database system

  • Radius: a remote authentication and accounting server

  • ...

Many commercial applications are available for Linux, and for more
information about these packages we refer to their specific documentation.
Throughout this guide we will only discuss freely available software, which
comes (in most cases) with a GNU license.

To install missing or new packages, you will need some form of software
management. The most common implementations include RPM and dpkg. RPM is the
RedHat Package Manager, which is used on a variety of Linux systems,
eventhough the name does not suggest this. Dpkg is the Debian package
management system, which uses an interface called apt-get, that can manage
RPM packages as well. Novell Ximian Red Carpet is a third party
implementation of RPM with a graphical front-end. Other third party software
vendors may have their own installation procedures, sometimes resembling the
InstallShield and such, as known on MS Windows and other platforms. As you
advance into Linux, you will likely get in touch with one or more of these
programs.



1.5.2. GNU/Linux

The Linux kernel (the bones of your system, see Section 3.2.3.1) is not part
of the GNU project but uses the same license as GNU software. A great
majority of utilities and development tools (the meat of your system), which
are not Linux-specific, are taken from the GNU project. Because any usable
system must contain both the kernel and at least a minimal set of utilities,
some people argue that such a system should be called a GNU/Linux system.

In order to obtain the highest possible degree of independence between
distributions, this is the sort of Linux that we will discuss throughout this
course. If we are not talking about a GNU/Linux system, the specific
distribution, version or program name will be mentioned.



1.5.3. Which distribution should I install?

Prior to installation, the most important factor is your hardware. Since
every Linux distribution contains the basic packages and can be built to meet
almost any requirement (because they all use the Linux kernel), you only need
to consider if the distribution will run on your hardware. LinuxPPC for
example has been made to run on Apple and other PowerPCs and does not run on
an ordinary x86 based PC. LinuxPPC does run on the new Macs, but you can't
use it for some of the older ones with ancient bus technology. Another tricky
case is Sun hardware, which could be an old SPARC CPU or a newer UltraSparc,
both requiring different versions of Linux.

Some Linux distributions are optimized for certain processors, such as Athlon
CPUs, while they will at the same time run decent enough on the standard 486,
586 and 686 Intel processors. Sometimes distributions for special CPUs are
not as reliable, since they are tested by fewer people.

Most Linux distributions offer a set of programs for generic PCs with special
packages containing optimized kernels for the x86 Intel based CPUs. These
distributions are well-tested and maintained on a regular basis, focusing on
reliant server implementation and easy installation and update procedures.
Examples are Debian, Ubuntu, Fedora, SuSE and Mandriva, which are by far the
most popular Linux systems and generally considered easy to handle for the
beginning user, while not blocking professionals from getting the most out of
their Linux machines. Linux also runs decently on laptops and middle-range
servers. Drivers for new hardware are included only after extensive testing,
which adds to the stability of a system.

While the standard desktop might be Gnome on one system, another might offer
KDE by default. Generally, both Gnome and KDE are available for all major
Linux distributions. Other window and desktop managers are available for more
advanced users.

The standard installation process allows users to choose between different
basic setups, such as a workstation, where all packages needed for everyday
use and development are installed, or a server installation, where different
network services can be selected. Expert users can install every combination
of packages they want during the initial installation process.

The goal of this guide is to apply to all Linux distributions. For your own
convenience, however, it is strongly advised that beginners stick to a
mainstream distribution, supporting all common hardware and applications by
default. The following are very good choices for novices:

  • [http://fedora.redhat.com] Fedora Core

  • [http://www.debian.org] Debian

  • [http://www.suse.de] SuSE Linux

  • [http://www.mandrake.com] Mandriva (former MandrakeSoft)

  • [http://www.knoppix.com] Knoppix: an operating system that runs from your
    CD-ROM, you don't need to install anything.

Downloadable ISO-images can be obtained from [http://www.linuxiso.org]
LinuxISO.org. The main distributions can be purchased in any decent computer
shop.



1.6. Summary

In this chapter, we learned that:

  • Linux is an implementation of UNIX.

  • The Linux operating system is written in the C programming language.

  • "De gustibus et coloribus non disputandum est": there's a Linux for
    everyone.

  • Linux uses GNU tools, a set of freely available standard tools for
    handling the operating system.



1.7. Exercises

 A practical exercise for starters: install Linux on your PC. Read the
installation manual for your distribution and/or the Installation HOWTO and
do it.

Note Read the docs!
     Most errors stem from not reading the information provided during the
     install. Reading the installation messages carefully is the first step
     on the road to success.

Things you must know BEFORE starting a Linux installation:

  • Will this distribution run on my hardware?

    Check with [http://www.tldp.org/HOWTO/Hardware-HOWTO/index.html]  http://
    www.tldp.org/HOWTO/Hardware-HOWTO/index.html when in doubt about
    compatibility of your hardware.

  • What kind of keyboard do I have (number of keys, layout)? What kind of
    mouse (serial/parallel, number of buttons)? How many MB of RAM?

  • Will I install a basic workstation or a server, or will I need to select
    specific packages myself?

  • Will I install from my hard disk, from a CD-ROM, or using the network?
    Should I adapt the BIOS for any of this? Does the installation method
    require a boot disk?

  • Will Linux be the only system on this computer, or will it be a dual boot
    installation? Should I make a large partition in order to install virtual
    systems later on, or is this a virtual installation itself?

  • Is this computer in a network? What is its hostname, IP address? Are
    there any gateway servers or other important networked machines my box
    should communicate with?

    Caution Linux expects to be networked
            Not using the network or configuring it incorrectly may result in
            slow startup.

  • Is this computer a gateway/router/firewall? (If you have to think about
    this question, it probably isn't.)

  • Partitioning: let the installation program do it for you this time, we
    will discuss partitions in detail in Chapter 3. There is system-specific
    documentation available if you want to know everything about it. If your
    Linux distribution does not offer default partitioning, that probably
    means it is not suited for beginners.

  • Will this machine start up in text mode or in graphical mode?

  • Think of a good password for the administrator of this machine (root).
    Create a non-root user account (non-privileged access to the system).

  • Do I need a rescue disk? (recommended)

  • Which languages do I want?

The full checklist can be found at [http://www.tldp.org/HOWTO/
Installation-HOWTO/index.html]  http://www.tldp.org/HOWTO/Installation-HOWTO/
index.html.

In the following chapters we will find out if the installation has been
successful.



Chapter 2. Quickstart

    In order to get the most out of this guide, we will immediately start
    with a practical chapter on connecting to the Linux system and doing some
    basic things.

    We will discuss:

     

      □ Connecting to the system

      □ Disconnecting from the system

      □ Text and graphic mode

      □ Changing your password

      □ Navigating through the file system

      □ Determining file type

      □ Looking at text files

      □ Finding help



2.1. Logging in, activating the user interface and logging out

2.1.1. Introduction

In order to work on a Linux system directly, you will need to provide a user
name and password. You always need to authenticate to the system. As we
already mentioned in the exercise from Chapter 1, most PC-based Linux systems
have two basic modes for a system to run in: either quick and sober in text
console mode, which looks like DOS with mouse, multitasking and multi-user
features, or in graphical mode, which looks better but eats more system
resources.



2.1.2. Graphical mode

This is the default nowadays on most desktop computers. You know you will
connect to the system using graphical mode when you are first asked for your
user name, and then, in a new window, to type your password.

To log in, make sure the mouse pointer is in the login window, provide your
user name and password to the system and click OK or press Enter.

Warning Careful with that root account!
        It is generally considered a bad idea to connect (graphically) using
        the root user name, the system adminstrator's account, since the use
        of graphics includes running a lot of extra programs, in root's case
        with a lot of extra permissions. To keep all risks as low as
        possible, use a normal user account to connect graphically. But there
        are enough risks to keep this in mind as a general advice, for all
        use of the root account: only log in as root when extra privileges
        are required.

After entering your user name/password combination, it can take a little
while before the graphical environment is started, depending on the CPU speed
of your computer, on the software you use and on your personal settings.

To continue, you will need to open a terminal window or xterm for short (X
being the name for the underlying software supporting the graphical
environment). This program can be found in the Applications->Utilities,
System Tools or Internet menu, depending on what window manager you are
using. There might be icons that you can use as a shortcut to get an xterm
window as well, and clicking the right mouse button on the desktop background
will usually present you with a menu containing a terminal window
application.

While browsing the menus, you will notice that a lot of things can be done
without entering commands via the keyboard. For most users, the good old
point-'n'-click method of dealing with the computer will do. But this guide
is for future network and system administrators, who will need to meddle with
the heart of the system. They need a stronger tool than a mouse to handle all
the tasks they will face. This tool is the shell, and when in graphical mode,
we activate our shell by opening a terminal window.

The terminal window is your control panel for the system. Almost everything
that follows is done using this simple but powerful text tool. A terminal
window should always show a command prompt when you open one. This terminal
shows a standard prompt, which displays the user's login name, and the
current working directory, represented by the twiddle (~):

Figure 2-1. Terminal window

[xterm]

Another common form for a prompt is this one:

[user@host dir]

In the above example, user will be your login name, hosts the name of the
machine you are working on, and dir an indication of your current location in
the file system.

Later we will discuss prompts and their behavior in detail. For now, it
suffices to know that prompts can display all kinds of information, but that
they are not part of the commands you are giving to your system.

To disconnect from the system in graphical mode, you need to close all
terminal windows and other applications. After that, hit the logout icon or
find Log Out in the menu. Closing everything is not really necessary, and the
system can do this for you, but session management might put all currently
open applications back on your screen when you connect again, which takes
longer and is not always the desired effect. However, this behavior is
configurable.

When you see the login screen again, asking to enter user name and password,
logout was successful.

Note Gnome or KDE?
     We mentioned both the Gnome and KDE desktops already a couple of times.
     These are the two most popular ways of managing your desktop, although
     there are many, many others. Whatever desktop you chose to work with is
     fine - as long as you know how to open a terminal window. However, we
     will continue to refer to both Gnome and KDE for the most popular ways
     of achieving certain tasks.



2.1.3. Text mode

You know you're in text mode when the whole screen is black, showing (in most
cases white) characters. A text mode login screen typically shows some
information about the machine you are working on, the name of the machine and
a prompt waiting for you to log in:


RedHat Linux Release 8.0 (Psyche)

blast login: _



The login is different from a graphical login, in that you have to hit the
Enter key after providing your user name, because there are no buttons on the
screen that you can click with the mouse. Then you should type your password,
followed by another Enter. You won't see any indication that you are entering
something, not even an asterisk, and you won't see the cursor move. But this
is normal on Linux and is done for security reasons.

When the system has accepted you as a valid user, you may get some more
information, called the message of the day, which can be anything.
Additionally, it is popular on UNIX systems to display a fortune cookie,
which contains some general wise or unwise (this is up to you) thoughts.
After that, you will be given a shell, indicated with the same prompt that
you would get in graphical mode.

Warning Don't log in as root
        Also in text mode: log in as root only to do setup and configuration
        that absolutely requires administrator privileges, such as adding
        users, installing software packages, and performing network and other
        system configuration. Once you are finished, immediately leave the
        special account and resume your work as a non-privileged user.
        Alternatively, some systems, like Ubuntu, force you to use sudo, so
        that you do not need direct access to the administrative account.

Logging out is done by entering the logout command, followed by Enter. You
are successfully disconnected from the system when you see the login screen
again.

Caution The power button
        While Linux was not meant to be shut off without application of the
        proper procedures for halting the system, hitting the power button is
        equivalent to starting those procedures on newer systems. However,
        powering off an old system without going through the halting process
        might cause severe damage! If you want to be sure, always use the
        Shut down option when you log out from the graphical interface, or,
        when on the login screen (where you have to give your user name and
        password) look around for a shutdown button.

Now that we know how to connect to and disconnect from the system, we're
ready for our first commands.



2.2. Absolute basics

2.2.1. The commands

These are the quickies, which we need to get started; we will discuss them
later in more detail.

Table 2-1. Quickstart commands


Command   Meaning                                                         

ls        Displays a list of files in the current working directory, like 
          the dir command in DOS                                          

cd        change directories                                              
directory                                                                 

passwd    change the password for the current user                        

file      display file type of file with name filename                    
filename                                                                  

cat       throws content of textfile on the screen                        
textfile                                                                  

pwd       display present working directory                               

exit or   leave this session                                              
logout                                                                    

man       read man pages on command                                       
command                                                                   

info      read Info pages on command                                      
command                                                                   

apropos   search the whatis database for strings                          
string                                                                    



2.2.2. General remarks

You type these commands after the prompt, in a terminal window in graphical
mode or in text mode, followed by Enter.

Commands can be issued by themselves, such as ls. A command behaves different
when you specify an option, usually preceded with a dash (-), as in ls -a.
The same option character may have a different meaning for another command.
GNU programs take long options, preceded by two dashes (--), like ls --all.
Some commands have no options.

The argument(s) to a command are specifications for the object(s) on which
you want the command to take effect. An example is ls /etc, where the
directory /etc is the argument to the ls command. This indicates that you
want to see the content of that directory, instead of the default, which
would be the content of the current directory, obtained by just typing ls
followed by Enter. Some commands require arguments, sometimes arguments are
optional.

You can find out whether a command takes options and arguments, and which
ones are valid, by checking the online help for that command, see Section 2.3
.

In Linux, like in UNIX, directories are separated using forward slashes, like
the ones used in web addresses (URLs). We will discuss directory structure
in-depth later.

The symbols . and .. have special meaning when directories are concerned. We
will try to find out about those during the exercises, and more in the next
chapter.

Try to avoid logging in with or using the system administrator's account,
root. Besides doing your normal work, most tasks, including checking the
system, collecting information etc., can be executed using a normal user
account with no special permissions at all. If needed, for instance when
creating a new user or installing new software, the preferred way of
obtaining root access is by switching user IDs, see Section 3.2.1 for an
example.

Almost all commands in this book can be executed without system administrator
privileges. In most cases, when issuing a command or starting a program as a
non-privileged user, the system will warn you or prompt you for the root
password when root access is required. Once you're done, leave the
application or session that gives you root privileges immediately.

Reading documentation should become your second nature. Especially in the
beginning, it is important to read system documentation, manuals for basic
commands, HOWTOs and so on. Since the amount of documentation is so enormous,
it is impossible to include all related documentation. This book will try to
guide you to the most appropriate documentation on every subject discussed,
in order to stimulate the habit of reading the man pages.



2.2.3. Using Bash features

Several special key combinations allow you to do things easier and faster
with the GNU shell, Bash, which is the default on almost any Linux system,
see Section 3.2.3.2. Below is a list of the most commonly used features; you
are strongly suggested to make a habit out of using them, so as to get the
most out of your Linux experience from the very beginning.

Table 2-2. Key combinations in Bash


Key or key    Function                                                    
combination                                                               

Ctrl+A        Move cursor to the beginning of the command line.           

Ctrl+C        End a running program and return the prompt, see Chapter 4. 

Ctrl+D        Log out of the current shell session, equal to typing exit  
              or logout.                                                  

Ctrl+E        Move cursor to the end of the command line.                 

Ctrl+H        Generate backspace character.                               

Ctrl+L        Clear this terminal.                                        

Ctrl+R        Search command history, see Section 3.3.3.4.                

Ctrl+Z        Suspend a program, see Chapter 4.                           

ArrowLeft and Move the cursor one place to the left or right on the       
ArrowRight    command line, so that you can insert characters at other    
              places than just at the beginning and the end.              

ArrowUp and   Browse history. Go to the line that you want to repeat, edit
ArrowDown     details if necessary, and press Enter to save time.         

Shift+PageUp  Browse terminal buffer (to see text that has "scrolled off" 
and Shift+    the screen).                                                
PageDown                                                                  

              Command or filename completion; when multiple choices are   
Tab           possible, the system will either signal with an audio or    
              visual bell, or, if too many choices are possible, ask you  
              if you want to see them all.                                

Tab Tab       Shows file or command completion possibilities.             


The last two items in the above table may need some extra explanations. For
instance, if you want to change into the directory
directory_with_a_very_long_name, you are not going to type that very long
name, no. You just type on the command line cd dir, then you press Tab and
the shell completes the name for you, if no other files are starting with the
same three characters. Of course, if there are no other items starting with
"d", then you might just as wel type cd d and then Tab. If more than one file
starts with the same characters, the shell will signal this to you, upon
which you can hit Tab twice with short interval, and the shell presents the
choices you have:

your_prompt> cd st
starthere        stuff          stuffit

In the above example, if you type "a" after the first two characters and hit
Tab again, no other possibilities are left, and the shell completes the
directory name, without you having to type the string "rthere":

your_prompt> cd starthere

Of course, you'll still have to hit Enter to accept this choice.

In the same example, if you type "u", and then hit Tab, the shell will add
the "ff" for you, but then it protests again, because multiple choices are
possible. If you type Tab Tab again, you'll see the choices; if you type one
or more characters that make the choice unambiguous to the system, and Tab
again, or Enter when you've reach the end of the file name that you want to
choose, the shell completes the file name and changes you into that directory
- if indeed it is a directory name.

This works for all file names that are arguments to commands.

The same goes for command name completion. Typing ls and then hitting the Tab
key twice, lists all the commands in your PATH (see Section 3.2.1) that start
with these two characters:

your_prompt> ls
ls           lsdev        lspci        lsraid       lsw
lsattr       lsmod        lspgpot      lss16toppm
lsb_release  lsof         lspnp        lsusb



2.3. Getting help

2.3.1. Be warned

GNU/Linux is all about becoming more self-reliant. And as usual with this
system, there are several ways to achieve the goal. A common way of getting
help is finding someone who knows, and however patient and peace-loving the
Linux-using community will be, almost everybody will expect you to have tried
one or more of the methods in this section before asking them, and the ways
in which this viewpoint is expressed may be rather harsh if you prove not to
have followed this basic rule.



2.3.2. The man pages

A lot of beginning users fear the man (manual) pages, because they are an
overwhelming source of documentation. They are, however, very structured, as
you will see from the example below on: man man.

Reading man pages is usually done in a terminal window when in graphical
mode, or just in text mode if you prefer it. Type the command like this at
the prompt, followed by Enter:

yourname@yourcomp ~> man man

The documentation for man will be displayed on your screen after you press
Enter:

man(1)                                                         man(1)



NAME
 man - format and display the on-line manual pages
 manpath - determine user's search path for man pages

SYNOPSIS
 man [-acdfFhkKtwW] [--path] [-m system] [-p string] [-C config_file]
 [-M pathlist] [-P pager] [-S section_list] [section] name ...


DESCRIPTION
 man formats and displays the on-line manual pages.  If you specify
 section, man only looks in that section of the manual.
 name is normally the name of the manual page, which is typically the
 name of a  command, function, or file.  However, if name contains a
 slash (/) then man interprets it as a file specification, so that you
 can do man ./foo.5 or even man /cd/foo/bar.1.gz.

 See  below  for  a  description  of where man looks for the manual
 page files.

OPTIONS
 -C  config_file
lines 1-27

Browse to the next page using the space bar. You can go back to the previous
page using the b-key. When you reach the end, man will usually quit and you
get the prompt back. Type q if you want to leave the man page before reaching
the end, or if the viewer does not quit automatically at the end of the page.

Note Pagers
     The available key combinations for manipulating the man pages depend on
     the pager used in your distribution. Most distributions use less to view
     the man pages and to scroll around. See Section 3.3.4.2 for more info on
     pagers.

Each man page usually contains a couple of standard sections, as we can see
from the man man example:

  • The first line contains the name of the command you are reading about,
    and the id of the section in which this man page is located. The man
    pages are ordered in chapters. Commands are likely to have multiple man
    pages, for example the man page from the user section, the man page from
    the system admin section, and the man page from the programmer section.

  • The name of the command and a short description are given, which is used
    for building an index of the man pages. You can look for any given search
    string in this index using the apropos command.

  • The synopsis of the command provides a technical notation of all the
    options and/or arguments this command can take. You can think of an
    option as a way of executing the command. The argument is what you
    execute it on. Some commands have no options or no arguments. Optional
    options and arguments are put in between "[" and "]" to indicate that
    they can be left out.

  • A longer description of the command is given.

  • Options with their descriptions are listed. Options can usually be
    combined. If not so, this section will tell you about it.

  • Environment describes the shell variables that influence the behavior of
    this command (not all commands have this).

  • Sometimes sections specific to this command are provided.

  • A reference to other man pages is given in the "SEE ALSO" section. In
    between parentheses is the number of the man page section in which to
    find this command. Experienced users often switch to the "SEE ALSO" part
    using the / command followed by the search string SEE and press Enter.

  • Usually there is also information about known bugs (anomalies) and where
    to report new bugs you may find.

  • There might also be author and copyright information.

Some commands have multiple man pages. For instance, the passwd command has a
man page in section 1 and another in section 5. By default, the man page with
the lowest number is shown. If you want to see another section than the
default, specify it after the man command:

man 5 passwd

If you want to see all man pages about a command, one after the other, use
the -a to man:

man -a passwd

This way, when you reach the end of the first man page and press SPACE again,
the man page from the next section will be displayed.



2.3.3. More info

2.3.3.1. The Info pages

In addition to the man pages, you can read the Info pages about a command,
using the info command. These usually contain more recent information and are
somewhat easier to use. The man pages for some commands refer to the Info
pages.

Get started by typing info info in a terminal window:

File: info.info,  Node: Top,  Next: Getting Started,  Up: (dir)

Info: An Introduction
*********************

   Info is a program, which you are using now, for reading
documentation of computer programs.  The GNU Project distributes most
of its on-line manuals in the Info format, so you need a program called
"Info reader" to read the manuals.  One of such programs you are using
now.

   If you are new to Info and want to learn how to use it, type the
command `h' now.  It brings you to a programmed instruction sequence.

   To learn advanced Info commands, type `n' twice.  This brings you to
`Info for Experts', skipping over the `Getting Started' chapter.

* Menu:

* Getting Started::             Getting started using an Info reader.
* Advanced Info::               Advanced commands within Info.
* Creating an Info File::       How to make your own Info file.
--zz-Info: (info.info.gz)Top, 24 lines --Top-------------------------------
Welcome to Info version 4.2. Type C-h for help, m for menu item.

Use the arrow keys to browse through the text and move the cursor on a line
starting with an asterisk, containing the keyword about which you want info,
then hit Enter. Use the P and N keys to go to the previous or next subject.
The space bar will move you one page further, no matter whether this starts a
new subject or an Info page for another command. Use Q to quit. The info
program has more information.



2.3.3.2. The whatis and apropos commands

A short index of explanations for commands is available using the whatis
command, like in the examples below:

[your_prompt] whatis ls
ls                   (1)  - list directory contents

This displays short information about a command, and the first section in the
collection of man pages that contains an appropriate page.

If you don't know where to get started and which man page to read, apropos
gives more information. Say that you don't know how to start a browser, then
you could enter the following command:

another prompt> apropos browser
Galeon [galeon](1)  - gecko-based GNOME web browser
lynx           (1)  - a general purpose distributed information browser
                    for the World Wide Web
ncftp          (1)  - Browser program for the File Transfer Protocol
opera          (1)  - a graphical web browser
pilot          (1)  - simple file system browser in the style of the
                    Pine Composer
pinfo          (1)  - curses based lynx-style info browser
pinfo [pman]   (1)  - curses based lynx-style info browser
viewres        (1x)  - graphical class browser for Xt

After pressing Enter you will see that a lot of browser related stuff is on
your machine: not only web browsers, but also file and FTP browsers, and
browsers for documentation. If you have development packages installed, you
may also have the accompanying man pages dealing with writing programs having
to do with browsers. Generally, a command with a man page in section one, so
one marked with "(1)", is suitable for trying out as a user. The user who
issued the above apropos might consequently try to start the commands galeon,
lynx or opera, since these clearly have to do with browsing the world wide
web.



2.3.3.3. The --help option

Most GNU commands support the --help, which gives a short explanation about
how to use the command and a list of available options. Below is the output
of this option with the cat command:

userprompt@host: cat --help
Usage: cat [OPTION] [FILE]...
Concatenate FILE(s), or standard input, to standard output.

  -A, --show-all           equivalent to -vET
  -b, --number-nonblank    number nonblank output lines
  -e                       equivalent to -vE
  -E, --show-ends          display $ at end of each line
  -n, --number             number all output lines
  -s, --squeeze-blank      never more than one single blank line
  -t                       equivalent to -vT
  -T, --show-tabs          display TAB characters as ^I
  -u                       (ignored)
  -v, --show-nonprinting   use ^ and M- notation,
                           except for LFD and TAB
      --help     display this help and exit
      --version  output version information and exit

With no FILE, or when FILE is -, read standard input.

Report bugs to <bug-textutils@gnu.org>.



2.3.3.4. Graphical help

Don't despair if you prefer a graphical user interface. Konqueror, the
default KDE file manager, provides painless and colourful access to the man
and Info pages. You may want to try "info:info" in the Location address bar,
and you will get a browsable Info page about the info command. Similarly,
"man:ls" will present you with the man page for the ls command. You even get
command name completion: you will see the man pages for all the commands
starting with "ls" in a scroll-down menu. Entering "info:/dir" in the address
location toolbar displays all the Info pages, arranged in utility categories.
Excellent Help content, including the Konqueror Handbook. Start up from the
menu or by typing the command konqueror in a terminal window, followed by
Enter; see the screenshot below.

Figure 2-2. Konqueror as help browser

[konqueror]

The Gnome Help Browser is very user friendly as well. You can start it
selecting Applications->Help from the Gnome menu, by clicking the lifeguard
icon on your desktop or by entering the command gnome-help in a terminal
window. The system documentation and man pages are easily browsable with a
plain interface.

The nautilus file manager provides a searchable index of the man and Info
pages, they are easily browsable and interlinked. Nautilus is started from
the command line, or clicking your home directory icon, or from the Gnome
menu.

The big advantage of GUIs for system documentation is that all information is
completely interlinked, so you can click through in the "SEE ALSO" sections
and wherever links to other man pages appear, and thus browse and acquire
knowledge without interruption for hours at the time.



2.3.3.5. Exceptions

Some commands don't have separate documentation, because they are part of
another command. cd, exit, logout and pwd are such exceptions. They are part
of your shell program and are called shell built-in commands. For information
about these, refer to the man or info page of your shell. Most beginning
Linux users have a Bash shell. See Section 3.2.3.2 for more about shells.

If you have been changing your original system configuration, it might also
be possible that man pages are still there, but not visible because your
shell environment has changed. In that case, you will need to check the
MANPATH variable. How to do this is explained in Section 7.2.1.2.

Some programs or packages only have a set of instructions or references in
the directory /usr/share/doc. See Section 3.3.4 to display.

In the worst case, you may have removed the documentation from your system by
accident (hopefully by accident, because it is a very bad idea to do this on
purpose). In that case, first try to make sure that there is really nothing
appropriate left using a search tool, read on in Section 3.3.3. If so, you
may have to re-install the package that contains the command to which the
documentation applied, see Section 7.5.



2.4. Summary

Linux traditionally operates in text mode or in graphical mode. Since CPU
power and RAM are not the cost anymore these days, every Linux user can
afford to work in graphical mode and will usually do so. This does not mean
that you don't have to know about text mode: we will work in the text
environment throughout this course, using a terminal window.

Linux encourages its users to acquire knowledge and to become independent.
Inevitably, you will have to read a lot of documentation to achieve that
goal; that is why, as you will notice, we refer to extra documentation for
almost every command, tool and problem listed in this book. The more docs you
read, the easier it will become and the faster you will leaf through manuals.
Make reading documentation a habit as soon as possible. When you don't know
the answer to a problem, refering to the documentation should become a second
nature.

We already learned some commands:

Table 2-3. New commands in chapter 2: Basics


CommandMeaning                                       

aproposSearch information about a command or subject.

cat    Show content of one or more files.            

cd     Change into another directory.                

exit   Leave a shell session.                        

file   Get information about the content of a file.  

info   Read Info pages about a command.              

logout Leave a shell session.                        

ls     List directory content.                       

man    Read manual pages of a command.               

passwd Change your password.                         

pwd    Display the current working directory.        



2.5. Exercises

Most of what we learn is by making mistakes and by seeing how things can go
wrong. These exercises are made to get you to read some error messages. The
order in which you do these exercises is important.

Don't forget to use the Bash features on the command line: try to do the
exercises typing as few characters as possible!



2.5.1. Connecting and disconnecting

 

  • Determine whether you are working in text or in graphical mode.

    I am working in text/graphical mode. (cross out what's not applicable)

  • Log in with the user name and password you made for yourself during the
    installation.

  • Log out.

  • Log in again, using a non-existent user name

    -> What happens?



2.5.2. Passwords

Log in again with your user name and password.

  • Change your password into P6p3.aa! and hit the Enter key.

    -> What happens?

  • Try again, this time enter a password that is ridiculously easy, like 123
    or aaa.

    -> What happens?

  • Try again, this time don't enter a password but just hit the Enter key.

    -> What happens?

  • Try the command psswd instead of passwd

    -> What happens?

Warning New password
        Unless you change your password back again to what it was before this
        exercise, it will be "P6p3.aa!". Change your password after this
        exercise!
 
        Note that some systems might not allow to recycle passwords, i.e.
        restore the original one within a certain amount of time or a certain
        amount of password changes, or both.



2.5.3. Directories

These are some exercises to help you get the feel.

  • Enter the command cd blah

    -> What happens?

  • Enter the command cd ..

    Mind the space between "cd" and ".."! Use the pwd command.

    -> What happens?

  • List the directory contents with the ls command.

    -> What do you see?

    -> What do you think these are?

    -> Check using the pwd command.

  • Enter the cd command.

    -> What happens?

  • Repeat step 2 two times.

    -> What happens?

  • Display the content of this directory.

  • Try the command cd root

    -> What happens?

    -> To which directories do you have access?

  • Repeat step 4.

    Do you know another possibility to get where you are now?



2.5.4. Files

  • Change directory to / and then to etc. Type ls; if the output is longer
    than your screen, make the window longer, or try Shift+PageUp and Shift+
    PageDown.

    The file inittab contains the answer to the first question in this list.
    Try the file command on it.

    -> The file type of my inittab is .....

  • Use the command cat inittab and read the file.

    -> What is the default mode of your computer?

  • Return to your home directory using the cd command.

  • Enter the command file .

    -> Does this help to find the meaning of "."?

  • Can you look at "." using the cat command?

  • Display help for the cat program, using the --help option. Use the option
    for numbering of output lines to count how many users are listed in the
    file /etc/passwd.



2.5.5. Getting help

  • Read man intro

  • Read man ls

  • Read info passwd

  • Enter the apropos pwd command.

  • Try man or info on cd.

    -> How would you find out more about cd?

  • Read ls --help and try it out.



Chapter 3. About files and the file system

    After the initial exploration in Chapter 2, we are ready to discuss the
    files and directories on a Linux system in more detail. Many users have
    difficulties with Linux because they lack an overview of what kind of
    data is kept in which locations. We will try to shine some light on the
    organization of files in the file system.

    We will also list the most important files and directories and use
    different methods of viewing the content of those files, and learn how
    files and directories can be created, moved and deleted.

    After completion of the exercises in this chapter, you will be able to:

     

      □ Describe the layout of a Linux file system

      □ Display and set paths

      □ Describe the most important files, including kernel and shell

      □ Find lost and hidden files

      □ Create, move and delete files and directories

      □ Display contents of files

      □ Understand and use different link types

      □ Find out about file properties and change file permissions



3.1. General overview of the Linux file system

3.1.1. Files

3.1.1.1. General

A simple description of the UNIX system, also applicable to Linux, is this:

"On a UNIX system, everything is a file; if something is not a file, it is a
process."

This statement is true because there are special files that are more than
just files (named pipes and sockets, for instance), but to keep things
simple, saying that everything is a file is an acceptable generalization. A
Linux system, just like UNIX, makes no difference between a file and a
directory, since a directory is just a file containing names of other files.
Programs, services, texts, images, and so forth, are all files. Input and
output devices, and generally all devices, are considered to be files,
according to the system.

In order to manage all those files in an orderly fashion, man likes to think
of them in an ordered tree-like structure on the hard disk, as we know from
MS-DOS (Disk Operating System) for instance. The large branches contain more
branches, and the branches at the end contain the tree's leaves or normal
files. For now we will use this image of the tree, but we will find out later
why this is not a fully accurate image.



3.1.1.2. Sorts of files

Most files are just files, called regular files; they contain normal data,
for example text files, executable files or programs, input for or output
from a program and so on.

While it is reasonably safe to suppose that everything you encounter on a
Linux system is a file, there are some exceptions.

  • Directories: files that are lists of other files.

  • Special files: the mechanism used for input and output. Most special
    files are in /dev, we will discuss them later.

  • Links: a system to make a file or directory visible in multiple parts of
    the system's file tree. We will talk about links in detail.

  • (Domain) sockets: a special file type, similar to TCP/IP sockets,
    providing inter-process networking protected by the file system's access
    control.

  • Named pipes: act more or less like sockets and form a way for processes
    to communicate with each other, without using network socket semantics.

The -l option to ls displays the file type, using the first character of each
input line:

jaime:~/Documents> ls -l
total 80
-rw-rw-r--   1 jaime   jaime   31744 Feb 21 17:56 intro Linux.doc
-rw-rw-r--   1 jaime   jaime   41472 Feb 21 17:56 Linux.doc
drwxrwxr-x   2 jaime   jaime    4096 Feb 25 11:50 course

This table gives an overview of the characters determining the file type:

Table 3-1. File types in a long list


SymbolMeaning      

-     Regular file 

d     Directory    

l     Link         

c     Special file 

s     Socket       

p     Named pipe   

b     Block device 


In order not to always have to perform a long listing for seeing the file
type, a lot of systems by default don't issue just ls, but ls -F, which
suffixes file names with one of the characters "/=*|@" to indicate the file
type. To make it extra easy on the beginning user, both the -F and --color
options are usually combined, see Section 3.3.1.1. We will use ls -F
throughout this document for better readability.

As a user, you only need to deal directly with plain files, executable files,
directories and links. The special file types are there for making your
system do what you demand from it and are dealt with by system administrators
and programmers.

 Now, before we look at the important files and directories, we need to know
more about partitions.



3.1.2. About partitioning

3.1.2.1. Why partition?

Most people have a vague knowledge of what partitions are, since every
operating system has the ability to create or remove them. It may seem
strange that Linux uses more than one partition on the same disk, even when
using the standard installation procedure, so some explanation is called for.

One of the goals of having different partitions is to achieve higher data
security in case of disaster. By dividing the hard disk in partitions, data
can be grouped and separated. When an accident occurs, only the data in the
partition that got the hit will be damaged, while the data on the other
partitions will most likely survive.

This principle dates from the days when Linux didn't have journaled file
systems and power failures might have lead to disaster. The use of partitions
remains for security and robustness reasons, so a breach on one part of the
system doesn't automatically mean that the whole computer is in danger. This
is currently the most important reason for partitioning. A simple example: a
user creates a script, a program or a web application that starts filling up
the disk. If the disk contains only one big partition, the entire system will
stop functioning if the disk is full. If the user stores the data on a
separate partition, then only that (data) partition will be affected, while
the system partitions and possible other data partitions keep functioning.

Mind that having a journaled file system only provides data security in case
of power failure and sudden disconnection of storage devices. This does not
protect your data against bad blocks and logical errors in the file system.
In those cases, you should use a RAID (Redundant Array of Inexpensive Disks)
solution.



3.1.2.2. Partition layout and types

There are two kinds of major partitions on a Linux system:

  • data partition: normal Linux system data, including the root partition
    containing all the data to start up and run the system; and

  • swap partition: expansion of the computer's physical memory, extra memory
    on hard disk.

Most systems contain a root partition, one or more data partitions and one or
more swap partitions. Systems in mixed environments may contain partitions
for other system data, such as a partition with a FAT or VFAT file system for
MS Windows data.

Most Linux systems use fdisk at installation time to set the partition type.
As you may have noticed during the exercise from Chapter 1, this usually
happens automatically. On some occasions, however, you may not be so lucky.
In such cases, you will need to select the partition type manually and even
manually do the actual partitioning. The standard Linux partitions have
number 82 for swap and 83 for data, which can be journaled (ext3) or normal
(ext2, on older systems). The fdisk utility has built-in help, should you
forget these values.

Apart from these two, Linux supports a variety of other file system types,
such as the relatively new Reiser file system, JFS, NFS, FATxx and many other
file systems natively available on other (proprietary) operating systems.

The standard root partition (indicated with a single forward slash, /) is
about 100-500 MB, and contains the system configuration files, most basic
commands and server programs, system libraries, some temporary space and the
home directory of the administrative user. A standard installation requires
about 250 MB for the root partition.

Swap space (indicated with swap) is only accessible for the system itself,
and is hidden from view during normal operation. Swap is the system that
ensures, like on normal UNIX systems, that you can keep on working, whatever
happens. On Linux, you will virtually never see irritating messages like Out
of memory, please close some applications first and try again, because of
this extra memory. The swap or virtual memory procedure has long been adopted
by operating systems outside the UNIX world by now.

Using memory on a hard disk is naturally slower than using the real memory
chips of a computer, but having this little extra is a great comfort. We will
learn more about swap when we discuss processes in Chapter 4.

Linux generally counts on having twice the amount of physical memory in the
form of swap space on the hard disk. When installing a system, you have to
know how you are going to do this. An example on a system with 512 MB of RAM:

  • 1st possibility: one swap partition of 1 GB

  • 2nd possibility: two swap partitions of 512 MB

  • 3rd possibility: with two hard disks: 1 partition of 512 MB on each disk.

The last option will give the best results when a lot of I/O is to be
expected.

Read the software documentation for specific guidelines. Some applications,
such as databases, might require more swap space. Others, such as some
handheld systems, might not have any swap at all by lack of a hard disk. Swap
space may also depend on your kernel version.

The kernel is on a separate partition as well in many distributions, because
it is the most important file of your system. If this is the case, you will
find that you also have a /boot partition, holding your kernel(s) and
accompanying data files.

The rest of the hard disk(s) is generally divided in data partitions,
although it may be that all of the non-system critical data resides on one
partition, for example when you perform a standard workstation installation.
When non-critical data is separated on different partitions, it usually
happens following a set pattern:

  • a partition for user programs (/usr)

  • a partition containing the users' personal data (/home)

  • a partition to store temporary data like print- and mail-queues (/var)

  • a partition for third party and extra software (/opt)

Once the partitions are made, you can only add more. Changing sizes or
properties of existing partitions is possible but not advisable.

The division of hard disks into partitions is determined by the system
administrator. On larger systems, he or she may even spread one partition
over several hard disks, using the appropriate software. Most distributions
allow for standard setups optimized for workstations (average users) and for
general server purposes, but also accept customized partitions. During the
installation process you can define your own partition layout using either
your distribution specific tool, which is usually a straight forward
graphical interface, or fdisk, a text-based tool for creating partitions and
setting their properties.

A workstation or client installation is for use by mainly one and the same
person. The selected software for installation reflects this and the stress
is on common user packages, such as nice desktop themes, development tools,
client programs for E-mail, multimedia software, web and other services.
Everything is put together on one large partition, swap space twice the
amount of RAM is added and your generic workstation is complete, providing
the largest amount of disk space possible for personal use, but with the
disadvantage of possible data integrity loss during problem situations.

On a server, system data tends to be separate from user data. Programs that
offer services are kept in a different place than the data handled by this
service. Different partitions will be created on such systems:

  • a partition with all data necessary to boot the machine

  • a partition with configuration data and server programs

  • one or more partitions containing the server data such as database
    tables, user mails, an ftp archive etc.

  • a partition with user programs and applications

  • one or more partitions for the user specific files (home directories)

  • one or more swap partitions (virtual memory)

Servers usually have more memory and thus more swap space. Certain server
processes, such as databases, may require more swap space than usual; see the
specific documentation for detailed information. For better performance, swap
is often divided into different swap partitions.



3.1.2.3. Mount points

All partitions are attached to the system via a mount point. The mount point
defines the place of a particular data set in the file system. Usually, all
partitions are connected through the root partition. On this partition, which
is indicated with the slash (/), directories are created. These empty
directories will be the starting point of the partitions that are attached to
them. An example: given a partition that holds the following directories:

videos/         cd-images/      pictures/

We want to attach this partition in the filesystem in a directory called /opt
/media. In order to do this, the system administrator has to make sure that
the directory /opt/media exists on the system. Preferably, it should be an
empty directory. How this is done is explained later in this chapter. Then,
using the mount command, the administrator can attach the partition to the
system. When you look at the content of the formerly empty directory /opt/
media, it will contain the files and directories that are on the mounted
medium (hard disk or partition of a hard disk, CD, DVD, flash card, USB or
other storage device).

During system startup, all the partitions are thus mounted, as described in
the file /etc/fstab. Some partitions are not mounted by default, for instance
if they are not constantly connected to the system, such like the storage
used by your digital camera. If well configured, the device will be mounted
as soon as the system notices that it is connected, or it can be
user-mountable, i.e. you don't need to be system administrator to attach and
detach the device to and from the system. There is an example in Section 9.3.

On a running system, information about the partitions and their mount points
can be displayed using the df command (which stands for disk full or disk
free). In Linux, df is the GNU version, and supports the -h or human readable
option which greatly improves readability. Note that commercial UNIX machines
commonly have their own versions of df and many other commands. Their
behavior is usually the same, though GNU versions of common tools often have
more and better features.

The df command only displays information about active non-swap partitions.
These can include partitions from other networked systems, like in the
example below where the home directories are mounted from a file server on
the network, a situation often encountered in corporate environments.

freddy:~> df -h
Filesystem          Size  Used Avail Use% Mounted on
/dev/hda8           496M  183M  288M  39% /
/dev/hda1           124M  8.4M  109M   8% /boot
/dev/hda5            19G   15G  2.7G  85% /opt
/dev/hda6           7.0G  5.4G  1.2G  81% /usr
/dev/hda7           3.7G  2.7G  867M  77% /var
fs1:/home           8.9G  3.7G  4.7G  44% /.automount/fs1/root/home



3.1.3. More file system layout

3.1.3.1. Visual

For convenience, the Linux file system is usually thought of in a tree
structure. On a standard Linux system you will find the layout generally
follows the scheme presented below.

Figure 3-1. Linux file system layout

[FS-layout]

This is a layout from a RedHat system. Depending on the system admin, the
operating system and the mission of the UNIX machine, the structure may vary,
and directories may be left out or added at will. The names are not even
required; they are only a convention.

The tree of the file system starts at the trunk or slash, indicated by a
forward slash (/). This directory, containing all underlying directories and
files, is also called the root directory or "the root" of the file system.

Directories that are only one level below the root directory are often
preceded by a slash, to indicate their position and prevent confusion with
other directories that could have the same name. When starting with a new
system, it is always a good idea to take a look in the root directory. Let's
see what you could run into:

emmy:~> cd /
emmy:/> ls
bin/   dev/  home/    lib/         misc/  opt/     root/  tmp/  var/
boot/  etc/  initrd/  lost+found/  mnt/   proc/    sbin/  usr/

Table 3-2. Subdirectories of the root directory


Directory Content                                                         

/bin      Common programs, shared by the system, the system administrator 
          and the users.                                                  

          The startup files and the kernel, vmlinuz. In some recent       
/boot     distributions also grub data. Grub is the GRand Unified Boot    
          loader and is an attempt to get rid of the many different       
          boot-loaders we know today.                                     

/dev      Contains references to all the CPU peripheral hardware, which   
          are represented as files with special properties.               

          Most important system configuration files are in /etc, this     
/etc      directory contains data similar to those in the Control Panel in
          Windows                                                         

/home     Home directories of the common users.                           

/initrd   (on some distributions) Information for booting. Do not remove! 

/lib      Library files, includes files for all kinds of programs needed  
          by the system and the users.                                    

/         Every partition has a lost+found in its upper directory. Files  
lost+foundthat were saved during failures are here.                       

/misc     For miscellaneous purposes.                                     

/mnt      Standard mount point for external file systems, e.g. a CD-ROM or
          a digital camera.                                               

/net      Standard mount point for entire remote file systems             

/opt      Typically contains extra and third party software.              

          A virtual file system containing information about system       
          resources. More information about the meaning of the files in   
/proc     proc is obtained by entering the command man proc in a terminal 
          window. The file proc.txt discusses the virtual file system in  
          detail.                                                         

          The administrative user's home directory. Mind the difference   
/root     between /, the root directory and /root, the home directory of  
          the root user.                                                  

/sbin     Programs for use by the system and the system administrator.    

/tmp      Temporary space for use by the system, cleaned upon reboot, so  
          don't use this for saving any work!                             

/usr      Programs, libraries, documentation etc. for all user-related    
          programs.                                                       

          Storage for all variable files and temporary files created by   
/var      users, such as log files, the mail queue, the print spooler     
          area, space for temporary storage of files downloaded from the  
          Internet, or to keep an image of a CD before burning it.        


How can you find out which partition a directory is on? Using the df command
with a dot (.) as an option shows the partition the current directory belongs
to, and informs about the amount of space used on this partition:

sandra:/lib> df -h .
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda7             980M  163M  767M  18% /

As a general rule, every directory under the root directory is on the root
partition, unless it has a separate entry in the full listing from df (or df
-h with no other options).

Read more in man hier.



3.1.3.2. The file system in reality

For most users and for most common system administration tasks, it is enough
to accept that files and directories are ordered in a tree-like structure.
The computer, however, doesn't understand a thing about trees or
tree-structures.

Every partition has its own file system. By imagining all those file systems
together, we can form an idea of the tree-structure of the entire system, but
it is not as simple as that. In a file system, a file is represented by an
inode, a kind of serial number containing information about the actual data
that makes up the file: to whom this file belongs, and where is it located on
the hard disk.

Every partition has its own set of inodes; throughout a system with multiple
partitions, files with the same inode number can exist.

Each inode describes a data structure on the hard disk, storing the
properties of a file, including the physical location of the file data. When
a hard disk is initialized to accept data storage, usually during the initial
system installation process or when adding extra disks to an existing system,
a fixed number of inodes per partition is created. This number will be the
maximum amount of files, of all types (including directories, special files,
links etc.) that can exist at the same time on the partition. We typically
count on having 1 inode per 2 to 8 kilobytes of storage.

At the time a new file is created, it gets a free inode. In that inode is the
following information:

  • Owner and group owner of the file.

  • File type (regular, directory, ...)

  • Permissions on the file Section 3.4.1

  • Date and time of creation, last read and change.

  • Date and time this information has been changed in the inode.

  • Number of links to this file (see later in this chapter).

  • File size

  • An address defining the actual location of the file data.

The only information not included in an inode, is the file name and
directory. These are stored in the special directory files. By comparing file
names and inode numbers, the system can make up a tree-structure that the
user understands. Users can display inode numbers using the -i option to ls.
The inodes have their own separate space on the disk.



3.2. Orientation in the file system

3.2.1. The path

When you want the system to execute a command, you almost never have to give
the full path to that command. For example, we know that the ls command is in
the /bin directory (check with which -a ls), yet we don't have to enter the
command /bin/ls for the computer to list the content of the current
directory.

The PATH environment variable takes care of this. This variable lists those
directories in the system where executable files can be found, and thus saves
the user a lot of typing and memorizing locations of commands. So the path
naturally contains a lot of directories containing bin somewhere in their
names, as the user below demonstrates. The echo command is used to display
the content ("$") of the variable PATH:

rogier:> echo $PATH
/opt/local/bin:/usr/X11R6/bin:/usr/bin:/usr/sbin/:/bin

In this example, the directories /opt/local/bin, /usr/X11R6/bin, /usr/bin, /
usr/sbin and /bin are subsequently searched for the required program. As soon
as a match is found, the search is stopped, even if not every directory in
the path has been searched. This can lead to strange situations. In the first
example below, the user knows there is a program called sendsms to send an
SMS message, and another user on the same system can use it, but she can't.
The difference is in the configuration of the PATH variable:

[jenny@blob jenny]$ sendsms
bash: sendsms: command not found
[jenny@blob jenny]$ echo $PATH
/bin:/usr/bin:/usr/bin/X11:/usr/X11R6/bin:/home/jenny/bin
[jenny@blob jenny]$ su - tony
Password:
tony:~>which sendsms
sendsms is /usr/local/bin/sendsms

tony:~>echo $PATH
/home/tony/bin.Linux:/home/tony/bin:/usr/local/bin:/usr/local/sbin:\
/usr/X11R6/bin:/usr/bin:/usr/sbin:/bin:/sbin

Note the use of the su (switch user) facility, which allows you to run a
shell in the environment of another user, on the condition that you know the
user's password.

A backslash indicates the continuation of a line on the next, without an
Enter separating one line from the other.

In the next example, a user wants to call on the wc (word count) command to
check the number of lines in a file, but nothing happens and he has to break
off his action using the Ctrl+C combination:

jumper:~> wc -l test

(Ctrl-C)
jumper:~> which wc
wc is hashed (/home/jumper/bin/wc)

jumper:~> echo $PATH
/home/jumper/bin:/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin

The use of the which command shows us that this user has a bin-directory in
his home directory, containing a program that is also called wc. Since the
program in his home directory is found first when searching the paths upon a
call for wc, this "home-made" program is executed, with input it probably
doesn't understand, so we have to stop it. To resolve this problem there are
several ways (there are always several ways to solve a problem in UNIX/
Linux): one answer could be to rename the user's wc program, or the user can
give the full path to the exact command he wants, which can be found by using
the -a option to the which command.

If the user uses programs in the other directories more frequently, he can
change his path to look in his own directories last:

jumper:~> export PATH=/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin:/home/jumper/bin

Note Changes are not permanent!
     Note that when using the export command in a shell, the changes are
     temporary and only valid for this session (until you log out). Opening
     new sessions, even while the current one is still running, will not
     result in a new path in the new session. We will see in Section 7.2 how
     we can make these kinds of changes to the environment permanent, adding
     these lines to the shell configuration files.



3.2.2. Absolute and relative paths

A path, which is the way you need to follow in the tree structure to reach a
given file, can be described as starting from the trunk of the tree (the / or
root directory). In that case, the path starts with a slash and is called an
absolute path, since there can be no mistake: only one file on the system can
comply.

In the other case, the path doesn't start with a slash and confusion is
possible between ~/bin/wc (in the user's home directory) and bin/wc in /usr,
from the previous example. Paths that don't start with a slash are always
relative.

In relative paths we also use the . and .. indications for the current and
the parent directory. A couple of practical examples:

  • When you want to compile source code, the installation documentation
    often instructs you to run the command ./configure, which runs the
    configure program located in the current directory (that came with the
    new code), as opposed to running another configure program elsewhere on
    the system.

  • In HTML files, relative paths are often used to make a set of pages
    easily movable to another place:

    <img alt="Garden with trees" src="../images/garden.jpg">

  • Notice the difference one more time:

    theo:~> ls /mp3
    ls: /mp3: No such file or directory
    theo:~>ls mp3/
    oriental/  pop/  sixties/



3.2.3. The most important files and directories

3.2.3.1. The kernel

The kernel is the heart of the system. It manages the communication between
the underlying hardware and the peripherals. The kernel also makes sure that
processes and daemons (server processes) are started and stopped at the exact
right times. The kernel has a lot of other important tasks, so many that
there is a special kernel-development mailing list on this subject only,
where huge amounts of information are shared. It would lead us too far to
discuss the kernel in detail. For now it suffices to know that the kernel is
the most important file on the system.



3.2.3.2. The shell

3.2.3.2.1. What is a shell?

When I was looking for an appropriate explanation on the concept of a shell,
it gave me more trouble than I expected. All kinds of definitions are
available, ranging from the simple comparison that "the shell is the steering
wheel of the car", to the vague definition in the Bash manual which says that
"bash is an sh-compatible command language interpreter," or an even more
obscure expression, "a shell manages the interaction between the system and
its users". A shell is much more than that.

A shell can best be compared with a way of talking to the computer, a
language. Most users do know that other language, the point-and-click
language of the desktop. But in that language the computer is leading the
conversation, while the user has the passive role of picking tasks from the
ones presented. It is very difficult for a programmer to include all options
and possible uses of a command in the GUI-format. Thus, GUIs are almost
always less capable than the command or commands that form the backend.

The shell, on the other hand, is an advanced way of communicating with the
system, because it allows for two-way conversation and taking initiative.
Both partners in the communication are equal, so new ideas can be tested. The
shell allows the user to handle a system in a very flexible way. An
additional asset is that the shell allows for task automation.



3.2.3.2.2. Shell types

Just like people know different languages and dialects, the computer knows
different shell types:

  • sh or Bourne Shell: the original shell still used on UNIX systems and in
    UNIX related environments. This is the basic shell, a small program with
    few features. When in POSIX-compatible mode, bash will emulate this
    shell.

  • bash or Bourne Again SHell: the standard GNU shell, intuitive and
    flexible. Probably most advisable for beginning users while being at the
    same time a powerful tool for the advanced and professional user. On
    Linux, bash is the standard shell for common users. This shell is a
    so-called superset of the Bourne shell, a set of add-ons and plug-ins.
    This means that the Bourne Again SHell is compatible with the Bourne
    shell: commands that work in sh, also work in bash. However, the reverse
    is not always the case. All examples and exercises in this book use bash.

  • csh or C Shell: the syntax of this shell resembles that of the C
    programming language. Sometimes asked for by programmers.

  • tcsh or Turbo C Shell: a superset of the common C Shell, enhancing
    user-friendliness and speed.

  • ksh or the Korn shell: sometimes appreciated by people with a UNIX
    background. A superset of the Bourne shell; with standard configuration a
    nightmare for beginning users.

The file /etc/shells gives an overview of known shells on a Linux system:

mia:~> cat /etc/shells
/bin/bash
/bin/sh
/bin/tcsh
/bin/csh

Note Fake Bourne shell
     Note that /bin/sh is usually a link to Bash, which will execute in
     Bourne shell compatible mode when called on this way.

Your default shell is set in the /etc/passwd file, like this line for user
mia:

mia:L2NOfqdlPrHwE:504:504:Mia Maya:/home/mia:/bin/bash

To switch from one shell to another, just enter the name of the new shell in
the active terminal. The system finds the directory where the name occurs
using the PATH settings, and since a shell is an executable file (program),
the current shell activates it and it gets executed. A new prompt is usually
shown, because each shell has its typical appearance:

mia:~> tcsh
[mia@post21 ~]$



3.2.3.2.3. Which shell am I using?

If you don't know which shell you are using, either check the line for your
account in /etc/passwd or type the command

echo $SHELL



3.2.3.3. Your home directory

Your home directory is your default destination when connecting to the
system. In most cases it is a subdirectory of /home, though this may vary.
Your home directory may be located on the hard disk of a remote file server;
in that case your home directory may be found in /nethome/your_user_name. In
another case the system administrator may have opted for a less
comprehensible layout and your home directory may be on /disk6/HU/07/
jgillard.

Whatever the path to your home directory, you don't have to worry too much
about it. The correct path to your home directory is stored in the HOME
environment variable, in case some program needs it. With the echo command
you can display the content of this variable:

orlando:~> echo $HOME
/nethome/orlando

You can do whatever you like in your home directory. You can put as many
files in as many directories as you want, although the total amount of data
and files is naturally limited because of the hardware and size of the
partitions, and sometimes because the system administrator has applied a
quota system. Limiting disk usage was common practice when hard disk space
was still expensive. Nowadays, limits are almost exclusively applied in large
environments. You can see for yourself if a limit is set using the quota
command:

pierre@lamaison:/> quota -v
Diskquotas for user pierre (uid 501): none

In case quotas have been set, you get a list of the limited partitions and
their specific limitations. Exceeding the limits may be tolerated during a
grace period with fewer or no restrictions at all. Detailed information can
be found using the info quota or man quota commands.

Note No Quota?
     If your system can not find the quota, then no limitation of file system
     usage is being applied.

Your home directory is indicated by a tilde (~), shorthand for /path_to_home/
user_name. This same path is stored in the HOME variable, so you don't have
to do anything to activate it. A simple application: switch from /var/music/
albums/arno/2001 to images in your home directory using one elegant command:

rom:/var/music/albums/arno/2001> cd ~/images

rom:~/images> pwd
/home/rom/images

Later in this chapter we will talk about the commands for managing files and
directories in order to keep your home directory tidy.



3.2.4. The most important configuration files

As we mentioned before, most configuration files are stored in the /etc
directory. Content can be viewed using the cat command, which sends text
files to the standard output (usually your monitor). The syntax is straight
forward:

cat file1 file2 ... fileN

In this section we try to give an overview of the most common configuration
files. This is certainly not a complete list. Adding extra packages may also
add extra configuration files in /etc. When reading the configuration files,
you will find that they are usually quite well commented and
self-explanatory. Some files also have man pages which contain extra
documentation, such as man group.

Table 3-3. Most common configuration files


File         Information/service                                          

             Mail aliases file for use with the Sendmail and Postfix mail 
             server. Running a mail server on each and every system has   
             long been common use in the UNIX world, and almost every     
aliases      Linux distribution still comes with a Sendmail package. In   
             this file local user names are matched with real names as    
             they occur in E-mail addresses, or with other local          
             addresses.                                                   

apache       Config files for the Apache web server.                      

             The system-wide configuration file for the Bourne Again      
bashrc       SHell. Defines functions and aliases for all users. Other    
             shells may have their own system-wide config files, like     
             cshrc.                                                       

crontab and  Configuration of tasks that need to be executed periodically 
the cron.*   - backups, updates of the system databases, cleaning of the  
directories  system, rotating logs etc.                                   

default      Default options for certain commands, such as useradd.       

filesystems  Known file systems: ext3, vfat, iso9660 etc.                 

fstab        Lists partitions and their mount points.                     

ftp*         Configuration of the ftp-server: who can connect, what parts 
             of the system are accessible etc.                            

             Configuration file for user groups. Use the shadow utilities 
group        groupadd, groupmod and groupdel to edit this file. Edit      
             manually only if you really know what you are doing.         

             A list of machines that can be contacted using the network,  
hosts        but without the need for a domain name service. This has     
             nothing to do with the system's network configuration, which 
             is done in /etc/sysconfig.                                   

inittab      Information for booting: mode, number of text consoles etc.  

issue        Information about the distribution (release version and/or   
             kernel info).                                                

ld.so.conf   Locations of library files.                                  

lilo.conf,                                                                
silo.conf,   Boot information for the LInux LOader, the system for booting
aboot.conf   that is now gradually being replaced with GRUB.              
etc.                                                                      

logrotate.*  Rotation of the logs, a system preventing the collection of  
             huge amounts of log files.                                   

mail         Directory containing instructions for the behavior of the    
             mail server.                                                 

modules.conf Configuration of modules that enable special features        
             (drivers).                                                   

             Message Of The Day: Shown to everyone who connects to the    
motd         system (in text mode), may be used by the system admin to    
             announce system services/maintenance etc.                    

mtab         Currently mounted file systems. It is advised to never edit  
             this file.                                                   

nsswitch.confOrder in which to contact the name resolvers when a process  
             demands resolving of a host name.                            

pam.d        Configuration of authentication modules.                     

             Lists local users. Use the shadow utilities useradd, usermod 
passwd       and userdel to edit this file. Edit manually only when you   
             really know what you are doing.                              

             Outdated but still frequently used printer configuration     
printcap     file. Don't edit this manually unless you really know what   
             you are doing.                                               

             System wide configuration of the shell environment:          
profile      variables, default properties of new files, limitation of    
             resources etc.                                               

rc*          Directories defining active services for each run level.     

resolv.conf  Order in which to contact DNS servers (Domain Name Servers   
             only).                                                       

sendmail.cf  Main config file for the Sendmail server.                    

services     Connections accepted by this machine (open ports).           

sndconfig or Configuration of the sound card and sound events.            
sound                                                                     

ssh          Directory containing the config files for secure shell client
             and server.                                                  

             Directory containing the system configuration files: mouse,  
sysconfig    keyboard, network, desktop, system clock, power management   
             etc. (specific to RedHat)                                    

             Settings for the graphical server, X. RedHat uses XFree,     
             which is reflected in the name of the main configuration     
X11          file, XFree86Config. Also contains the general directions for
             the window managers available on the system, for example gdm,
             fvwm, twm, etc.                                              

xinetd.* or  Configuration files for Internet services that are run from  
inetd.conf   the system's (extended) Internet services daemon (servers    
             that don't run an independent daemon).                       


Throughout this guide we will learn more about these files and study some of
them in detail.



3.2.5. The most common devices

Devices, generally every peripheral attachment of a PC that is not the CPU
itself, is presented to the system as an entry in the /dev directory. One of
the advantages of this UNIX-way of handling devices is that neither the user
nor the system has to worry much about the specification of devices.

Users that are new to Linux or UNIX in general are often overwhelmed by the
amount of new names and concepts they have to learn. That is why a list of
common devices is included in this introduction.

Table 3-4. Common devices


Name                        Device                                        

cdrom                       CD drive                                      

console                     Special entry for the currently used console. 

cua*                        Serial ports                                  

dsp*                        Devices for sampling and recording            

                            Entries for most kinds of floppy drives, the  
fd*                         default is /dev/fd0, a floppy drive for 1.44  
                            MB floppies.                                  

hd[a-t][1-16]               Standard support for IDE drives with maximum  
                            amount of partitions each.                    

ir*                         Infrared devices                              

isdn*                       Management of ISDN connections                

js*                         Joystick(s)                                   

lp*                         Printers                                      

mem                         Memory                                        

midi*                       midi player                                   

mixer* and music            Idealized model of a mixer (combines or adds  
                            signals)                                      

modem                       Modem                                         

mouse (also msmouse,                                                      
logimouse, psmouse, input/  All kinds of mouses                           
mice, psaux)                                                              

null                        Bottomless garbage can                        

par*                        Entries for parallel port support             

pty*                        Pseudo terminals                              

radio*                      For Radio Amateurs (HAMs).                    

ram*                        boot device                                   

sd*                         SCSI disks with their partitions              

                            For audio applications using the synthesizer  
sequencer                   features of the sound card (MIDI-device       
                            controller)                                   

tty*                        Virtual consoles simulating vt100 terminals.  

usb*                        USB card and scanner                          

video*                      For use with a graphics card supporting video.



3.2.6. The most common variable files

In the /var directory we find a set of directories for storing specific
non-constant data (as opposed to the ls program or the system configuration
files, which change relatively infrequently or never at all). All files that
change frequently, such as log files, mailboxes, lock files, spoolers etc.
are kept in a subdirectory of /var.

As a security measure these files are usually kept in separate parts from the
main system files, so we can keep a close eye on them and set stricter
permissions where necessary. A lot of these files also need more permissions
than usual, like /var/tmp, which needs to be writable for everyone. A lot of
user activity might be expected here, which might even be generated by
anonymous Internet users connected to your system. This is one reason why the
/var directory, including all its subdirectories, is usually on a separate
partition. This way, there is for instance no risk that a mail bomb, for
instance, fills up the rest of the file system, containing more important
data such as your programs and configuration files.

Note /var/tmp and /tmp
     Files in /tmp can be deleted without notice, by regular system tasks or
     because of a system reboot. On some (customized) systems, also /var/tmp
     might behave unpredictably. Nevertheless, since this is not the case by
     default, we advise to use the /var/tmp directory for saving temporary
     files. When in doubt, check with your system administrator. If you
     manage your own system, you can be reasonably sure that this is a safe
     place if you did not consciously change settings on /var/tmp (as root, a
     normal user can not do this).

     Whatever you do, try to stick to the privileges granted to a normal user
     - don't go saving files directly under the root (/) of the file system,
     don't put them in /usr or some subdirectory or in another reserved
     place. This pretty much limits your access to safe file systems.

One of the main security systems on a UNIX system, which is naturally
implemented on every Linux machine as well, is the log-keeping facility,
which logs all user actions, processes, system events etc. The configuration
file of the so-called syslogdaemon determines which and how long logged
information will be kept. The default location of all logs is /var/log,
containing different files for access log, server logs, system messages etc.

In /var we typically find server data, which is kept here to separate it from
critical data such as the server program itself and its configuration files.
A typical example on Linux systems is /var/www, which contains the actual
HTML pages, scripts and images that a web server offers. The FTP-tree of an
FTP server (data that can be downloaded by a remote client) is also best kept
in one of /var's subdirectories. Because this data is publicly accessible and
often changeable by anonymous users, it is safer to keep it here, away from
partitions or directories with sensitive data.

On most workstation installations, /var/spool will at least contain an at and
a cron directory, containing scheduled tasks. In office environments this
directory usually contains lpd as well, which holds the print queue(s) and
further printer configuration files, as well as the printer log files.

On server systems we will generally find /var/spool/mail, containing incoming
mails for local users, sorted in one file per user, the user's "inbox". A
related directory is mqueue, the spooler area for unsent mail messages. These
parts of the system can be very busy on mail servers with a lot of users.
News servers also use the /var/spool area because of the enormous amounts of
messages they have to process.

The /var/lib/rpm directory is specific to RPM-based (RedHat Package Manager)
distributions; it is where RPM package information is stored. Other package
managers generally also store their data somewhere in /var.



3.3. Manipulating files

3.3.1. Viewing file properties

3.3.1.1. More about ls

Besides the name of the file, ls can give a lot of other information, such as
the file type, as we already discussed. It can also show permissions on a
file, file size, inode number, creation date and time, owners and amount of
links to the file. With the -a option to ls, files that are normally hidden
from view can be displayed as well. These are files that have a name starting
with a dot. A couple of typical examples include the configuration files in
your home directory. When you've worked with a certain system for a while,
you will notice that tens of files and directories have been created that are
not automatically listed in a directory index. Next to that, every directory
contains a file named just dot (.) and one with two dots (..), which are used
in combination with their inode number to determine the directory's position
in the file system's tree structure.

You should really read the Info pages about ls, since it is a very common
command with a lot of useful options. Options can be combined, as is the case
with most UNIX commands and their options. A common combination is ls -al; it
shows a long list of files and their properties as well as the destinations
that any symbolic links point to. ls -latr displays the same files, only now
in reversed order of the last change, so that the file changed most recently
occurs at the bottom of the list. Here are a couple of examples:

krissie:~/mp3> ls
Albums/  Radio/  Singles/  gene/  index.html

krissie:~/mp3> ls -a
./   .thumbs  Radio     gene/
../  Albums/  Singles/  index.html

krissie:~/mp3> ls -l Radio/
total 8
drwxr-xr-x    2 krissie krissie  4096 Oct 30  1999 Carolina/
drwxr-xr-x    2 krissie krissie  4096 Sep 24  1999 Slashdot/

krissie:~/mp3> ls -ld Radio/
drwxr-xr-x    4 krissie krissie  4096 Oct 30  1999 Radio/

krissie:~/mp3> ls -ltr
total 20
drwxr-xr-x    4 krissie krissie  4096 Oct 30  1999 Radio/
-rw-r--r--    1 krissie krissie   453 Jan  7  2001 index.html
drwxrwxr-x   30 krissie krissie  4096 Oct 20 17:32 Singles/
drwxr-xr-x    2 krissie krissie  4096 Dec  4 23:22 gene/
drwxrwxr-x   13 krissie krissie  4096 Dec 21 11:40 Albums/

On most Linux versions ls is aliased to color-ls by default. This feature
allows to see the file type without using any options to ls. To achieve this,
every file type has its own color. The standard scheme is in /etc/DIR_COLORS:

Table 3-5. Color-ls default color scheme


Color       File type          

blue        directories        

red         compressed archives

white       text files         

pink        images             

cyan        links              

yellow      devices            

green       executables        

flashing redbroken links       


More information is in the man page. The same information was in earlier days
displayed using suffixes to every non-standard file name. For mono-color use
(like printing a directory listing) and for general readability, this scheme
is still in use:

Table 3-6. Default suffix scheme for ls


CharacterFile type       

nothing  regular file    

/        directory       

*        executable file 

@        link            

=        socket          

|        named pipe      


A description of the full functionality and features of the ls command can be
read with info coreutils ls.



3.3.1.2. More tools

To find out more about the kind of data we are dealing with, we use the file
command. By applying certain tests that check properties of a file in the
file system, magic numbers and language tests, file tries to make an educated
guess about the format of a file. Some examples:

mike:~> file Documents/
Documents/: directory

mike:~> file high-tech-stats.pdf
high-tech-stats.pdf: PDF document, version 1.2

mike:~> file Nari-288.rm
Nari-288.rm: RealMedia file

mike:~> file bijlage10.sdw
bijlage10.sdw: Microsoft Office Document

mike:~> file logo.xcf
logo.xcf: GIMP XCF image data, version 0, 150 x 38, RGB Color

mike:~> file cv.txt
cv.txt: ISO-8859 text

mike:~> file image.png
image.png: PNG image data, 616 x 862, 8-bit grayscale, non-interlaced

mike:~> file figure
figure: ASCII text

mike:~> file me+tux.jpg
me+tux.jpg: JPEG image data, JFIF standard 1.01, resolution (DPI),
            "28 Jun 1999", 144 x 144

mike:~> file 42.zip.gz
42.zip.gz: gzip compressed data, deflated, original filename,
         `42.zip', last modified: Thu Nov  1 23:45:39 2001, os: Unix

mike:~> file vi.gif
vi.gif: GIF image data, version 89a, 88 x 31

mike:~> file slide1
slide1: HTML document text

mike:~> file template.xls
template.xls: Microsoft Office Document

mike:~> file abook.ps
abook.ps: PostScript document text conforming at level 2.0

mike:~> file /dev/log
/dev/log: socket

mike:~> file /dev/hda
/dev/hda: block special (3/0)

The file command has a series of options, among others the -z option to look
into compressed files. See info file for a detailed description. Keep in mind
that the results of file are not absolute, it is only a guess. In other
words, file can be tricked.

Note Why all the fuss about file types and formats?
     Shortly, we will discuss a couple of command-line tools for looking at
     plain text files. These tools will not work when used on the wrong type
     of files. In the worst case, they will crash your terminal and/or make a
     lot of beeping noises. If this happens to you, just close the terminal
     session and start a new one. But try to avoid it, because it is usually
     very disturbing for other people.



3.3.2. Creating and deleting files and directories

3.3.2.1. Making a mess...

... Is not a difficult thing to do. Today almost every system is networked,
so naturally files get copied from one machine to another. And especially
when working in a graphical environment, creating new files is a piece of
cake and is often done without the approval of the user. To illustrate the
problem, here's the full content of a new user's directory, created on a
standard RedHat system:

[newuser@blob user]$ ls -al
total 32
drwx------   3 user     user        4096 Jan 16 13:32 .
drwxr-xr-x   6 root     root        4096 Jan 16 13:32 ..
-rw-r--r--   1 user     user      24 Jan 16 13:32 .bash_logout
-rw-r--r--   1 user     user     191 Jan 16 13:32 .bash_profile
-rw-r--r--   1 user     user     124 Jan 16 13:32 .bashrc
drwxr-xr-x   3 user     user    4096 Jan 16 13:32 .kde
-rw-r--r--   1 user     user    3511 Jan 16 13:32 .screenrc
-rw-------   1 user     user      61 Jan 16 13:32 .xauthDqztLr

On first sight, the content of a "used" home directory doesn't look that bad
either:

olduser:~> ls
app-defaults/ crossover/   Fvwm@     mp3/      OpenOffice.org638/
articles/     Desktop/     GNUstep/  Nautilus/ staroffice6.0/
bin/          Desktop1/    images/   nqc/      training/
brol/         desktoptest/ Machines@ ns_imap/  webstart/
C/            Documents/   mail/     nsmail/   xml/
closed/       Emacs@       Mail/     office52/ Xrootenv.0

But when all the directories and files starting with a dot are included,
there are 185 items in this directory. This is because most applications have
their own directories and/or files, containing user-specific settings, in the
home directory of that user. Usually these files are created the first time
you start an application. In some cases you will be notified when a
non-existent directory needs to be created, but most of the time everything
is done automatically.

Furthermore, new files are created seemingly continuously because users want
to save files, keep different versions of their work, use Internet
applications, and download files and attachments to their local machine. It
doesn't stop. It is clear that one definitely needs a scheme to keep an
overview on things.

In the next section, we will discuss our means of keeping order. We only
discuss text tools available to the shell, since the graphical tools are very
intuitive and have the same look and feel as the well known point-and-click
MS Windows-style file managers, including graphical help functions and other
features you expect from this kind of applications. The following list is an
overview of the most popular file managers for GNU/Linux. Most file managers
can be started from the menu of your desktop manager, or by clicking your
home directory icon, or from the command line, issuing these commands:

  • nautilus: The default file manager in Gnome, the GNU desktop. Excellent
    documentation about working with this tool can be found at [http://
    www.gnome.org/learn/users-guide/latest/gosnautilus-1.html] http://
    www.gnome.org.

  • konqueror: The file manager typically used on a KDE desktop. The handbook
    is at [http://docs.kde.org/en/3.1/kdebase/konqueror/] http://
    docs.kde.org.

  • mc: Midnight Commander, the Unix file manager after the fashion of Norton
    Commander. All documentation available from [http://www.gnu.org/directory
    /midnightcommander.html] http://gnu.org/directory/ or a mirror, such as
    [http://www.ibiblio.org] http://www.ibiblio.org.

These applications are certainly worth giving a try and usually impress
newcomers to Linux, if only because there is such a wide variety: these are
only the most popular tools for managing directories and files, and many
other projects are being developed. Now let's find out about the internals
and see how these graphical tools use common UNIX commands.



3.3.2.2. The tools

3.3.2.2.1. Creating directories

A way of keeping things in place is to give certain files specific default
locations by creating directories and subdirectories (or folders and
sub-folders if you wish). This is done with the mkdir command:

richard:~> mkdir archive

richard:~> ls -ld archive
drwxrwxrwx  2 richard richard           4096 Jan 13 14:09 archive/

Creating directories and subdirectories in one step is done using the -p
option:

richard:~> cd archive

richard:~/archive> mkdir 1999 2000 2001

richard:~/archive> ls
1999/  2000/  2001/

richard:~/archive> mkdir 2001/reports/Restaurants-Michelin/
mkdir: cannot create directory `2001/reports/Restaurants-Michelin/':
No such file or directory

richard:~/archive> mkdir -p 2001/reports/Restaurants-Michelin/

richard:~/archive> ls 2001/reports/
Restaurants-Michelin/

If the new file needs other permissions than the default file creation
permissions, the new access rights can be set in one move, still using the
mkdir command, see the Info pages for more. We are going to discuss access
modes in the next section on file security.

The name of a directory has to comply with the same rules as those applied on
regular file names. One of the most important restrictions is that you can't
have two files with the same name in one directory (but keep in mind that
Linux is, like UNIX, a case sensitive operating system). There are virtually
no limits on the length of a file name, but it is usually kept shorter than
80 characters, so it can fit on one line of a terminal. You can use any
character you want in a file name, although it is advised to exclude
characters that have a special meaning to the shell. When in doubt, check
with Appendix C.



3.3.2.2.2. Moving files

Now that we have properly structured our home directory, it is time to clean
up unclassified files using the mv command:

richard:~/archive> mv ../report[1-4].doc reports/Restaurants-Michelin/

This command is also applicable when renaming files:

richard:~> ls To_Do
-rw-rw-r--    1 richard richard      2534 Jan 15 12:39 To_Do

richard:~> mv To_Do done

richard:~> ls -l done
-rw-rw-r--    1 richard richard      2534 Jan 15 12:39 done

It is clear that only the name of the file changes. All other properties
remain the same.

Detailed information about the syntax and features of the mv command can be
found in the man or Info pages. The use of this documentation should always
be your first reflex when confronted with a problem. The answer to your
problem is likely to be in the system documentation. Even experienced users
read man pages every day, so beginning users should read them all the time.
After a while, you will get to know the most common options to the common
commands, but you will still need the documentation as a primary source of
information. Note that the information contained in the HOWTOs, FAQs, man
pages and other sources is slowly being merged into the Info pages, which are
today the most up-to-date source of online (as in readily available on the
system) documentation.



3.3.2.2.3. Copying files

Copying files and directories is done with the cp command. A useful option is
recursive copy (copy all underlying files and subdirectories), using the -R
option to cp. The general syntax is

cp [-R] fromfile tofile

As an example the case of user newguy, who wants the same Gnome desktop
settings user oldguy has. One way to solve the problem is to copy the
settings of oldguy to the home directory of newguy:

victor:~> cp -R ../oldguy/.gnome/ .

This gives some errors involving file permissions, but all the errors have to
do with private files that newguy doesn't need anyway. We will discuss in the
next part how to change these permissions in case they really are a problem.



3.3.2.2.4. Removing files

Use the rm command to remove single files, rmdir to remove empty directories.
(Use ls -a to check whether a directory is empty or not). The rm command also
has options for removing non-empty directories with all their subdirectories,
read the Info pages for these rather dangerous options.

Note How empty can a directory be?
     It is normal that the directories . (dot) and .. (dot-dot) can't be
     removed, since they are also necessary in an empty directory to
     determine the directories ranking in the file system hierarchy.

On Linux, just like on UNIX, there is no garbage can - at least not for the
shell, although there are plenty of solutions for graphical use. So once
removed, a file is really gone, and there is generally no way to get it back
unless you have backups, or you are really fast and have a real good system
administrator. To protect the beginning user from this malice, the
interactive behavior of the rm, cp and mv commands can be activated using the
-i option. In that case the system won't immediately act upon request.
Instead it will ask for confirmation, so it takes an additional click on the
Enter key to inflict the damage:

mary:~> rm -ri archive/
rm: descend into directory `archive'? y
rm: descend into directory `archive/reports'? y
rm: remove directory `archive/reports'? y
rm: descend into directory `archive/backup'? y
rm: remove `archive/backup/sysbup200112.tar'? y
rm: remove directory `archive/backup'? y
rm: remove directory `archive'? y

We will discuss how to make this option the default in Chapter 7, which
discusses customizing your shell environment.



3.3.3. Finding files

3.3.3.1. Using shell features

In the example on moving files we already saw how the shell can manipulate
multiple files at once. In that example, the shell finds out automatically
what the user means by the requirements between the square braces "[" and "]
". The shell can substitute ranges of numbers and upper or lower case
characters alike. It also substitutes as many characters as you want with an
asterisk, and only one character with a question mark.

All sorts of substitutions can be used simultaneously; the shell is very
logical about it. The Bash shell, for instance, has no problem with
expressions like ls dirname/*/*/*[2-3].

In other shells, the asterisk is commonly used to minimize the efforts of
typing: people would enter cd dir* instead of cd directory. In Bash however,
this is not necessary because the GNU shell has a feature called file name
completion. It means that you can type the first few characters of a command
(anywhere) or a file (in the current directory) and if no confusion is
possible, the shell will find out what you mean. For example in a directory
containing many files, you can check if there are any files beginning with
the letter A just by typing ls A and pressing the Tab key twice, rather than
pressing Enter. If there is only one file starting with "A", this file will
be shown as the argument to ls (or any shell command, for that matter)
immediately.



3.3.3.2. Which

A very simple way of looking up files is using the which command, to look in
the directories listed in the user's search path for the required file. Of
course, since the search path contains only paths to directories containing
executable programs, which doesn't work for ordinary files. The which command
is useful when troubleshooting "Command not Found" problems. In the example
below, user tina can't use the acroread program, while her colleague has no
troubles whatsoever on the same system. The problem is similar to the PATH
problem in the previous part: Tina's colleague tells her that he can see the
required program in /opt/acroread/bin, but this directory is not in her path:

tina:~> which acroread
/usr/bin/which: no acroread in (/bin:/usr/bin:/usr/bin/X11)

The problem can be solved by giving the full path to the command to run, or
by re-exporting the content of the PATH variable:

tina:~> export PATH=$PATH:/opt/acroread/bin

tina:~> echo $PATH
/bin:/usr/bin:/usr/bin/X11:/opt/acroread/bin

Using the which command also checks to see if a command is an alias for
another command:

gerrit:~> which -a ls
ls is aliased to `ls -F --color=auto'
ls is /bin/ls

If this does not work on your system, use the alias command:

tille@www:~/mail$ alias ls
alias ls='ls --color'



3.3.3.3. Find and locate

These are the real tools, used when searching other paths beside those listed
in the search path. The find tool, known from UNIX, is very powerful, which
may be the cause of a somewhat more difficult syntax. GNU find, however,
deals with the syntax problems. This command not only allows you to search
file names, it can also accept file size, date of last change and other file
properties as criteria for a search. The most common use is for finding file
names:

find <path> -name <searchstring>

This can be interpreted as "Look in all files and subdirectories contained in
a given path, and print the names of the files containing the search string
in their name" (not in their content).

Another application of find is for searching files of a certain size, as in
the example below, where user peter wants to find all files in the current
directory or one of its subdirectories, that are bigger than 5 MB:

peter:~> find . -size +5000k
psychotic_chaos.mp3

If you dig in the man pages, you will see that find can also perform
operations on the found files. A common example is removing files. It is best
to first test without the -exec option that the correct files are selected,
after that the command can be rerun to delete the selected files. Below, we
search for files ending in .tmp:

peter:~>  find . -name "*.tmp" -exec rm {} \;

peter:~>

Tip Optimize!
    This command will call on rm as many times as a file answering the
    requirements is found. In the worst case, this might be thousands or
    millions of times. This is quite a load on your system.
 
    A more realistic way of working would be the use of a pipe (|) and the
    xargs tool with rm as an argument. This way, the rm command is only
    called when the command line is full, instead of for every file. See
    Chapter 5 for more on using I/O redirection to ease everyday tasks.

Later on (in 1999 according to the man pages, after 20 years of find), locate
was developed. This program is easier to use, but more restricted than find,
since its output is based on a file index database that is updated only once
every day. On the other hand, a search in the locate database uses less
resources than find and therefore shows the results nearly instantly.

Most Linux distributions use slocate these days, security enhanced locate,
the modern version of locate that prevents users from getting output they
have no right to read. The files in root's home directory are such an
example, these are not normally accessible to the public. A user who wants to
find someone who knows about the C shell may issue the command locate .cshrc,
to display all users who have a customized configuration file for the C
shell. Supposing the users root and jenny are running C shell, then only the
file /home/jenny/.cshrc will be displayed, and not the one in root's home
directory. On most systems, locate is a symbolic link to the slocate program:

billy:~> ls -l /usr/bin/locate
lrwxrwxrwx 1 root slocate  7 Oct 28 14:18 /usr/bin/locate -> slocate*

User tina could have used locate to find the application she wanted:

tina:~> locate acroread
/usr/share/icons/hicolor/16x16/apps/acroread.png
/usr/share/icons/hicolor/32x32/apps/acroread.png
/usr/share/icons/locolor/16x16/apps/acroread.png
/usr/share/icons/locolor/32x32/apps/acroread.png
/usr/local/bin/acroread
/usr/local/Acrobat4/Reader/intellinux/bin/acroread
/usr/local/Acrobat4/bin/acroread

Directories that don't contain the name bin can't contain the program - they
don't contain executable files. There are three possibilities left. The file
in /usr/local/bin is the one tina would have wanted: it is a link to the
shell script that starts the actual program:

tina:~> file /usr/local/bin/acroread
/usr/local/bin/acroread: symbolic link to ../Acrobat4/bin/acroread

tina:~> file /usr/local/Acrobat4/bin/acroread
/usr/local/Acrobat4/bin/acroread: Bourne shell script text executable

tina:~> file /usr/local/Acrobat4/Reader/intellinux/bin/acroread
/usr/local/Acrobat4/Reader/intellinux/bin/acroread: ELF 32-bit LSB
executable, Intel 80386, version 1, dynamically linked (uses
shared libs), not stripped

In order to keep the path as short as possible, so the system doesn't have to
search too long every time a user wants to execute a command, we add /usr/
local/bin to the path and not the other directories, which only contain the
binary files of one specific program, while /usr/local/bin contains other
useful programs as well.

Again, a description of the full features of find and locate can be found in
the Info pages.



3.3.3.4. The grep command

3.3.3.4.1. General line filtering

A simple but powerful program, grep is used for filtering input lines and
returning certain patterns to the output. There are literally thousands of
applications for the grep program. In the example below, jerry uses grep to
see how he did the thing with find:

jerry:~> grep -a find .bash_history
find . -name userinfo
man find
find ../ -name common.cfg

Tip Search history
    Also useful in these cases is the search function in bash, activated by
    pressing Ctrl+R at once, such as in the example where we want to check
    how we did that last find again:

    thomas ~> ^R
    (reverse-i-search)`find': find `/home/thomas` -name *.xml

    Type your search string at the search prompt. The more characters you
    type, the more restricted the search gets. This reads the command history
    for this shell session (which is written to .bash_history in your home
    directory when you quit that session). The most recent occurrence of your
    search string is shown. If you want to see previous commands containing
    the same string, type Ctrl+R again.

    See the Info pages on bash for more.

All UNIXes with just a little bit of decency have an online dictionary. So
does Linux. The dictionary is a list of known words in a file named words,
located in /usr/share/dict. To quickly check the correct spelling of a word,
no graphical application is needed:

william:~> grep pinguin /usr/share/dict/words

william:~> grep penguin /usr/share/dict/words
penguin
penguins

Tip Dictionary vs. word list
    Some distributions offer the dict command, which offers more features
    than simply searching words in a list.

Who is the owner of that home directory next to mine? Hey, there's his
telephone number!

lisa:~> grep gdbruyne /etc/passwd
gdbruyne:x:981:981:Guy Debruyne, tel 203234:/home/gdbruyne:/bin/bash

And what was the E-mail address of Arno again?

serge:~/mail> grep -i arno *
sent-mail: To: <Arno.Hintjens@celeb.com>
sent-mail: On Mon, 24 Dec 2001, Arno.Hintjens@celeb.com wrote:

find and locate are often used in combination with grep to define some
serious queries. For more information, see Chapter 5 on I/O redirection.



3.3.3.4.2. Special characters

Characters that have a special meaning to the shell have to be escaped. The
escape character in Bash is backslash, as in most shells; this takes away the
special meaning of the following character. The shell knows about quite some
special characters, among the most common /, ., ? and *. A full list can be
found in the Info pages and documentation for your shell.

For instance, say that you want to display the file "*" instead of all the
files in a directory, you would have to use

less \*

The same goes for filenames containing a space:

cat This\ File



3.3.4. More ways to view file content

3.3.4.1. General

Apart from cat, which really doesn't do much more than sending files to the
standard output, there are other tools to view file content.

The easiest way of course would be to use graphical tools instead of command
line tools. In the introduction we already saw a glimpse of an office
application, OpenOffice.org. Other examples are the GIMP (start up with gimp
from the command line), the GNU Image Manipulation Program; xpdf to view
Portable Document Format files (PDF); GhostView (gv) for viewing PostScript
files; Mozilla/FireFox, links (a text mode browser), Konqueror, Opera and
many others for web content; XMMS, CDplay and others for multimedia file
content; AbiWord, Gnumeric, KOffice etc. for all kinds of office applications
and so on. There are thousands of Linux applications; to list them all would
take days.

Instead we keep concentrating on shell- or text-mode applications, which form
the basics for all other applications. These commands work best in a text
environment on files containing text. When in doubt, check first using the
file command.

So let's see what text tools we have that are useful to look inside files.

Note Font problems
     Plain text tools such as the ones we will now be discussing, often have
     problems with "plain" text files because of the font encoding used in
     those files. Special characters, such as accented alphabetical
     characters, Chinese characters and other characters from languages using
     different character sets than the default en_US encoding and so on, are
     then displayed the wrong way or replaced by unreadable rubbish. These
     problems are discussed in Section 7.4.



3.3.4.2. "less is more"

Undoubtedly you will hear someone say this phrase sooner or later when
working in a UNIX environment. A little bit of UNIX history explains this:

  • First there was cat. Output was streamed in an uncontrollable way.

  • Then there was pg, which may still be found on older UNIXes. This command
    puts text to the output one page at the time.

  • The more program was a revised version of pg. This command is still
    available on every Linux system.

  • less is the GNU version of more and has extra features allowing
    highlighting of search strings, scrolling back etc. The syntax is very
    simple:

    less name_of_file

    More information is located in the Info pages.

You already know about pagers by now, because they are used for viewing the
man pages.



3.3.4.3. The head and tail commands

These two commands display the n first/last lines of a file respectively. To
see the last ten commands entered:

tony:~> tail -10 .bash_history 
locate configure | grep bin
man bash
cd
xawtv &
grep usable /usr/share/dict/words
grep advisable /usr/share/dict/words
info quota
man quota
echo $PATH
frm

head works similarly. The tail command has a handy feature to continuously
show the last n lines of a file that changes all the time. This -f option is
often used by system administrators to check on log files. More information
is located in the system documentation files.



3.3.5. Linking files

3.3.5.1. Link types

Since we know more about files and their representation in the file system,
understanding links (or shortcuts) is a piece of cake. A link is nothing more
than a way of matching two or more file names to the same set of file data.
There are two ways to achieve this:

  • Hard link: Associate two or more file names with the same inode. Hard
    links share the same data blocks on the hard disk, while they continue to
    behave as independent files.

    There is an immediate disadvantage: hard links can't span partitions,
    because inode numbers are only unique within a given partition.

  • Soft link or symbolic link (or for short: symlink): a small file that is
    a pointer to another file. A symbolic link contains the path to the
    target file instead of a physical location on the hard disk. Since inodes
    are not used in this system, soft links can span across partitions.

The two link types behave similar, but are not the same, as illustrated in
the scheme below:

Figure 3-2. Hard and soft link mechanism

[links]

Note that removing the target file for a symbolic link makes the link
useless.

Each regular file is in principle a hardlink. Hardlinks can not span across
partitions, since they refer to inodes, and inode numbers are only unique
within a given partition.

It may be argued that there is a third kind of link, the user-space link,
which is similar to a shortcut in MS Windows. These are files containing
meta-data which can only be interpreted by the graphical file manager. To the
kernel and the shell these are just normal files. They may end in a .desktop
or .lnk suffix; an example can be found in ~/.gnome-desktop:

[dupont@boulot .gnome-desktop]$ cat La\ Maison\ Dupont
[Desktop Entry]
Encoding=Legacy-Mixed
Name=La Maison Dupont
Type=X-nautilus-home
X-Nautilus-Icon=temp-home
URL=file:///home/dupont

This example is from a KDE desktop:

[lena@venus Desktop]$ cat camera
[Desktop Entry]
Dev=/dev/sda1
FSType=auto
Icon=memory
MountPoint=/mnt/camera
Type=FSDevice
X-KDE-Dynamic-Device=true

Creating this kind of link is easy enough using the features of your
graphical environment. Should you need help, your system documentation should
be your first resort.

In the next section, we will study the creation of UNIX-style symbolic links
using the command line.



3.3.5.2. Creating symbolic links

The symbolic link is particularly interesting for beginning users: they are
fairly obvious to see and you don't need to worry about partitions.

The command to make links is ln. In order to create symlinks, you need to use
the -s option:

ln -s targetfile linkname

In the example below, user freddy creates a link in a subdirectory of his
home directory to a directory on another part of the system:

freddy:~/music> ln -s /opt/mp3/Queen/ Queen

freddy:~/music> ls -l
lrwxrwxrwx  1 freddy  freddy  17 Jan 22 11:07 Queen -> /opt/mp3/Queen

Symbolic links are always very small files, while hard links have the same
size as the original file.

The application of symbolic links is widespread. They are often used to save
disk space, to make a copy of a file in order to satisfy installation
requirements of a new program that expects the file to be in another
location, they are used to fix scripts that suddenly have to run in a new
environment and can generally save a lot of work. A system admin may decide
to move the home directories of the users to a new location, disk2 for
instance, but if he wants everything to work like before, like the /etc/
passwd file, with a minimum of effort he will create a symlink from /home to
the new location /disk2/home.



3.4. File security

3.4.1. Access rights: Linux's first line of defense

The Linux security model is based on the one used on UNIX systems, and is as
rigid as the UNIX security model (and sometimes even more), which is already
quite robust. On a Linux system, every file is owned by a user and a group
user. There is also a third category of users, those that are not the user
owner and don't belong to the group owning the file. For each category of
users, read, write and execute permissions can be granted or denied.

We already used the long option to list files using the ls -l command, though
for other reasons. This command also displays file permissions for these
three user categories; they are indicated by the nine characters that follow
the first character, which is the file type indicator at the beginning of the
file properties line. As seen in the examples below, the first three
characters in this series of nine display access rights for the actual user
that owns the file. The next three are for the group owner of the file, the
last three for other users. The permissions are always in the same order:
read, write, execute for the user, the group and the others. Some examples:

marise:~> ls -l To_Do
-rw-rw-r--    1 marise  users      5 Jan 15 12:39 To_Do
marise:~> ls -l /bin/ls
-rwxr-xr-x    1 root    root   45948 Aug  9 15:01 /bin/ls*

The first file is a regular file (first dash). Users with user name marise or
users belonging to the group users can read and write (change/move/delete)
the file, but they can't execute it (second and third dash). All other users
are only allowed to read this file, but they can't write or execute it
(fourth and fifth dash).

The second example is an executable file, the difference: everybody can run
this program, but you need to be root to change it.

The Info pages explain how the ls command handles display of access rights in
detail, see the section What information is listed.

For easy use with commands, both access rights or modes and user groups have
a code. See the tables below.

Table 3-7. Access mode codes


Code  Meaning                                                             

0 or -The access right that is supposed to be on this place is not        
      granted.                                                            

4 or rread access is granted to the user category defined in this place   

2 or wwrite permission is granted to the user category defined in this    
      place                                                               

1 or xexecute permission is granted to the user category defined in this  
      place                                                               


Table 3-8. User group codes


CodeMeaning                

u   user permissions       

g   group permissions      

o   permissions for others 


This straight forward scheme is applied very strictly, which allows a high
level of security even without network security. Among other functions, the
security scheme takes care of user access to programs, it can serve files on
a need-to-know basis and protect sensitive data such as home directories and
system configuration files.

You should know what your user name is. If you don't, it can be displayed
using the id command, which also displays the default group you belong to and
eventually other groups of which you are a member:

tilly:~> id
uid=504(tilly) gid=504(tilly) groups=504(tilly),100(users),2051(org)

Your user name is also stored in the environment variable USER:

tilly:~> echo $USER
tilly



3.4.2. The tools

3.4.2.1. The chmod command

A normal consequence of applying strict file permissions, and sometimes a
nuisance, is that access rights will need to be changed for all kinds of
reasons. We use the chmod command to do this, and eventually to chmod has
become an almost acceptable English verb, meaning the changing of the access
mode of a file. The chmod command can be used with alphanumeric or numeric
options, whatever you like best.

The example below uses alphanumeric options in order to solve a problem that
commonly occurs with new users:

asim:~> ./hello
bash: ./hello: bad interpreter: Permission denied

asim:~> cat hello
#!/bin/bash
echo "Hello, World"

asim:~> ls -l hello
-rw-rw-r--    1 asim    asim    32 Jan 15 16:29 hello

asim:~> chmod u+x hello

asim:~> ./hello
Hello, World

asim:~> ls -l hello
-rwxrw-r--   1 asim    asim    32 Jan 15 16:29 hello*

The + and - operators are used to grant or deny a given right to a given
group. Combinations separated by commas are allowed. The Info and man pages
contain useful examples. Here's another one, which makes the file from the
previous example a private file to user asim:

asim:~> chmod u+rwx,go-rwx hello

asim:~> ls -l hello
-rwx------    1 asim    asim    32 Jan 15 16:29 hello*

The kind of problem resulting in an error message saying that permission is
denied somewhere is usually a problem with access rights in most cases. Also,
comments like, "It worked yesterday," and "When I run this as root it works,"
are most likely caused by the wrong file permissions.

When using chmod with numeric arguments, the values for each granted access
right have to be counted together per group. Thus we get a 3-digit number,
which is the symbolic value for the settings chmod has to make. The following
table lists the most common combinations:

Table 3-9. File protection with chmod


Command     Meaning                                                       

chmod 400   To protect a file against accidental overwriting.             
file                                                                      

chmod 500   To protect yourself from accidentally removing, renaming or   
directory   moving files from this directory.                             

chmod 600   A private file only changeable by the user who entered this   
file        command.                                                      

chmod 644   A publicly readable file that can only be changed by the      
file        issuing user.                                                 

chmod 660   Users belonging to your group can change this file, others    
file        don't have any access to it at all.                           

chmod 700   Protects a file against any access from other users, while the
file        issuing user still has full access.                           

chmod 755   For files that should be readable and executable by others,   
directory   but only changeable by the issuing user.                      

chmod 775   Standard file sharing mode for a group.                       
file                                                                      

chmod 777   Everybody can do everything to this file.                     
file                                                                      


If you enter a number with less than three digits as an argument to chmod,
omitted characters are replaced with zeros starting from the left. There is
actually a fourth digit on Linux systems, that precedes the first three and
sets special access modes. Everything about these and many more are located
in the Info pages.



3.4.2.2. Logging on to another group

When you type id on the command line, you get a list of all the groups that
you can possibly belong to, preceded by your user name and ID and the group
name and ID that you are currently connected with. However, on many Linux
systems you can only be actively logged in to one group at the time. By
default, this active or primary group is the one that you get assigned from
the /etc/passwd file. The fourth field of this file holds users' primary
group ID, which is looked up in the /etc/group file. An example:

asim:~> id
uid=501(asim) gid=501(asim) groups=100(users),501(asim),3400(web)

asim:~> grep asim /etc/passwd
asim:x:501:501:Asim El Baraka:/home/asim:/bin/bash

asim:~> grep 501 /etc/group
asim:x:501:

The fourth field in the line from /etc/passwd contains the value "501", which
represents the group asim in the above example. From /etc/group we can get
the name matching this group ID. When initially connecting to the system,
this is the group that asim will belong to.

Note User private group scheme
     In order to allow more flexibility, most Linux systems follow the
     so-called user private group scheme, that assigns each user primarily to
     his or her own group. This group is a group that only contains this
     particular user, hence the name "private group". Usually this group has
     the same name as the user login name, which can be a bit confusing.

Apart from his own private group, user asim can also be in the groups users
and web. Because these are secondary groups to this user, he will need to use
the newgrp to log into any of these groups (use gpasswd for setting the group
password first). In the example, asim needs to create files that are owned by
the group web.

asim:/var/www/html> newgrp web

asim:/var/www/html> id
uid=501(asim) gid=3400(web) groups=100(users),501(asim),3400(web)

When asim creates new files now, they will be in group ownership of the group
web instead of being owned by the group asim:

asim:/var/www/html> touch test

asim:/var/www/html> ls -l test
-rw-rw-r--  1 asim web   0 Jun 10 15:38 test

Logging in to a new group prevents you from having to use chown (see Section
3.4.2.4) or calling your system administrator to change ownerships for you.

See the manpage for newgrp for more information.



3.4.2.3. The file mask

When a new file is saved somewhere, it is first subjected to the standard
security procedure. Files without permissions don't exist on Linux. The
standard file permission is determined by the mask for new file creation. The
value of this mask can be displayed using the umask command:

bert:~> umask
0002

Instead of adding the symbolic values to each other, as with chmod, for
calculating the permission on a new file they need to be subtracted from the
total possible access rights. In the example above, however, we see 4 values
displayed, yet there are only 3 permission categories: user, group and other.
The first zero is part of the special file attributes settings, which we will
discuss in Section 3.4.2.4 and Section 4.1.6. It might just as well be that
this first zero is not displayed on your system when entering the umask
command, and that you only see 3 numbers representing the default file
creation mask.

Each UNIX-like system has a system function for creating new files, which is
called each time a user uses a program that creates new files, for instance,
when downloading a file from the Internet, when saving a new text document
and so on. This function creates both new files and new directories. Full
read, write and execute permission is granted to everybody when creating a
new directory. When creating a new file, this function will grant read and
write permissions for everybody, but set execute permissions to none for all
user categories. This, before the mask is applied, a directory has
permissions 777 or rwxrwxrwx, a plain file 666 or rw-rw-rw-.

The umask value is subtracted from these default permissions after the
function has created the new file or directory. Thus, a directory will have
permissions of 775 by default, a file 664, if the mask value is (0)002. This
is demonstrated in the example below:

bert:~> mkdir newdir

bert:~> ls -ld newdir
drwxrwxr-x    2 bert    bert            4096 Feb 28 13:45 newdir/

bert:~> touch newfile

bert:~> ls -l newfile
-rw-rw-r--    1 bert    bert               0 Feb 28 13:52 newfile

Note Files versus directories
     A directory gets more permissions by default: it always has the execute
     permission. If it wouldn't have that, it would not be accessible. Try
     this out by chmodding a directory 644!

If you log in to another group using the newgrp command, the mask remains
unchanged. Thus, if it is set to 002, files and directories that you create
while being in the new group will also be accessible to the other members of
that group; you don't have to use chmod.

The root user usually has stricter default file creation permissions:

[root@estoban root]# umask
022

These defaults are set system-wide in the shell resource configuration files,
for instance /etc/bashrc or /etc/profile. You can change them in your own
shell configuration file, see Chapter 7 on customizing your shell
environment.



3.4.2.4. Changing user and group ownership

When a file is owned by the wrong user or group, the error can be repaired
with the chown (change owner) and chgrp (change group) commands. Changing
file ownership is a frequent system administrative task in environments where
files need to be shared in a group. Both commands are very flexible, as you
can find out by using the --help option.

The chown command can be applied to change both user and group ownership of a
file, while chgrp only changes group ownership. Of course the system will
check if the user issuing one of these commands has sufficient permissions on
the file(s) she wants to change.

In order to only change the user ownership of a file, use this syntax:

chown newuser file

If you use a colon after the user name (see the Info pages), group ownership
will be changed as well, to the primary group of the user issuing the
command. On a Linux system, each user has his own group, so this form can be
used to make files private:

jacky:~> id
uid=1304(jacky) gid=(1304) groups=1304(jacky),2034(pproject)

jacky:~> ls -l my_report
-rw-rw-r--  1 jacky   project       29387 Jan 15 09:34 my_report

jacky:~> chown jacky: my_report

jacky:~> chmod o-r my_report

jacky:~> ls -l my_report
-rw-rw----  1 jacky   jacky         29387 Jan 15 09:34 my_report

If jacky would like to share this file, without having to give everybody
permission to write it, he can use the chgrp command:

jacky:~> ls -l report-20020115.xls
-rw-rw---- 1 jacky   jacky   45635 Jan 15 09:35 report-20020115.xls

jacky:~> chgrp project report-20020115.xls

jacky:~> chmod o= report-20020115.xls

jacky:~> ls -l report-20020115.xls
-rw-rw---- 1 jacky   project 45635 Jan 15 09:35 report-20020115.xls

This way, users in the group project will be able to work on this file. Users
not in this group have no business with it at all.

Both chown and chgrp can be used to change ownership recursively, using the
-R option. In that case, all underlying files and subdirectories of a given
directory will belong to the given user and/or group.

Note Restrictions
     On most systems, the use of the chown and chgrp commands is restricted
     for non-privileged users. If you are not the administrator of the
     system, you can not change user nor group ownerships for security
     reasons. If the usage of these commands would not be restricted,
     malicious users could assign ownership of files to other users and/or
     groups and change behavior of those users' environments and even cause
     damage to other users' files.



3.4.2.5. Special modes

For the system admin to not be bothered solving permission problems all the
time, special access rights can be given to entire directories, or to
separate programs. There are three special modes:

  • Sticky bit mode: After execution of a job, the command is kept in the
    system memory. Originally this was a feature used a lot to save memory:
    big jobs are loaded into memory only once. But these days memory is
    inexpensive and there are better techniques to manage it, so it is not
    used anymore for its optimizing capabilities on single files. When
    applied to an entire directory, however, the sticky bit has a different
    meaning. In that case, a user can only change files in this directory
    when she is the user owner of the file or when the file has appropriate
    permissions. This feature is used on directories like /var/tmp, that have
    to be accessible for everyone, but where it is not appropriate for users
    to change or delete each other's data. The sticky bit is indicated by a t
    at the end of the file permission field:

    mark:~> ls -ld /var/tmp
    drwxrwxrwt   19 root     root         8192 Jan 16 10:37 /var/tmp/

    The sticky bit is set using the command chmod o+t directory. The historic
    origin of the "t" is in UNIX' save Text access feature.

  • SUID (set user ID) and SGID (set group ID): represented by the character
    s in the user or group permission field. When this mode is set on an
    executable file, it will run with the user and group permissions on the
    file instead of with those of the user issuing the command, thus giving
    access to system resources. We will discuss this further in Chapter 4.

  • SGID (set group ID) on a directory: in this special case every file
    created in the directory will have the same group owner as the directory
    itself (while normal behavior would be that new files are owned by the
    users who create them). This way, users don't need to worry about file
    ownership when sharing directories:

    mimi:~> ls -ld /opt/docs
    drwxrws---  4 root    users          4096 Jul 25 2001 docs/

    mimi:~> ls -l /opt/docs
    -rw-rw----  1 mimi    users        345672 Aug 30 2001-Council.doc

    This is the standard way of sharing files in UNIX.

    Note Existing files are left unchanged!
         Files that are being moved to a SGID directory but were created
         elsewhere keep their original user and group owner. This may be
         confusing.



3.5. Summary

On UNIX, as on Linux, all entities are in some way or another presented to
the system as files with the appropriate file properties. Use of (predefined)
paths allows the users and the system admin to find, read and manipulate
files.

We've made our first steps toward becoming an expert: we discussed the real
and the fake structure of the file system, and we know about the Linux file
security model, as well as several other security precautions that are taken
on every system by default.

The shell is the most important tool for interaction with the system. We
learned several shell commands in this chapter, which are listed in the table
below.

Table 3-10. New commands in chapter 3: Files and the file system


Command                  Meaning                                          

bash                     GNU shell program.                               

cat file(s)              Send content of file(s) to standard output.      

cd directory             Enter directory. cd is a bash built-in command.  

chgrp newgroup file(s)   Change the group ownership of file(s) to newgroup

chmod mode file(s)       Change access permissions on file(s)             

chown newowner[:         Change file owner and group ownership.           
[newgroup]] file(s)                                                       

cp sourcefile targetfile Copy sourcefile to targetfile.                   

df file                  Reports on used disk space on the partition      
                         containing file.                                 

echo string              Display a line of text                           

export                   Part of bash that announces variables and their  
                         values to the system.                            

file filename            Determine file type of filename.                 

find path expression     Find files in the file system hierarchy          

grep PATTERN file        Print lines in file containing the search        
                         pattern.                                         

head file                Send the first part of file to standard output   

id                       Prints real and effective user name and groups.  

info command             Read documentation about command.                

less file                View file with a powerful viewer.                

ln targetfile linkname   Make a link with name linkname to targetfile.    

locate searchstring      Print all accessible files matching the search   
                         pattern.                                         

ls file(s)               Prints directory content.                        

man command              Format and display online (system) manual pages  
                         for command.                                     

mkdir newdir             Make a new empty directory.                      

mv oldfile newfile       Rename or move oldfile.                          

newgrp groupname         Log in to a new group.                           

pwd                      Print the present or current working directory.  

quota                    Show disk usage and limits.                      

rm file                  Removes files and directories.                   

rmdir file               Removes directories.                             

tail file                Print the last part of file.                     

umask [value]            Show or change new file creation mode.           

wc file                  Counts lines, words and characters in file.      

which command            Shows the full path to command.                  


We also stressed the fact that you should READ THE MAN PAGES. This
documentation is your first-aid kit and contains the answers to many
questions. The above list contains the basic commands that you will use on a
daily basis, but they can do much more than the tasks we've discussed here.
Reading the documentation will give you the control you need.

Last but not least, a handy overview of file permissions:

Table 3-11. File permissions


Who\Whatr(ead)w(rite)(e)x(ecute)

u(ser)  4     2      1          

g(roup) 4     2      1          

o(ther) 4     2      1          



3.6. Exercises

Just login with your common user ID.



3.6.1. Partitions

  • On which partition is your home directory?

  • How many partitions are on your system?

  • What is the total size of your Linux installation?



3.6.2. Paths

  • Display your search path.

  • Export a senseless path by entering, for instance, export PATH=blah and
    try listing directory content.

  • What is the path to your home directory? How would another user reach
    your home directory starting from his own home directory, using a
    relative path?

  • Go to the tmp directory in /var.

  • Now go to share in /usr using only one command. Change to doc. What is
    your present working directory?



3.6.3. Tour of the system

  • Change to the /proc directory.

  • What CPU(s) is the system running on?

  • How much RAM does it currently use?

  • How much swap space do you have?

  • What drivers are loaded?

  • How many hours has the system been running?

  • Which filesystems are known by your system?

  • Change to /etc/rc.d | /etc/init.d | /etc/runlevels and choose the
    directory appropriate for your run level.

  • What services should be running in this level?

  • Which services run in graphical mode that don't run in text mode?

  • Change to /etc

  • How long does the system keep the log file in which user logins are
    monitored?

  • Which release are you running?

  • Are there any issues or messages of the day?

  • How many users are defined on your system? Don't count them, let the
    computer do it for you!

  • How many groups?

  • Where is the time zone information kept?

  • Are the HOWTOs installed on your system?

  • Change to /usr/share/doc.

  • Name three programs that come with the GNU coreutils package.

  • Which version of bash is installed on this system?



3.6.4. Manipulating files

  • Create a new directory in your home directory.

  • Can you move this directory to the same level as your home directory?

  • Copy all XPM files from /usr/share/pixmaps to the new directory. What
    does XPM mean?

  • List the files in reverse alphabetical order.

  • Change to your home directory. Create a new directory and copy all the
    files of the /etc directory into it. Make sure that you also copy the
    files and directories which are in the subdirectories of /etc! (recursive
    copy)

  • Change into the new directory and make a directory for files starting
    with an upper case character and one for files starting with a lower case
    character. Move all the files to the appropriate directories. Use as few
    commands as possible.

  • Remove the remaining files.

  • Delete the directory and its entire content using a single command.

  • Use grep to find out which script starts the Font Server in the graphical
    run level.

  • Where is the sendmail server program?

  • Make a symbolic link in your home directory to /var/tmp. Check that it
    really works.

  • Make another symbolic link in your home directory to this link. Check
    that it works. Remove the first link and list directory content. What
    happened to the second link?



3.6.5. File permissions

  • Can you change file permissions on /home?

  • What is your standard file creation mode?

  • Change ownership of /etc to your own user and group.

  • Change file permissions of ~/.bashrc so that only you and your primary
    group can read it.

  • Issue the command locate root. Do you notice anything special?

  • Make a symbolic link to /root. Can it be used?



Chapter 4. Processes

    Next to files, processes are the most important things on a UNIX/Linux
    system. In this chapter, we will take a closer look at those processes.
    We will learn more about:

     

      □ Multi-user processing and multi-tasking

      □ Process types

      □ Controlling processes with different signals

      □ Process attributes

      □ The life cycle of a process

      □ System startup and shutdown

      □ SUID and SGID

      □ System speed and response

      □ Scheduling processes

      □ The Vixie cron system

      □ How to get the most out of your system



4.1. Processes inside out

4.1.1. Multi-user and multi-tasking

Now that we are more used to our environment and we are able to communicate a
little bit with our system, it is time to study the processes we can start in
more detail. Not every command starts a single process. Some commands
initiate a series of processes, such as mozilla; others, like ls, are
executed as a single command.

Furthermore, Linux is based on UNIX, where it has been common policy to have
multiple users running multiple commands, at the same time and on the same
system. It is obvious that measures have to be taken to have the CPU manage
all these processes, and that functionality has to be provided so users can
switch between processes. In some cases, processes will have to continue to
run even when the user who started them logs out. And users need a means to
reactivate interrupted processes.

We will explain the structure of Linux processes in the next sections.



4.1.2. Process types

4.1.2.1. Interactive processes

Interactive processes are initialized and controlled through a terminal
session. In other words, there has to be someone connected to the system to
start these processes; they are not started automatically as part of the
system functions. These processes can run in the foreground, occupying the
terminal that started the program, and you can't start other applications as
long as this process is running in the foreground. Alternatively, they can
run in the background, so that the terminal in which you started the program
can accept new commands while the program is running. Until now, we mainly
focussed on programs running in the foreground - the length of time taken to
run them was too short to notice - but viewing a file with the less command
is a good example of a command occupying the terminal session. In this case,
the activated program is waiting for you to do something. The program is
still connected to the terminal from where it was started, and the terminal
is only useful for entering commands this program can understand. Other
commands will just result in errors or unresponsiveness of the system.

While a process runs in the background, however, the user is not prevented
from doing other things in the terminal in which he started the program,
while it is running.

The shell offers a feature called job control which allows easy handling of
multiple processes. This mechanism switches processes between the foreground
and the background. Using this system, programs can also be started in the
background immediately.

Running a process in the background is only useful for programs that don't
need user input (via the shell). Putting a job in the background is typically
done when execution of a job is expected to take a long time. In order to
free the issuing terminal after entering the command, a trailing ampersand is
added. In the example, using graphical mode, we open an extra terminal window
from the existing one:

billy:~> xterm &
[1] 26558

billy:~> jobs
[1]+  Running                 xterm &

The full job control features are explained in detail in the bash Info pages,
so only the frequently used job control applications are listed here:

Table 4-1. Controlling processes


(part of)      Meaning                                                    
command                                                                   

regular_commandRuns this command in the foreground.                       

command &      Run this command in the background (release the terminal)  

jobs           Show commands running in the background.                   

Ctrl+Z         Suspend (stop, but not quit) a process running in the      
               foreground (suspend).                                      

Ctrl+C         Interrupt (terminate and quit) a process running in the    
               foreground.                                                

               Every process running in the background gets a number      
%n             assigned to it. By using the % expression a job can be     
               referred to using its number, for instance fg %2.          

bg             Reactivate a suspended program in the background.          

fg             Puts the job back in the foreground.                       

kill           End a process (also see Shell Builtin Commands in the Info 
               pages of bash)                                             


More practical examples can be found in the exercises.

Most UNIX systems are likely to be able to run screen, which is useful when
you actually want another shell to execute commands. Upon calling screen, a
new session is created with an accompanying shell and/or commands as
specified, which you can then put out of the way. In this new session you may
do whatever it is you want to do. All programs and operations will run
independent of the issuing shell. You can then detach this session, while the
programs you started in it continue to run, even when you log out of the
originating shell, and pick your screen up again any time you like.

This program originates from a time when virtual consoles were not invented
yet, and everything needed to be done using one text terminal. To addicts, it
still has meaning in Linux, even though we've had virtual consoles for almost
ten years.



4.1.2.2. Automatic processes

Automatic or batch processes are not connected to a terminal. Rather, these
are tasks that can be queued into a spooler area, where they wait to be
executed on a FIFO (first-in, first-out) basis. Such tasks can be executed
using one of two criteria:

  • At a certain date and time: done using the at command, which we will
    discuss in the second part of this chapter.

  • At times when the total system load is low enough to accept extra jobs:
    done using the batch command. By default, tasks are put in a queue where
    they wait to be executed until the system load is lower than 0.8. In
    large environments, the system administrator may prefer batch processing
    when large amounts of data have to be processed or when tasks demanding a
    lot of system resources have to be executed on an already loaded system.
    Batch processing is also used for optimizing system performance.



4.1.2.3. Daemons

Daemons are server processes that run continuously. Most of the time, they
are initialized at system startup and then wait in the background until their
service is required. A typical example is the networking daemon, xinetd,
which is started in almost every boot procedure. After the system is booted,
the network daemon just sits and waits until a client program, such as an FTP
client, needs to connect.



4.1.3. Process attributes

A process has a series of characteristics, which can be viewed with the ps
command:

  • The process ID or PID: a unique identification number used to refer to
    the process.

  • The parent process ID or PPID: the number of the process (PID) that
    started this process.

  • Nice number: the degree of friendliness of this process toward other
    processes (not to be confused with process priority, which is calculated
    based on this nice number and recent CPU usage of the process).

  • Terminal or TTY: terminal to which the process is connected.

  • User name of the real and effective user (RUID and EUID): the owner of
    the process. The real owner is the user issuing the command, the
    effective user is the one determining access to system resources. RUID
    and EUID are usually the same, and the process has the same access rights
    the issuing user would have. An example to clarify this: the browser
    mozilla in /usr/bin is owned by user root:

    theo:~> ls -l /usr/bin/mozilla
    -rwxr-xr-x  1 root   root      4996 Nov 20 18:28 /usr/bin/mozilla*

    theo:~> mozilla &
    [1] 26595

    theo:~> ps -af
    UID     PID  PPID C STIME TTY       TIME CMD
    theo  26601 26599 0 15:04 pts/5 00:00:00 /usr/lib/mozilla/mozilla-bin
    theo  26613 26569 0 15:04 pts/5 00:00:00 ps -af

    When user theo starts this program, the process itself and all processes
    started by the initial process, will be owned by user theo and not by the
    system administrator. When mozilla needs access to certain files, that
    access will be determined by theo's permissions and not by root's.

  • Real and effective group owner (RGID and EGID): The real group owner of a
    process is the primary group of the user who started the process. The
    effective group owner is usually the same, except when SGID access mode
    has been applied to a file.



4.1.4. Displaying process information

The ps command is one of the tools for visualizing processes. This command
has several options which can be combined to display different process
attributes.

With no options specified, ps only gives information about the current shell
and eventual processes:

theo:~> ps
  PID TTY          TIME CMD
 4245 pts/7    00:00:00 bash
 5314 pts/7    00:00:00 ps

Since this does not give enough information - generally, at least a hundred
processes are running on your system - we will usually select particular
processes out of the list of all processes, using the grep command in a pipe,
see Section 5.1.2.1, as in this line, which will select and display all
processes owned by a particular user:

ps -ef | grep username

This example shows all processes with a process name of bash, the most common
login shell on Linux systems:

theo:> ps auxw | grep bash
brenda   31970  0.0  0.3  6080 1556 tty2   S  Feb23   0:00 -bash
root     32043  0.0  0.3  6112 1600 tty4   S  Feb23   0:00 -bash
theo     32581  0.0  0.3  6384 1864 pts/1  S  Feb23   0:00 bash
theo     32616  0.0  0.3  6396 1896 pts/2  S  Feb23   0:00 bash
theo     32629  0.0  0.3  6380 1856 pts/3  S  Feb23   0:00 bash
theo      2214  0.0  0.3  6412 1944 pts/5  S  16:18   0:02 bash
theo      4245  0.0  0.3  6392 1888 pts/7  S  17:26   0:00 bash
theo      5427  0.0  0.1  3720  548 pts/7  S  19:22   0:00 grep bash

In these cases, the grep command finding lines containing the string bash is
often displayed as well on systems that have a lot of idletime. If you don't
want this to happen, use the pgrep command.

Bash shells are a special case: this process list also shows which ones are
login shells (where you have to give your username and password, such as when
you log in in textmode or do a remote login, as opposed to non-login shells,
started up for instance by clicking a terminal window icon). Such login
shells are preceded with a dash (-).

Note |?
     We will explain about the | operator in the next chapter, see Chapter 5.

More info can be found the usual way: ps --help or man ps. GNU ps supports
different styles of option formats; the above examples don't contain errors.

Note that ps only gives a momentary state of the active processes, it is a
one-time recording. The top program displays a more precise view by updating
the results given by ps (with a bunch of options) once every five seconds,
generating a new list of the processes causing the heaviest load
periodically, meanwhile integrating more information about the swap space in
use and the state of the CPU, from the proc file system:

 12:40pm up 9 days, 6:00, 4 users, load average: 0.21, 0.11, 0.03
89 processes: 86 sleeping, 3 running, 0 zombie, 0 stopped
CPU states:  2.5% user,  1.7% system,  0.0% nice, 95.6% idle
Mem:   255120K av, 239412K used, 15708K free, 756K shrd, 22620K buff
Swap: 1050176K av, 76428K used, 973748K free, 82756K cached

  PID USER  PRI NI SIZE  RSS SHARE STAT %CPU %MEM TIME COMMAND
 5005 root  14  0 91572  15M 11580 R    1.9  6.0  7:53 X
19599 jeff  14  0  1024 1024   796 R    1.1  0.4  0:01 top
19100 jeff   9  0  5288 4948  3888 R    0.5  1.9  0:24 gnome-terminal
19328 jeff   9  0 37884  36M 14724 S    0.5 14.8  1:30 mozilla-bin
    1 root   8  0   516  472   464 S    0.0  0.1  0:06 init
    2 root   9  0     0    0     0 SW   0.0  0.0  0:02 keventd
    3 root   9  0     0    0     0 SW   0.0  0.0  0:00 kapm-idled
    4 root  19 19     0    0     0 SWN  0.0  0.0  0:00 ksoftirqd_CPU0
    5 root   9  0     0    0     0 SW   0.0  0.0  0:33 kswapd
    6 root   9  0     0    0     0 SW   0.0  0.0  0:00 kreclaimd
    7 root   9  0     0    0     0 SW   0.0  0.0  0:00 bdflush
    8 root   9  0     0    0     0 SW   0.0  0.0  0:05 kupdated
    9 root  -1-20     0    0     0 SW<  0.0  0.0  0:00 mdrecoveryd
   13 root   9  0     0    0     0 SW   0.0  0.0  0:01 kjournald
   89 root   9  0     0    0     0 SW   0.0  0.0  0:00 khubd
  219 root   9  0     0    0     0 SW   0.0  0.0  0:00 kjournald
  220 root   9  0     0    0     0 SW   0.0  0.0  0:00 kjournald

The first line of top contains the same information displayed by the uptime
command:

jeff:~> uptime
  3:30pm, up 12 days, 23:29, 6 users, load average: 0.01, 0.02, 0.00

The data for these programs is stored among others in /var/run/utmp
(information about currently connected users) and in the virtual file system
/proc, for example /proc/loadavg (average load information). There are all
sorts of graphical applications to view this data, such as the Gnome System
Monitor and lavaps. Over at [http://www.freshmeat.net] FreshMeat and [http://
www.sourceforge.org] SourceForge you will find tens of applications that
centralize this information along with other server data and logs from
multiple servers on one (web) server, allowing monitoring of the entire IT
infrastructure from one workstation.

The relations between processes can be visualized using the pstree command:

sophie:~> pstree
init-+-amd
     |-apmd
     |-2*[artsd]
     |-atd
     |-crond
     |-deskguide_apple
     |-eth0
     |-gdm---gdm-+-X
     |           `-gnome-session-+-Gnome
     |                           |-ssh-agent
     |                           `-true
     |-geyes_applet
     |-gkb_applet
     |-gnome-name-serv
     |-gnome-smproxy
     |-gnome-terminal-+-bash---vim
     |                |-bash
     |                |-bash---pstree
     |                |-bash---ssh
     |                |-bash---mozilla-bin---mozilla-bin---3*[mozilla-bin]
     |                `-gnome-pty-helper
     |-gpm
     |-gweather
     |-kapm-idled
     |-3*[kdeinit]
     |-keventd
     |-khubd
     |-5*[kjournald]
     |-klogd
     |-lockd---rpciod
     |-lpd
     |-mdrecoveryd
     |-6*[mingetty]
     |-8*[nfsd]
     |-nscd---nscd---5*[nscd]
     |-ntpd
     |-3*[oafd]
     |-panel
     |-portmap
     |-rhnsd
     |-rpc.mountd
     |-rpc.rquotad
     |-rpc.statd
     |-sawfish
     |-screenshooter_a
     |-sendmail
     |-sshd---sshd---bash---su---bash
     |-syslogd
     |-tasklist_applet
     |-vmnet-bridge
     |-xfs
     `-xinetd-ipv6

The -u and -a options give additional information. For more options and what
they do, refer to the Info pages.

In the next section, we will see how one process can create another.



4.1.5. Life and death of a process

4.1.5.1. Process creation

A new process is created because an existing process makes an exact copy of
itself. This child process has the same environment as its parent, only the
process ID number is different. This procedure is called forking.

After the forking process, the address space of the child process is
overwritten with the new process data. This is done through an exec call to
the system.

The fork-and-exec mechanism thus switches an old command with a new, while
the environment in which the new program is executed remains the same,
including configuration of input and output devices, environment variables
and priority. This mechanism is used to create all UNIX processes, so it also
applies to the Linux operating system. Even the first process, init, with
process ID 1, is forked during the boot procedure in the so-called
bootstrapping procedure.

This scheme illustrates the fork-and-exec mechanism. The process ID changes
after the fork procedure:

Figure 4-1. Fork-and-exec mechanism

[fork-and-e]

There are a couple of cases in which init becomes the parent of a process,
while the process was not started by init, as we already saw in the pstree
example. Many programs, for instance, daemonize their child processes, so
they can keep on running when the parent stops or is being stopped. A window
manager is a typical example; it starts an xterm process that generates a
shell that accepts commands. The window manager then denies any further
responsibility and passes the child process to init. Using this mechanism, it
is possible to change window managers without interrupting running
applications.

Every now and then things go wrong, even in good families. In an exceptional
case, a process might finish while the parent does not wait for the
completion of this process. Such an unburied process is called a zombie
process.



4.1.5.2. Ending processes

When a process ends normally (it is not killed or otherwise unexpectedly
interrupted), the program returns its exit status to the parent. This exit
status is a number returned by the program providing the results of the
program's execution. The system of returning information upon executing a job
has its origin in the C programming language in which UNIX has been written.

The return codes can then be interpreted by the parent, or in scripts. The
values of the return codes are program-specific. This information can usually
be found in the man pages of the specified program, for example the grep
command returns -1 if no matches are found, upon which a message on the lines
of "No files found" can be printed. Another example is the Bash builtin
command true, which does nothing except return an exit status of 0, meaning
success.



4.1.5.3. Signals

Processes end because they receive a signal. There are multiple signals that
you can send to a process. Use the kill command to send a signal to a
process. The command kill -l shows a list of signals. Most signals are for
internal use by the system, or for programmers when they write code. As a
user, you will need the following signals:

Table 4-2. Common signals


Signal    Signal     Meaning                                             
name      number                                                         

SIGTERM   15         Terminate the process in an orderly way.            

SIGINT    2          Interrupt the process. A process can ignore this    
                     signal.                                             

SIGKILL   9          Interrupt the process. A process can not ignore this
                     signal.                                             

SIGHUP    1          For daemons: reread the configuration file.         


You can read more about default actions that are taken when sending a signal
to a process in man 7 signal.



4.1.6. SUID and SGID

As promised in the previous chapter, we will now discuss the special modes
SUID and SGID in more detail. These modes exist to provide normal users the
ability to execute tasks they would normally not be able to do because of the
tight file permission scheme used on UNIX based systems. In the ideal
situation special modes are used as sparsely as possible, since they include
security risks. Linux developers have generally tried to avoid them as much
as possible. The Linux ps version, for example, uses the information stored
in the /proc file system, which is accessible to everyone, thus avoiding
exposition of sensitive system data and resources to the general public.
Before that, and still on older UNIX systems, the ps program needed access to
files such as /dev/mem and /dev/kmem, which had disadvantages because of the
permissions and ownerships on these files:

rita:~> ls -l /dev/*mem
crw-r-----    1 root     kmem       1,   2 Aug 30 22:30 /dev/kmem
crw-r-----    1 root     kmem       1,   1 Aug 30 22:30 /dev/mem

With older versions of ps, it was not possible to start the program as a
common user, unless special modes were applied to it.

While we generally try to avoid applying any special modes, it is sometimes
necessary to use an SUID. An example is the mechanism for changing passwords.
Of course users will want to do this themselves instead of having their
password set by the system administrator. As we know, user names and
passwords are listed in the /etc/passwd file, which has these access
permissions and owners:

bea:~> ls -l /etc/passwd
-rw-r--r--    1 root     root     1267 Jan 16 14:43 /etc/passwd

Still, users need to be able to change their own information in this file.
This is achieved by giving the passwd program special permissions:

mia:~> which passwd
passwd is /usr/bin/passwd

mia:~> ls -l /usr/bin/passwd
-r-s--x--x    1 root     root    13476 Aug  7 06:03 /usr/bin/passwd*

When called, the passwd command will run using the access permissions of root
, thus enabling a common user to edit the password file which is owned by the
system admin.

SGID modes on a file don't occur nearly as frequently as SUID, because SGID
often involves the creation of extra groups. In some cases, however, we have
to go through this trouble in order to build an elegant solution (don't worry
about this too much - the necessary groups are usually created upon
installation). This is the case for the write and wall programs, which are
used to send messages to other users' terminals (ttys). The write command
writes a message to a single user, while wall writes to all connected users.

Sending text to another user's terminal or graphical display is normally not
allowed. In order to bypass this problem, a group has been created, which
owns all terminal devices. When the write and wall commands are granted SGID
permissions, the commands will run using the access rights as applicable to
this group, tty in the example. Since this group has write access to the
destination terminal, also a user having no permissions to use that terminal
in any way can send messages to it.

In the example below, user joe first finds out on which terminal his
correspondent is connected, using the who command. Then he sends her a
message using the write command. Also illustrated are the access rights on
the write program and on the terminals occupied by the receiving user: it is
clear that others than the user owner have no permissions on the device,
except for the group owner, which can write to it.

joe:~> which write
write is /usr/bin/write

joe:~> ls -l /usr/bin/write
-rwxr-sr-x    1 root     tty      8744 Dec  5 00:55 /usr/bin/write*

joe:~> who
jenny     tty1     Jan 23 11:41
jenny     pts/1    Jan 23 12:21 (:0)
jenny     pts/2    Jan 23 12:22 (:0)
jenny     pts/3    Jan 23 12:22 (:0)
joe       pts/0    Jan 20 10:13 (lo.callhost.org)

joe:~> ls -l /dev/tty1
crw--w----    1 jenny   tty  4,     1 Jan 23 11:41 /dev/tty1

joe:~> write jenny tty1
hey Jenny, shall we have lunch together?
^C

User jenny gets this on her screen:

Message from joe@lo.callhost.org on ptys/1 at 12:36 ...
hey Jenny, shall we have lunch together?
EOF

After receiving a message, the terminal can be cleared using the Ctrl+L key
combination. In order to receive no messages at all (except from the system
administrator), use the mesg command. To see which connected users accept
messages from others use who -w. All features are fully explained in the Info
pages of each command.

Note Group names may vary
     The group scheme is specific to the distribution. Other distributions
     may use other names or other solutions.



4.2. Boot process, Init and shutdown

4.2.1. Introduction

One of the most powerful aspects of Linux concerns its open method of
starting and stopping the operating system, where it loads specified programs
using their particular configurations, permits you to change those
configurations to control the boot process, and shuts down in a graceful and
organized way.

Beyond the question of controlling the boot or shutdown process, the open
nature of Linux makes it much easier to determine the exact source of most
problems associated with starting up or shutting down your system. A basic
understanding of this process is quite beneficial to everybody who uses a
Linux system.

A lot of Linux systems use lilo, the LInux LOader for booting operating
systems. We will only discuss GRUB, however, which is easier to use and more
flexible. Should you need information about lilo, refer to the man pages and
HOWTOs. Both systems support dual boot installations, we refer to the HOWTOs
on this subject for practical examples and background information.



4.2.2. The boot process

When an x86 computer is booted, the processor looks at the end of the system
memory for the BIOS (Basic Input/Output System) and runs it. The BIOS program
is written into permanent read-only memory and is always available for use.
The BIOS provides the lowest level interface to peripheral devices and
controls the first step of the boot process.

The BIOS tests the system, looks for and checks peripherals, and then looks
for a drive to use to boot the system. Usually it checks the floppy drive (or
CD-ROM drive on many newer systems) for bootable media, if present, and then
it looks to the hard drive. The order of the drives used for booting is
usually controlled by a particular BIOS setting on the system. Once Linux is
installed on the hard drive of a system, the BIOS looks for a Master Boot
Record (MBR) starting at the first sector on the first hard drive, loads its
contents into memory, then passes control to it.

This MBR contains instructions on how to load the GRUB (or LILO) boot-loader,
using a pre-selected operating system. The MBR then loads the boot-loader,
which takes over the process (if the boot-loader is installed in the MBR). In
the default Red Hat Linux configuration, GRUB uses the settings in the MBR to
display boot options in a menu. Once GRUB has received the correct
instructions for the operating system to start, either from its command line
or configuration file, it finds the necessary boot file and hands off control
of the machine to that operating system.



4.2.3. GRUB features

This boot method is called direct loading because instructions are used to
directly load the operating system, with no intermediary code between the
boot-loaders and the operating system's main files (such as the kernel). The
boot process used by other operating systems may differ slightly from the
above, however. For example, Microsoft's DOS and Windows operating systems
completely overwrite anything on the MBR when they are installed without
incorporating any of the current MBR's configuration. This destroys any other
information stored in the MBR by other operating systems, such as Linux. The
Microsoft operating systems, as well as various other proprietary operating
systems, are loaded using a chain loading boot method. With this method, the
MBR points to the first sector of the partition holding the operating system,
where it finds the special files necessary to actually boot that operating
system.

 GRUB supports both boot methods, allowing you to use it with almost any
operating system, most popular file systems, and almost any hard disk your
BIOS can recognize.

GRUB contains a number of other features; the most important include:

  • GRUB provides a true command-based, pre-OS environment on x86 machines to
    allow maximum flexibility in loading operating systems with certain
    options or gathering information about the system.

  • GRUB supports Logical Block Addressing (LBA) mode, needed to access many
    IDE and all SCSI hard disks. Before LBA, hard drives could encounter a
    1024-cylinder limit, where the BIOS could not find a file after that
    point.

  • GRUB's configuration file is read from the disk every time the system
    boots, preventing you from having to write over the MBR every time you
    change the boot options.

A full description of GRUB may be found by issuing the info grub command or
at [http://www.gnu.org/software/grub/] the GRUB site. The Linux Documentation
Project has a [http://www.tldp.org/HOWTO/mini/Multiboot-with-GRUB.html]
Multiboot with GRUB Mini-HOWTO.



4.2.4. Init

The kernel, once it is loaded, finds init in sbin and executes it.

When init starts, it becomes the parent or grandparent of all of the
processes that start up automatically on your Linux system. The first thing
init does, is reading its initialization file, /etc/inittab. This instructs
init to read an initial configuration script for the environment, which sets
the path, starts swapping, checks the file systems, and so on. Basically,
this step takes care of everything that your system needs to have done at
system initialization: setting the clock, initializing serial ports and so
forth.

Then init continues to read the /etc/inittab file, which describes how the
system should be set up in each run level and sets the default run level. A
run level is a configuration of processes. All UNIX-like systems can be run
in different process configurations, such as the single user mode, which is
referred to as run level 1 or run level S (or s). In this mode, only the
system administrator can connect to the system. It is used to perform
maintenance tasks without risks of damaging the system or user data.
Naturally, in this configuration we don't need to offer user services, so
they will all be disabled. Another run level is the reboot run level, or run
level 6, which shuts down all running services according to the appropriate
procedures and then restarts the system.

Use the who to check what your current run level is:

willy@ubuntu:~$ who -r
        run-level 2 2006-10-17 23:22            last=S

More about run levels in the next section, see Section 4.2.5.

After having determined the default run level for your system, init starts
all of the background processes necessary for the system to run by looking in
the appropriate rc directory for that run level. init runs each of the kill
scripts (their file names start with a K) with a stop parameter. It then runs
all of the start scripts (their file names start with an S) in the
appropriate run level directory so that all services and applications are
started correctly. In fact, you can execute these same scripts manually after
the system is finished booting with a command like /etc/init.d/httpd stop or
service httpd stop logged in as root, in this case stopping the web server.

Note Special case
     Note that on system startup, the scripts in rc2.d and rc3.d are usually
     executed. In that case, no services are stopped (at least not
     permanently). There are only services that are started.

None of the scripts that actually start and stop the services are located in
/etc/rc<x>.d. Rather, all of the files in /etc/rc<x>.d are symbolic links
that point to the actual scripts located in /etc/init.d. A symbolic link is
nothing more than a file that points to another file, and is used in this
case because it can be created and deleted without affecting the actual
scripts that kill or start the services. The symbolic links to the various
scripts are numbered in a particular order so that they start in that order.
You can change the order in which the services start up or are killed by
changing the name of the symbolic link that refers to the script that
actually controls the service. You can use the same number multiple times if
you want a particular service started or stopped right before or after
another service, as in the example below, listing the content of /etc/rc5.d,
where crond and xfs are both started from a linkname starting with "S90". In
this case, the scripts are started in alphabetical order.

[jean@blub /etc/rc5.d] ls
K15httpd@     K45named@    S08ipchains@  S25netfs@      S85gpm@
K16rarpd@     K46radvd@    S08iptables@  S26apmd@       S90crond@
K20nfs@       K61ldap@     S09isdn@      S28autofs@     S90xfs@
K20rstatd@    K65identd@   S10network@   S30nscd@       S95anacron@
K20rusersd@   K74ntpd@     S12syslog@    S55sshd@       S95atd@
K20rwalld@    K74ypserv@   S13portmap@   S56rawdevices@ S97rhnsd@
K20rwhod@     K74ypxfrd@   S14nfslock@   S56xinetd@     S99local@
K25squid@     K89bcm5820@  S17keytable@  S60lpd@
K34yppasswdd@  S05kudzu@    S20random@    S80sendmail@

After init has progressed through the run levels to get to the default run
level, the /etc/inittab script forks a getty process for each virtual console
(login prompt in text mode). getty opens tty lines, sets their modes, prints
the login prompt, gets the user's name, and then initiates a login process
for that user. This allows users to authenticate themselves to the system and
use it. By default, most systems offer 6 virtual consoles, but as you can see
from the inittab file, this is configurable.

 /etc/inittab can also tell init how it should handle a user pressing Ctrl+
Alt+Delete at the console. As the system should be properly shut down and
restarted rather than immediately power-cycled, init is told to execute the
command /sbin/shutdown -t3 -r now, for instance, when a user hits those keys.
In addition, /etc/inittab states what init should do in case of power
failures, if your system has a UPS unit attached to it.

 On most RPM-based systems the graphical login screen is started in run level
5, where /etc/inittab runs a script called /etc/X11/prefdm. The prefdm script
runs the preferred X display manager, based on the contents of the /etc/
sysconfig/desktop directory. This is typically gdm if you run GNOME or kdm if
you run KDE, but they can be mixed, and there's also the xdm that comes with
a standard X installation.

But there are other possibilities as well. On Debian, for instance, there is
an initscript for each of the display managers, and the content of the /etc/
X11/default-display-manager is used to determine which one to start. More
about the graphical interface can be read in Section 7.3. Ultimately, your
system documentation will explain the details about the higher level aspects
of init.

The /etc/default and/or /etc/sysconfig directories contain entries for a
range of functions and services, these are all read at boot time. The
location of the directory containing system defaults might be somewhat
different depending on your Linux distribution.

Besides the graphical user environment, a lot of other services may be
started as well. But if all goes well, you should be looking at a login
prompt or login screen when the boot process has finished.

Note Other procedures
     We explained how SysV init works on x86 based machines. Startup
     procedures may vary on other architectures and distributions. Other
     systems may use the BSD-style init, where startup files are not split up
     into multiple /etc/rc<LEVEL>.d directories. It might also be possible
     that your system uses /etc/rc.d/init.d instead of /etc/init.d.



4.2.5. Init run levels

The idea behind operating different services at different run levels
essentially revolves around the fact that different systems can be used in
different ways. Some services cannot be used until the system is in a
particular state, or mode, such as being ready for more than one user or
having networking available.

 There are times in which you may want to operate the system in a lower mode.
Examples are fixing disk corruption problems in run level 1 so no other users
can possibly be on the system, or leaving a server in run level 3 without an
X session running. In these cases, running services that depend upon a higher
system mode to function does not make sense because they will not work
correctly anyway. By already having each service assigned to start when its
particular run level is reached, you ensure an orderly start up process, and
you can quickly change the mode of the machine without worrying about which
services to manually start or stop.

Available run levels are generally described in /etc/inittab, which is
partially shown below:


#
# inittab   This file describes how the INIT process should set up
#           the system in a certain run-level.

# Default run level. The run levels are:
#   0 - halt (Do NOT set initdefault to this)
#   1 - Single user mode
#   2 - Multiuser, without NFS
#       (The same as 3, if you do not have networking)
#   3 - Full multiuser mode
#   4 - unused
#   5 - X11
#   6 - reboot (Do NOT set initdefault to this)
#
id:5:initdefault:
<--cut-->

Feel free to configure unused run levels (commonly run level 4) as you see
fit. Many users configure those run levels in a way that makes the most sense
for them while leaving the standard run levels as they are by default. This
allows them to quickly move in and out of their custom configuration without
disturbing the normal set of features at the standard run levels.

If your machine gets into a state where it will not boot due to a bad /etc/
inittab or will not let you log in because you have a corrupted /etc/passwd
file (or if you have simply forgotten your password), boot into single-user
mode.

Tip No graphics?
    When you are working in text mode because you didn't get presented a
    graphical login screen on the console of your machine, you can normally
    switch to console 7 or up to have a graphical login. If this is not the
    case, check the current run level using the command who -r. If it is set
    to something else than the original default from /etc/inittab, chances
    are that the system does not start up in graphical mode by default.
    Contact your system administrator or read man init in that case. Note
    that switching run levels is done preferably using the telinit command;
    switching from a text to a graphical console or vice versa does not
    involve a run level switch.

The discussion of run levels, scripts and configurations in this guide tries
to be as general as possible. Lots of variations exist. For instance, Gentoo
Linux stores scripts in /etc/run levels. Other systems might first run
through (a) lower run level(s) and execute all the scripts in there before
arriving at the final run level and executing those scripts. Refer to your
system documentation for more information. You might also read through the
scripts that are refered to in /etc/inittab to get a better comprehension of
what happens on your system.



4.2.5.1. Tools

The chkconfig or update-rc.d utilities, when installed on your system,
provide a simple command-line tool for maintaining the /etc/init.d directory
hierarchy. These relieve system administrators from having to directly
manipulate the numerous symbolic links in the directories under /etc/rc[x].d.

 In addition, some systems offer the ntsysv tool, which provides a text-based
interface; you may find this easier to use than chkconfig's command-line
interface. On SuSE Linux, you will find the yast and insserv tools. For
Mandrake easy configuration, you may want to try DrakConf, which allows among
other features switching between run levels 3 and 5. In Mandriva this became
the Mandriva Linux Control Center.

 Most distributions provide a graphical user interface for configuring
processes, check with your system documentation.

 All of these utilities must be run as root. The system administrator may
also manually create the appropriate links in each run level directory in
order to start or stop a service in a certain run level.



4.2.6. Shutdown

UNIX was not made to be shut down, but if you really must, use the shutdown
command. After completing the shutdown procedure, the -h option will halt the
system, while -r will reboot it.

The reboot and halt commands are now able to invoke shutdown if run when the
system is in run levels 1-5, and thus ensure proper shutdown of the
system,but it is a bad habit to get into, as not all UNIX/Linux versions have
this feature.

If your computer does not power itself down, you should not turn off the
computer until you see a message indicating that the system is halted or
finished shutting down, in order to give the system the time to unmount all
partitions. Being impatient may cause data loss.



4.3. Managing processes

4.3.1. Work for the system admin

While managing system resources, including processes, is a task for the local
system administrator, it doesn't hurt a common user to know something about
it, especially where his or her own processes and their optimal execution are
concerned.

We will explain a little bit on a theoretical level about system performance,
though not as far as hardware optimization and other advanced procedures.
Instead, we will study the daily problems a common user is confronted with,
and actions such a user can take to optimally use the resources available. As
we learn in the next section, this is mainly a matter of thinking before
acting.

Figure 4-2. Can't you go faster?

[trappen]



4.3.2. How long does it take?

Bash offers a built-in time command that displays how long a command takes to
execute. The timing is highly accurate and can be used on any command. In the
example below, it takes about a minute and a half to make this book:

tilly:~/xml/src> time make
Output written on abook.pdf (222 pages, 1619861 bytes).
Transcript written on abook.log.

real    1m41.056s
user    1m31.190s
sys     0m1.880s

The GNU time command in /usr/bin (as opposed to the shell built-in version)
displays more information that can be formatted in different ways. It also
shows the exit status of the command, and the total elapsed time. The same
command as the above using the independent time gives this output:

tilly:~/xml/src> /usr/bin/time make
Output written on abook.pdf (222 pages, 1595027 bytes).
Transcript written on abook.log.

Command exited with non-zero status 2
88.87user 1.74system 1:36.21elapsed 94%CPU
                                (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (2192major+30002minor)pagefaults 0swaps

Refer again to the Info pages for all the information.



4.3.3. Performance

To a user, performance means quick execution of commands. To a system
manager, on the other hand, it means much more: the system admin has to
optimize system performance for the whole system, including users, all
programs and daemons. System performance can depend on a thousand tiny things
which are not accounted for with the time command:

  • the program executing is badly written or doesn't use the computer
    appropriately

  • access to disks, controllers, display, all kinds of interfaces, etc.

  • reachability of remote systems (network performance)

  • amount of users on the system, amount of users actually working
    simultaneously

  • time of day

  • ...



4.3.4. Load

In short: the load depends on what is normal for your system. My old P133
running a firewall, SSH server, file server, a route daemon, a sendmail
server, a proxy server and some other services doesn't complain with 7 users
connected; the load is still 0 on average. Some (multi-CPU) systems I've seen
were quite happy with a load of 67. There is only one way to find out - check
the load regularly if you want to know what's normal. If you don't, you will
only be able to measure system load from the response time of the command
line, which is a very rough measurement since this speed is influenced by a
hundred other factors.

Keep in mind that different systems will behave different with the same load
average. For example, a system with a graphics card supporting hardware
acceleration will have no problem rendering 3D images, while the same system
with a cheap VGA card will slow down tremendously while rendering. My old
P133 will become quite uncomfortable when I start the X server, but on a
modern system you hardly notice the difference in the system load.



4.3.5. Can I do anything as a user?

A big environment can slow you down. If you have lots of environment
variables set (instead of shell variables), long search paths that are not
optimized (errors in setting the path environment variable) and more of those
settings that are usually made "on the fly", the system will need more time
to search and read data.

In X, window managers and desktop environments can be real CPU-eaters. A
really fancy desktop comes with a price, even when you can download it for
free, since most desktops provide add-ons ad infinitum. Modesty is a virtue
if you don't buy a new computer every year.



4.3.5.1. Priority

The priority or importance of a job is defined by it's nice number. A program
with a high nice number is friendly to other programs, other users and the
system; it is not an important job. The lower the nice number, the more
important a job is and the more resources it will take without sharing them.

Making a job nicer by increasing its nice number is only useful for processes
that use a lot of CPU time (compilers, math applications and the like).
Processes that always use a lot of I/O time are automatically rewarded by the
system and given a higher priority (a lower nice number), for example
keyboard input always gets highest priority on a system.

Defining the priority of a program is done with the nice command.

Most systems also provide the BSD renice command, which allows you to change
the niceness of a running command. Again, read the man page for your
system-specific information.

Caution Interactive programs
        It is NOT a good idea to nice or renice an interactive program or a
        job running in the foreground.

Use of these commands is usually a task for the system administrator. Read
the man page for more info on extra functionality available to the system
administrator.



4.3.5.2. CPU resources

On every Linux system, many programs want to use the CPU(s) at the same time,
even if you are the only user on the system. Every program needs a certain
amount of cycles on the CPU to run. There may be times when there are not
enough cycles because the CPU is too busy. The uptime command is wildly
inaccurate (it only displays averages, you have to know what is normal), but
far from being useless. There are some actions you can undertake if you think
your CPU is to blame for the unresponsiveness of your system:

  • Run heavy programs when the load is low. This may be the case on your
    system during the night. See next section for scheduling.

  • Prevent the system from doing unnecessary work: stop daemons and programs
    that you don't use, use locate instead of a heavy find, ...

  • Run big jobs with a low priority

If none of these solutions are an option in your particular situation, you
may want to upgrade your CPU. On a UNIX machine this is a job for the system
admin.



4.3.5.3. Memory resources

When the currently running processes expect more memory than the system has
physically available, a Linux system will not crash; it will start paging, or
swapping, meaning the process uses the memory on disk or in swap space,
moving contents of the physical memory (pieces of running programs or entire
programs in the case of swapping) to disk, thus reclaiming the physical
memory to handle more processes. This slows the system down enormously since
access to disk is much slower than access to memory. The top command can be
used to display memory and swap use. Systems using glibc offer the memusage
and memusagestat commands to visualize memory usage.

If you find that a lot of memory and swap space are being used, you can try:

  • Killing, stopping or renicing those programs that use a big chunk of
    memory

  • Adding more memory (and in some cases more swap space) to the system.

  • Tuning system performance, which is beyond the scope of this document.
    See the reading list in Appendix A for more.



4.3.5.4. I/O resources

While I/O limitations are a major cause of stress for system admins, the
Linux system offers rather poor utilities to measure I/O performance. The ps,
vmstat and top tools give some indication about how many programs are waiting
for I/O; netstat displays network interface statistics, but there are
virtually no tools available to measure the I/O response to system load, and
the iostat command gives a brief overview of general I/O usage. Various
graphical front-ends exist to put the output of these commands in a humanly
understandable form.

Each device has its own problems, but the bandwidth available to network
interfaces and the bandwidth available to disks are the two primary causes of
bottlenecks in I/O performance.

Network I/O problems:

  • Network overload:

    The amount of data transported over the network is larger than the
    network's capacity, resulting in slow execution of every network related
    task for all users. They can be solved by cleaning up the network (which
    mainly involves disabling protocols and services that you don't need) or
    by reconfiguring the network (for example use of subnets, replacing hubs
    with switches, upgrading interfaces and equipment).

  • Network integrity problems:

    Occurs when data is transferred incorrectly. Solving this kind of problem
    can only be done by isolating the faulty element and replacing it.

Disk I/O problems:

  • per-process transfer rate too low:

    Read or write speed for a single process is not sufficient.

  • aggregate transfer rate too low:

    The maximum total bandwidth that the system can provide to all programs
    that run is not enough.

This kind of problem is more difficult to detect, and usually takes extra
hardware in order to re-divide data streams over buses, controllers and
disks, if overloaded hardware is cause of the problem. One solution to solve
this is a RAID array configuration optimized for input and output actions.
This way, you get to keep the same hardware. An upgrade to faster buses,
controlers and disks is usually the other option.

If overload is not the cause, maybe your hardware is gradually failing, or
not well connected to the system. Check contacts, connectors and plugs to
start with.



4.3.5.5. Users

Users can be divided in several classes, depending on their behavior with
resource usage:

  • Users who run a (large) number of small jobs: you, the beginning Linux
    user, for instance.

  • Users who run relatively few but large jobs: users running simulations,
    calculations, emulators or other programs that eat a lot of memory, and
    usually these users have accompanying large data files.

  • Users who run few jobs but use a lot of CPU time (developers and the
    like).

You can see that system requirements may vary for each class of users, and
that it can be hard to satisfy everyone. If you are on a multi-user system,
it is useful (and fun) to find out habits of other users and the system, in
order to get the most out of it for your specific purposes.



4.3.5.6. Graphical tools

For the graphical environment, there are a whole bunch of monitoring tools
available. Below is a screen shot of the Gnome System Monitor, which has
features for displaying and searching process information, and monitoring
system resources:

Figure 4-3. Gnome System Monitor

[system-mon]

There are also a couple of handy icons you can install in the task bar, such
as a disk, memory and load monitor. xload is another small X application for
monitoring system load. Find your favorite!



4.3.5.7. Interrupting your processes

As a non-privileged user, you can only influence your own processes. We
already saw how you can display processes and filter out processes that
belong to a particular user, and what possible restrictions can occur. When
you see that one of your processes is eating too much of the system's
resources, there are two things that you can do:

 1. Make the process use less resources without interrupting it;

 2. Stop the process altogether.

In the case that you want the process to continue to run, but you also want
to give the other processes on the system a chance, you can renice the
process. Appart from using the nice or renice commands, top is an easy way of
spotting the troublesome process(es) and reducing priority.

Identify the process in the "NI" column, it will most likely have a negative
priority. Type r and enter the process ID of the process that you want to
renice. Then enter the nice value, for instance "20". That means that from
now on, this process will take 1/5 of the CPU cycles at the most.

Examples of processes that you want to keep on running are emulators, virtual
machines, compilers and so on.

If you want to stop a process because it hangs or is going totally berserk in
the way of I/O consumption, file creation or use of other system resources,
use the kill command. If you have the opportunity, first try to kill the
process softly, sending it the SIGTERM signal. This is an instruction to
terminate whatever it is doing, according to procedures as described in the
code of the program:

joe:~> ps -ef | grep mozilla
joe    25822    1  0 Mar11 ?    00:34:04 /usr/lib/mozilla-1.4.1/mozilla-

joe:~> kill -15 25822

In the example above, user joe stopped his Mozilla browser because it hung.

Some processes are a little bit harder to get rid of. If you have the time,
you might want to send them the SIGINT signal to interrupt them. If that does
not do the trick either, use the strongest signal, SIGKILL. In the example
below, joe stops a Mozilla that is frozen:

joe:~> ps -ef | grep mozilla
joe    25915    1  0 Mar11 ?    00:15:06 /usr/lib/mozilla-1.4.1/mozilla-

joe:~> kill -9 25915

joe:~> ps -ef | grep 25915
joe     2634 32273 0 18:09 pts/4   00:00:00 grep 25915

In such cases, you might want to check that the process is really dead, using
the grep filter again on the PID. If this only returns the grep process, you
can be sure that you succeeded in stopping the process.

Among processes that are hard to kill is your shell. And that is a good
thing: if they would be easy to kill, you woud loose your shell every time
you type Ctrl-C on the command line accidentally, since this is equivalent to
sending a SIGINT.

Note UNIX without pipes is almost unthinkable
     The usage of pipes (|) for using output of one command as input of
     another is explained in the next chapter, Chapter 5.

In a graphical environment, the xkill program is very easy to use. Just type
the name of the command, followed by an Enter and select the window of the
application that you want to stop. It is rather dangerous because it sends a
SIGKILL by default, so only use it when an application hangs.



4.4. Scheduling processes

4.4.1. Use that idle time!

A Linux system can have a lot to suffer from, but it usually suffers only
during office hours. Whether in an office environment, a server room or at
home, most Linux systems are just idling away during the morning, the
evening, the nights and weekends. Using this idle time can be a lot cheaper
than buying those machines you'd absolutely need if you want everything done
at the same time.

There are three types of delayed execution:

  • Waiting a little while and then resuming job execution, using the sleep
    command. Execution time depends on the system time at the moment of
    submission.

  • Running a command at a specified time, using the at command. Execution of
    the job(s) depends on system time, not the time of submission.

  • Regularly running a command on a monthly, weekly, daily or hourly basis,
    using the cron facilities.

The following sections discuss each possibility.



4.4.2. The sleep command

The Info page on sleep is probably one of the shortest there is. All sleep
does is wait. By default the time to wait is expressed in seconds.

So why does it exist? Some practical examples:

Somebody calls you on the phone, you say "Yes I'll be with you in half an
hour" but you're about drowned in work as it is and bound to forget your
lunch:

(sleep 1800; echo "Lunch time..") &

When you can't use the at command for some reason, it's five o'clock, you
want to go home but there's still work to do and right now somebody is eating
system resources:

(sleep 10000; myprogram) &

Make sure there's an auto-logout on your system, and that you log out or lock
your desktop/office when submitting this kind of job, or run it in a screen
session.

When you run a series of printouts of large files, but you want other users
to be able to print in between:

lp lotoftext; sleep 900; lp hugefile; sleep 900; lp anotherlargefile

Printing files is discussed in Chapter 8.

Programmers often use the sleep command to halt script or program execution
for a certain time.



4.4.3. The at command

The at command executes commands at a given time, using your default shell
unless you tell the command otherwise (see the man page).

The options to at are rather user-friendly, which is demonstrated in the
examples below:

steven@home:~> at tomorrow + 2 days
warning: commands will be executed using (in order) a) $SHELL
        b) login shell c) /bin/sh
at>  cat reports | mail myboss@mycompany
at> <EOT>
job 1 at 2001-06-16 12:36

Typing Ctrl+D quits the at utility and generates the "EOT" message.

User steven does a strange thing here combining two commands; we will study
this sort of practice in Chapter 5, Redirecting Input and Output.

steven@home:~> at 0237
warning: commands will be executed using (in order) a) $SHELL
        b) login shell c) /bin/sh
at>  cd new-programs
at>  ./configure; make
at> <EOT>
job 2 at 2001-06-14 02:00

The -m option sends mail to the user when the job is done, or explains when a
job can't be done. The command atq lists jobs; perform this command before
submitting jobs in order prevent them from starting at the same time as
others. With the atrm command you can remove scheduled jobs if you change
your mind.

It is a good idea to pick strange execution times, because system jobs are
often run at "round" hours, as you can see in Section 4.4.4 the next section.
For example, jobs are often run at exactly 1 o'clock in the morning (e.g.
system indexing to update a standard locate database), so entering a time of
0100 may easily slow your system down rather than fire it up. To prevent jobs
from running all at the same time, you may also use the batch command, which
queues processes and feeds the work in the queue to the system in an evenly
balanced way, preventing excessive bursts of system resource usage. See the
Info pages for more information.



4.4.4. Cron and crontab

The cron system is managed by the cron daemon. It gets information about
which programs and when they should run from the system's and users' crontab
entries. Only the root user has access to the system crontabs, while each
user should only have access to his own crontabs. On some systems (some)
users may not have access to the cron facility.

At system startup the cron daemon searches /var/spool/cron/ for crontab
entries which are named after accounts in /etc/passwd, it searches /etc/
cron.d/ and it searches /etc/crontab, then uses this information every minute
to check if there is something to be done. It executes commands as the user
who owns the crontab file and mails any output of commands to the owner.

On systems using Vixie cron, jobs that occur hourly, daily, weekly and
monthly are kept in separate directories in /etc to keep an overview, as
opposed to the standard UNIX cron function, where all tasks are entered into
one big file.

Example of a Vixie crontab file:

[root@blob /etc]# more crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# run-parts
# commands to execute every hour
01 * * * * root run-parts /etc/cron.hourly
# commands to execute every day
02 4 * * * root run-parts /etc/cron.daily
# commands to execute every week
22 4 * * 0 root run-parts /etc/cron.weekly
commands to execute every month
42 4 1 * * root run-parts /etc/cron.monthly

Note Alternative
     You could also use the crontab -l command to display crontabs.

Some variables are set, and after that there's the actual scheduling, one
line per job, starting with 5 time and date fields. The first field contains
the minutes (from 0 to 59), the second defines the hour of execution (0-23),
the third is day of the month (1-31), then the number of the month (1-12),
the last is day of the week (0-7, both 0 and 7 are Sunday). An asterisk in
these fields represents the total acceptable range for the field. Lists are
allowed; to execute a job from Monday to Friday enter 1-5 in the last field,
to execute a job on Monday, Wednesday and Friday enter 1,3,5.

Then comes the user who should run the processes which are listed in the last
column. The example above is from a Vixie cron configuration where root runs
the program run-parts on regular intervals, with the appropriate directories
as options. In these directories, the actual jobs to be executed at the
scheduled time are stored as shell scripts, like this little script that is
run daily to update the database used by the locate command:

billy@ahost cron.daily]$ cat slocate.cron
#!/bin/sh
renice +19 -p $$ >/dev/null 2>&1
/usr/bin/updatedb -f "nfs,smbfs,ncpfs,proc,devpts" -e \
"/tmp,/var/tmp, /usr/tmp,/afs,/net"

Users are supposed to edit their crontabs in a safe way using the crontab -e
command. This will prevent a user from accidentally opening more than one
copy of his/her crontab file. The default editor is vi (see Chapter 6, but
you can use any text editor, such as gvim or gedit if you feel more
comfortable with a GUI editor.

When you quit, the system will tell you that a new crontab is installed.

This crontab entry reminds billy to go to his sports club every Thursday
night:

billy:~> crontab -l
# DO NOT EDIT THIS FILE - edit the master and reinstall.
# (/tmp/crontab.20264 installed on Sun Jul 20 22:35:14 2003)
# (Cron version -- $Id: chap4.xml,v 1.28 2007/09/19 12:22:26 tille Exp $)
38 16 * * 3 mail -s "sports evening" billy

After adding a new scheduled task, the system will tell you that a new
crontab is installed. You do not need to restart the cron daemon for the
changes to take effect. In the example, billy added a new line pointing to a
backup script:

billy:~> crontab -e
45 15 * * 3 mail -s "sports evening" billy
4 4 * * 4,7 /home/billy/bin/backup.sh

<--write and quit-->

crontab: installing new crontab

billy:~>

The backup.sh script is executed every Thursday and Sunday. See Section 7.2.5
for an introduction to shell scripting. Keep in mind that output of commands,
if any, is mailed to the owner of the crontab file. If no mail service is
configured, you might find the output of your commands in your local mailbox,
/var/spool/mail/<your_username>, a plain text file.

Note Who runs my commands?
     You don't have to specify the user who should run the commands. They are
     executed with the user's own permissions by default.



4.5. Summary

Linux is a multi-user, multi-tasking operating system that has a UNIX-like
way of handling processes. Execution speed of commands can depend on a
thousand tiny things. Among others, we learned a lot of new commands to
visualize and handle processes. Here's a list:

Table 4-3. New commands in chapter 4: Processes


Command Meaning                                                           

at      Queue jobs for later execution.                                   

atq     Lists the user's pending jobs.                                    

atrm    Deletes jobs, determined by their job number.                     

batch   Executes commands when system load level permits.                 

crontab Maintain crontab files for individual users.                      

halt    Stop the system.                                                  

init runProcess control initialization.                                   
level                                                                     

jobs    Lists currently executing jobs.                                   

kill    Terminate a process.                                              

mesg    Control write access to your terminal.                            

netstat Display network connections, routing tables, interface statistics,
        masquerade connections and multicast memberships.                 

nice    Run a program with modified scheduling priority.                  

pgrep   Display processes.                                                

ps      Report process status.                                            

pstree  Display a tree of processes.                                      

reboot  Stop the system.                                                  

renice  Alter priority of running processes.                              

shutdownBring the system down.                                            

sleep   Delay for a specified time.                                       

time    Time a command or report resource usage.                          

top     Display top CPU processes.                                        

uptime  Show how long the system has been running.                        

vmstat  Report virtual memory statistics.                                 

w       Show who is logged on and what they are doing.                    

wall    Send a message to everybody's terminals.                          

who     Show who is logged on.                                            

write   Send a message to another user.                                   



4.6. Exercises

These are some exercises that will help you get the feel for processes
running on your system.



4.6.1. General

  • Run top in one terminal while you do the exercises in another.

  • Run the ps command.

  • Read the man pages to find out how to display all your processes.

  • Run the command find /. What effect does it have on system load? Stop
    this command.

  • In graphical mode, start the xclock program in the foreground. Then let
    it run in the background. Stop the program using the kill command.

  • Run the xcalc directly in the background, so that the prompt of the
    issuing terminal is released.

  • What does kill -9 -1 do?

  • Open two terminals or terminal windows again and use write to send a
    message from one to the other.

  • Issue the dmesg command. What does it tell?

  • How long does it take to execute ls in the current directory?

  • Based on process entries in /proc, owned by your UID, how would you work
    to find out which processes these actually represent?

  • How long has your system been running?

  • Which is your current TTY?

  • Name 3 processes that couldn't have had init as an initial parent.

  • Name 3 commands which use SUID mode. Explain why this is so.

  • Name the commands that are generally causing the highest load on your
    system.



4.6.2. Booting, init etc.

  • Can you reboot the system as a normal user? Why is that?

  • According to your current run level, name the steps that are taken during
    shutdown.

  • How do you change the system run level? Switch from your default run
    level to run level 1 and vice versa.

  • Make a list of all the services and daemons that are started up when your
    system has booted.

  • Which kernel is currently load at startup?

  • Suppose you have to start some exotic server at boot time. Up until now,
    you logged in after booting the system and started this server manually
    using a script named deliver_pizza in your home directory. What do you
    have to do in order to have the service start up automatically in run
    level 4, which you defined for this purpose only?



4.6.3. Scheduling

  • Use sleep to create a reminder that your pasta is ready in ten minutes.

  • Create an at job that copies all files in your home directory to /var/tmp
    within half an hour. You may want to create a sub-directory in /var/tmp.

  • Make a cronjob that does this task every Monday to Friday during lunch.

  • Check that it works.

  • Make a mistake in the crontab entry, like issuing the nonexistent command
    coppy instead of cp. What happens upon execution of the task?



Chapter 5. I/O redirection

    This chapter describes more about the powerful UNIX mechanism of
    redirecting input, output and errors. Topics include:

     

      □ Standard input, output and errors

      □ Redirection operators

      □ How to use output of one command as input for another

      □ How to put output of a command in a file for later referrence

      □ How to append output of multiple commands to a file

      □ Input redirection

      □ Handling standard error messages

      □ Combining redirection of input, output and error streams

      □ Output filters



5.1. Simple redirections

5.1.1. What are standard input and standard output?

 Most Linux commands read input, such as a file or another attribute for the
command, and write output. By default, input is being given with the
keyboard, and output is displayed on your screen. Your keyboard is your
standard input (stdin) device, and the screen or a particular terminal window
is the standard output (stdout) device.

However, since Linux is a flexible system, these default settings don't
necessarily have to be applied. The standard output, for example, on a
heavily monitored server in a large environment may be a printer.



5.1.2. The redirection operators

5.1.2.1. Output redirection with > and |

Sometimes you will want to put output of a command in a file, or you may want
to issue another command on the output of one command. This is known as
redirecting output. Redirection is done using either the ">" (greater-than
symbol), or using the "|" (pipe) operator which sends the standard output of
one command to another command as standard input.

As we saw before, the cat command concatenates files and puts them all
together to the standard output. By redirecting this output to a file, this
file name will be created - or overwritten if it already exists, so take
care.

nancy:~> cat test1
some words

nancy:~> cat test2
some other words

nancy:~> cat test1 test2 > test3

nancy:~> cat test3
some words
some other words

Warning Don't overwrite!
        Be careful not to overwrite existing (important) files when
        redirecting output. Many shells, including Bash, have a built-in
        feature to protect you from that risk: noclobber. See the Info pages
        for more information. In Bash, you would want to add the set -o 
        noclobber command to your .bashrc configuration file in order to
        prevent accidental overwriting of files.

Redirecting "nothing" to an existing file is equal to emptying the file:

nancy:~> ls -l list
-rw-rw-r--    1 nancy   nancy     117 Apr  2 18:09 list

nancy:~> > list

nancy:~> ls -l list
-rw-rw-r--    1 nancy   nancy       0 Apr  4 12:01 list

This process is called truncating.

The same redirection to an nonexistent file will create a new empty file with
the given name:

nancy:~> ls -l newlist
ls: newlist: No such file or directory

nancy:~> > newlist

nancy:~> ls -l newlist
-rw-rw-r--  1 nancy   nancy         0 Apr  4 12:05 newlist

Chapter 7 gives some more examples on the use of this sort of redirection.

Some examples using piping of commands:

To find a word within some text, display all lines matching "pattern1", and
exclude lines also matching "pattern2" from being displayed:

grep pattern1 file | grep -v pattern2

To display output of a directory listing one page at a time:

ls -la | less

To find a file in a directory:

ls -l | grep part_of_file_name



5.1.2.2. Input redirection

In another case, you may want a file to be the input for a command that
normally wouldn't accept a file as an option. This redirecting of input is
done using the "<" (less-than symbol) operator.

Below is an example of sending a file to somebody, using input redirection.

andy:~> mail mike@somewhere.org < to_do

If the user mike exists on the system, you don't need to type the full
address. If you want to reach somebody on the Internet, enter the fully
qualified address as an argument to mail.

This reads a bit more difficult than the beginner's cat file | mail someone,
but it is of course a much more elegant way of using the available tools.



5.1.2.3. Combining redirections

The following example combines input and output redirection. The file
text.txt is first checked for spelling mistakes, and the output is redirected
to an error log file:

spell < text.txt > error.log

The following command lists all commands that you can issue to examine
another file when using less:

mike:~> less --help | grep -i examine
  :e [file]      Examine a new file.
  :n          *  Examine the (N-th) next file from the command line.
  :p          *  Examine the (N-th) previous file from the command line.
  :x          *  Examine the first (or N-th) file from the command line.

The -i option is used for case-insensitive searches - remember that UNIX
systems are very case-sensitive.

If you want to save output of this command for future reference, redirect the
output to a file:

mike:~> less --help | grep -i examine > examine-files-in-less

mike:~> cat examine-files-in-less
  :e [file]      Examine a new file.
  :n          *  Examine the (N-th) next file from the command line.
  :p          *  Examine the (N-th) previous file from the command line.
  :x          *  Examine the first (or N-th) file from the command line.

Output of one command can be piped into another command virtually as many
times as you want, just as long as these commands would normally read input
from standard input and write output to the standard output. Sometimes they
don't, but then there may be special options that instruct these commands to
behave according to the standard definitions; so read the documentation (man
and Info pages) of the commands you use if you should encounter errors.

Again, make sure you don't use names of existing files that you still need.
Redirecting output to existing files will replace the content of those files.



5.1.2.4. The >> operator

Instead of overwriting file data, you can also append text to an existing
file using two subsequent greater-than signs:

Example:

mike:~> cat wishlist
more money
less work

mike:~> date >> wishlist

mike:~> cat wishlist
more money
less work
Thu Feb 28 20:23:07 CET 2002

The date command would normally put the last line on the screen; now it is
appended to the file wishlist.



5.2. Advanced redirection features

5.2.1. Use of file descriptors

There are three types of I/O, which each have their own identifier, called a
file descriptor:

  • standard input: 0

  • standard output: 1

  • standard error: 2

In the following descriptions, if the file descriptor number is omitted, and
the first character of the redirection operator is <, the redirection refers
to the standard input (file descriptor 0). If the first character of the
redirection operator is >, the redirection refers to the standard output
(file descriptor 1).

Some practical examples will make this more clear:

ls > dirlist 2>&1

will direct both standard output and standard error to the file dirlist,
while the command

ls 2>&1 > dirlist

will only direct standard output to dirlist. This can be a useful option for
programmers.

Things are getting quite complicated here, don't confuse the use of the
ampersand here with the use of it in Section 4.1.2.1, where the ampersand is
used to run a process in the background. Here, it merely serves as an
indication that the number that follows is not a file name, but rather a
location that the data stream is pointed to. Also note that the bigger-than
sign should not be separated by spaces from the number of the file
descriptor. If it would be separated, we would be pointing the output to a
file again. The example below demonstrates this:

[nancy@asus /var/tmp]$ ls 2> tmp

[nancy@asus /var/tmp]$ ls -l tmp
-rw-rw-r--  1 nancy nancy 0 Sept  7 12:58 tmp

[nancy@asus /var/tmp]$ ls 2 > tmp
ls: 2: No such file or directory

The first command that nancy executes is correct (eventhough no errors are
generated and thus the file to which standard error is redirected is empty).
The second command expects that 2 is a file name, which does not exist in
this case, so an error is displayed.

All these features are explained in detail in the Bash Info pages.



5.2.2. Examples

5.2.2.1. Analyzing errors

If your process generates a lot of errors, this is a way to thoroughly
examine them:

command 2>&1 | less

This is often used when creating new software using the make command, such as
in:

andy:~/newsoft> make all 2>&1 | less
--output ommitted--



5.2.2.2. Separating standard output from standard error

Constructs like these are often used by programmers, so that output is
displayed in one terminal window, and errors in another. Find out which
pseudo terminal you are using issuing the tty command first:

andy:~/newsoft> make all 2> /dev/pts/7



5.2.2.3. Writing to output and files simultaneously

You can use the tee command to copy input to standard output and one or more
output files in one move. Using the -a option to tee results in appending
input to the file(s). This command is useful if you want to both see and save
output. The > and >> operators do not allow to perform both actions
simultaneously.

This tool is usually called on through a pipe (|), as demonstrated in the
example below:

mireille ~/test> date | tee file1 file2
Thu Jun 10 11:10:34 CEST 2004

mireille ~/test> cat file1
Thu Jun 10 11:10:34 CEST 2004

mireille ~/test> cat file2
Thu Jun 10 11:10:34 CEST 2004

mireille ~/test> uptime | tee -a file2
 11:10:51 up 21 days, 21:21, 57 users,  load average: 0.04, 0.16, 0.26

mireille ~/test> cat file2
Thu Jun 10 11:10:34 CEST 2004
 11:10:51 up 21 days, 21:21, 57 users,  load average: 0.04, 0.16, 0.26



5.3. Filters

When a program performs operations on input and writes the result to the
standard output, it is called a filter. One of the most common uses of
filters is to restructure output. We'll discuss a couple of the most
important filters below.



5.3.1. More about grep

As we saw in Section 3.3.3.4, grep scans the output line per line, searching
for matching patterns. All lines containing the pattern will be printed to
standard output. This behavior can be reversed using the -v option.

Some examples: suppose we want to know which files in a certain directory
have been modified in February:

jenny:~> ls -la | grep Feb

The grep command, like most commands, is case sensitive. Use the -i option to
make no difference between upper and lower case. A lot of GNU extensions are
available as well, such as --colour, which is helpful to highlight
searchterms in long lines, and --after-context, which prints the number of
lines after the last matching line. You can issue a recursive grep that
searches all subdirectories of encountered directories using the -r option.
As usual, options can be combined.

Regular expressions can be used to further detail the exact character matches
you want to select out of all the input lines. The best way to start with
regular expressions is indeed to read the grep documentation. An excellent
chapter is included in the grep Info page. Since it would lead us too far
discussing the ins and outs of regular expressions, it is strongly advised to
start here if you want to know more about them.

Play around a bit with grep, it will be worth the trouble putting some time
in this most basic but very powerful filtering command. The exercises at the
end of this chapter will help you to get started, see Section 5.5.



5.3.2. Filtering output

The command sort arranges lines in alphabetical order by default:

thomas:~> cat people-I-like | sort
Auntie Emmy
Boyfriend
Dad
Grandma
Mum
My boss

But there are many more things sort can do. Looking at the file size, for
instance. With this command, directory content is sorted smallest files
first, biggest files last:

ls -la | sort -nk 5

Note Old sort syntax
     You might obtain the same result with ls -la | sort +4n, but this is an
     old form which does not comply with the current standards.

The sort command is also used in combination with the uniq program (or sort
-u) to sort output and filter out double entries:

thomas:~> cat itemlist
1
4
2
5
34
567
432
567
34
555

thomas:~> sort itemlist | uniq
1
2
34
4
432
5
555
567



5.4. Summary

In this chapter we learned how commands can be linked to each other, and how
input from one command can be used as output for another command.

Input/output redirection is a common task on UNIX and Linux machines. This
powerful mechanism allows flexible use of the building blocks UNIX is made
of.

The most commonly used redirections are > and |. Refer to Appendix C for an
overview of redirection commands and other shell constructs.

Table 5-1. New commands in chapter 5: I/O redirection


CommandMeaning                                   

date   Display time and date information.        

set    Configure shell options.                  

sort   Sort lines of text.                       

uniq   Remove duplicate lines from a sorted file.



5.5. Exercises

These exercises give more examples on how to combine commands. The main goal
is to try and use the Enter key as little as possible.

All exercises are done using a normal user ID, so as to generate some errors.
While you're at it, don't forget to read those man pages!

  • Use the cut command on the output of a long directory listing in order to
    display only the file permissions. Then pipe this output to sort and uniq
    to filter out any double lines. Then use the wc to count the different
    permission types in this directory.

  • Put the output of date in a file. Append the output of ls to this file.
    Send this file to your local mailbox (don't specify anything <@domain>,
    just the user name will do). When using Bash, you will see a new mail
    notice upon success.

  • List the devices in /dev which are currently used by your UID. Pipe
    through less to view them properly.

  • Issue the following commands as a non-privileged user. Determine standard
    input, output and error for each command.

      □ 
        cat nonexistentfile

      □ 
        file /sbin/ifconfig

      □ 
        grep root /etc/passwd /etc/nofiles > grepresults

      □ 
        /etc/init.d/sshd start > /var/tmp/output

      □ 
        /etc/init.d/crond start > /var/tmp/output 2>&1

      □ Now check your results by issuing the commands again, now redirecting
        standardoutput to the file /var/tmp/output and standard error to the
        file /var/tmp/error.

  • How many processes are you currently running?

  • How many invisible files are in your home directory?

  • Use locate to find documentation about the kernel.

  • Find out which file contains the following entry:

    root:x:0:0:root:/root:/bin/bash

    And this one:

    system:         root

  • See what happens upon issuing this command:

    > time; date >> time; cat < time

  • What command would you use to check which script in /etc/init.d starts a
    given process?



Chapter 6. Text editors

    In this chapter, we will discuss the importance of mastering an editor.
    We will focus mainly on the Improved vi editor.

    After finishing this chapter, you will be able to:

     

      □ Open and close files in text mode

      □ Edit files

      □ Search text

      □ Undo errors

      □ Merge files

      □ Recover lost files

      □ Find a program or suite for office use



6.1. Text editors

6.1.1. Why should I use an editor?

It is very important to be able to use at least one text mode editor. Knowing
how to use an editor on your system is the first step to independence.

We will need to master an editor by the next chapter as we need it to edit
files that influence our environment. As an advanced user, you may want to
start writing scripts, or books, develop websites or new programs. Mastering
an editor will immensely improve your productivity as well as your
capabilities.



6.1.2. Which editor should I use?

Our focus is on text editors, which can also be used on systems without a
graphical environment and in terminal windows. The additional advantage of
mastering a text editor is in using it on remote machines. Since you don't
need to transfer the entire graphical environment over the network, working
with text editors tremendously improves network speed.

There are, as usual, multiple ways to handle the problem. Let's see what
editors are commonly available:



6.1.2.1. GNU Emacs

Emacs is the extensible, customizable, self-documenting, real-time display
editor, known on many UNIX and other systems. The text being edited is
visible on the screen and is updated automatically as you type your commands.
It is a real-time editor because the display is updated very frequently,
usually after each character or pair of characters you type. This minimizes
the amount of information you must keep in your head as you edit. Emacs is
called advanced because it provides facilities that go beyond simple
insertion and deletion: controlling subprocesses; automatic indentation of
programs; viewing two or more files at once; editing formatted text; and
dealing in terms of characters, words, lines, sentences, paragraphs, and
pages, as well as expressions and comments in several different programming
languages.

Self-documenting means that at any time you can type a special character,
Ctrl+H, to find out what your options are. You can also use it to find out
what any command does, or to find all the commands that pertain to a topic.
Customizable means that you can change the definitions of Emacs commands in
little ways. For example, if you use a programming language in which comments
start with "<**" and end with "**>", you can tell the Emacs comment
manipulation commands to use those strings. Another sort of customization is
rearrangement of the command set. For example, if you prefer the four basic
cursor motion commands (up, down, left and right) on keys in a diamond
pattern on the keyboard, you can rebind the keys that way.

Extensible means that you can go beyond simple customization and write
entirely new commands, programs in the Lisp language that are run by Emacs's
own Lisp interpreter. Emacs is an online extensible system, which means that
it is divided into many functions that call each other, any of which can be
redefined in the middle of an editing session. Almost any part of Emacs can
be replaced without making a separate copy of all of Emacs. Most of the
editing commands of Emacs are written in Lisp already; the few exceptions
could have been written in Lisp but are written in C for efficiency. Although
only a programmer can write an extension, anybody can use it afterward.

When run under the X Window System (started as xemacs) Emacs provides its own
menus and convenient bindings to mouse buttons. But Emacs can provide many of
the benefits of a window system on a text-only terminal. For instance, you
can look at or edit several files at once, move text between files, and edit
files while running shell commands.



6.1.2.2. Vi(m)

Vim stands for "Vi IMproved". It used to be "Vi IMitation", but there are so
many improvements that a name change was appropriate. Vim is a text editor
which includes almost all the commands from the UNIX program vi and a lot of
new ones.

Commands in the vi editor are entered using only the keyboard, which has the
advantage that you can keep your fingers on the keyboard and your eyes on the
screen, rather than moving your arm repeatedly to the mouse. For those who
want it, mouse support and a GUI version with scrollbars and menus can be
activated.

We will refer to vi or vim throughout this book for editing files, while you
are of course free to use the editor of your choice. However, we recommend to
at least get the vi basics in the fingers, because it is the standard text
editor on almost all UNIX systems, while emacs can be an optional package.
There may be small differences between different computers and terminals, but
the main point is that if you can work with vi, you can survive on any UNIX
system.

Apart from the vim command, the vIm packages may also provide gvim, the Gnome
version of vim. Beginning users might find this easier to use, because the
menus offer help when you forgot or don't know how to perform a particular
editing task using the standard vim commands.



6.2. Using the Vim editor

6.2.1. Two modes

The vi editor is a very powerful tool and has a very extensive built-in
manual, which you can activate using the :help command when the program is
started (instead of using man or info, which don't contain nearly as much
information). We will only discuss the very basics here to get you started.

What makes vi confusing to the beginner is that it can operate in two modes:
command mode and insert mode. The editor always starts in command mode.
Commands move you through the text, search, replace, mark blocks and perform
other editing tasks, and some of them switch the editor to insert mode.

This means that each key has not one, but likely two meanings: it can either
represent a command for the editor when in command mode, or a character that
you want in a text when in insert mode.

Note Pronunciation
     It's pronounced "vee-eye".



6.2.2. Basic commands

6.2.2.1. Moving through the text

Moving through the text is usually possible with the arrow keys. If not, try:

  • h to move the cursor to the left

  • l to move it to the right

  • k to move up

  • j to move down

SHIFT-G will put the prompt at the end of the document.



6.2.2.2. Basic operations

These are some popular vi commands:

  • n dd will delete n lines starting from the current cursor position.

  • n dw will delete n words at the right side of the cursor.

  • x will delete the character on which the cursor is positioned

  • :n moves to line n of the file.

  • :w will save (write) the file

  • :q will exit the editor.

  • :q! forces the exit when you want to quit a file containing unsaved
    changes.

  • :wq will save and exit

  • :w newfile will save the text to newfile.

  • :wq! overrides read-only permission (if you have the permission to
    override permissions, for instance when you are using the root account.

  • /astring will search the string in the file and position the cursor on
    the first match below its position.

  • / will perform the same search again, moving the cursor to the next
    match.

  • :1, $s/word/anotherword/g will replace word with anotherword throughout
    the file.

  • yy will copy a block of text.

  • n p will paste it n times.

  • :recover will recover a file after an unexpected interruption.



6.2.2.3. Commands that switch the editor to insert mode

  • a will append: it moves the cursor one position to the right before
    switching to insert mode

  • i will insert

  • o will insert a blank line under the current cursor position and move the
    cursor to that line.

Pressing the Esc key switches back to command mode. If you're not sure what
mode you're in because you use a really old version of vi that doesn't
display an "INSERT" message, type Esc and you'll be sure to return to command
mode. It is possible that the system gives a little alert when you are
already in command mode when hitting Esc, by beeping or giving a visual bell
(a flash on the screen). This is normal behavior.



6.2.3. The easy way

Instead of reading the text, which is quite boring, you can use the vimtutor
to learn you first Vim commands. This is a thirty minute tutorial that
teaches the most basic Vim functionality in eight easy exercises. While you
can't learn everything about vim in just half an hour, the tutor is designed
to describe enough of the commands that you will be able to easily use Vim as
an all-purpose editor.

In UNIX and MS Windows, if Vim has been properly installed, you can start
this program from the shell or command line, entering the vimtutor command.
This will make a copy of the tutor file, so that you can edit it without the
risk of damaging the original. There are a few translated versions of the
tutor. To find out if yours is available, use the two-letter language code.
For French this would be vimtutor fr (if installed on the system).



6.3. Linux in the office

6.3.1. History

Throughout the last decade the office domain has typically been dominated by
MS Office, and, let's face it: the Microsoft Word, Excel and PowerPoint
formats are industry standards that you will have to deal with sooner or
later.

This monopoly situation of Microsoft proved to be a big disadvantage for
getting new users to Linux, so a group of German developers started the
StarOffice project, that was, and is still, aimed at making an MS Office
clone. Their company, StarDivision, was acquired by Sun Microsystems by the
end of the 1990s, just before the 5.2 release. Sun continues development but
restricted access to the sources. Nevertheless, development on the original
set of sources continues in the Open Source community, which had to rename
the project to OpenOffice. OpenOffice is now available for a variety of
platforms, including MS Windows, Linux, MacOS and Solaris. There is a
screenshot in Section 1.3.2.

Almost simultaneously, a couple of other quite famous projects took off. Also
a very common alternative to using MS Office is KOffice, the office suite
that used to be popular among SuSE users. Like the original, this clone
incorporates an MS Word and Excel compatible program, and much more.

Smaller projects deal with particular programs of the MS example suite, such
as Abiword and MS Wordview for compatibility with MS Word documents, and
Gnumeric for viewing and creating Excel compatible spreadsheets.



6.3.2. Suites and programs

Current distributions usually come with all the necessary tools. Since these
provide excellent guidelines and searchable indexes in the Help menus, we
won't discuss them in detail. For references, see you system documentation or
the web sites of the projects, such as

  • [http://www.openoffice.org] http://www.openoffice.org

  • [http://www.koffice.org] http://www.koffice.org

  • [http://freshmeat.net] Freshmeat and [http://sourceforge.org] SourceForge
    for various other projects.



6.3.3. Remarks

6.3.3.1. General use of office documents

Try to limit the use of office documents for the purposes they were meant
for: the office.

An example: it drives most Linux users crazy if you send them a mail that
says in the body something like: "Hello, I want to tell you something, see
attach", and then the attachement proves to be an MS Word compatible document
like: "Hello my friend, how is your new job going and will you have time to
have lunch with me tomorrow?" Also a bad idea is the attachment of your
signature in such a file, for instance. If you want to sign messages or
files, use GPG, the PGP-compatible GNU Privacy Guard or SSL (Secure Socket
Layer) certificates.

These users are not annoyed because they are unable to read these documents,
or because they are worried that these formats typically generate much larger
files, but rather because of the implication that they are using MS Windows,
and possibly because of the extra work of starting some additional programs.



6.3.3.2. System and user configuration files

In the next chapter, we start configuring our environment, and this might
include editing all kinds of files that determine how a program behave.

Don't edit these files with any office component!

The default file format specification would make the program add several
lines of code, defining the format of the file and the fonts used. These
lines won't be interpreted in the correct way by the programs depending on
them, resulting in errors or a crash of the program reading the file. In some
cases, you can save the file as plain text, but you'll run into trouble when
making this a habit.



6.3.3.3. But I want a graphical text editor!

If you really insist, try gedit, kedit, kwrite or xedit; these programs only
do text files, which is what we will be needing. If you plan on doing
anything serious, though, stick to a real text mode editor such as vim or
emacs.

An acceptable alternative is gvim, the Gnome version of vim. You still need
to use vi commands, but if you are stuck, you can look them up in the menus.



6.4. Summary

In this chapter we learned to use an editor. While it depends on your own
individual preference which one you use, it is necessary to at least know how
to use one editor.

The vi editor is available on every UNIX system.

Most Linux distributions include an office suite and a graphical text editor.



6.5. Exercises

This chapter has only one exercise: start the Vim tutor by entering vimtutor
in a terminal session, and get started.

You may alternatively start emacs and type Ctrl+H and then T to invoke the
self-paced Emacs tutorial.

Practice is the only way!



Chapter 7. Home sweet /home

    This chapter is about configuring your environment. Now that we know how
    to use an editor, we can change all kinds of files to make ourselves feel
    better at home. After completing this chapter, you will know more about:

     

      □ Organizing your environment

      □ Common shell setup files

      □ Shell configuration

      □ Configuring the prompt

      □ Configuring the graphical environment

      □ Sound and video applications

      □ Display and window managers

      □ How the X client-server system works

      □ Language and font settings

      □ Installing new software

      □ Updating existing packages



7.1. General good housekeeping

7.1.1. Introduction

As we mentioned before, it is easy enough to make a mess of the system. We
can't put enough stress on the importance of keeping the place tidy. When you
learn this from the start, it will become a good habit that will save you
time when programming on a Linux or UNIX system or when confronted with
system management tasks. Here are some ways of making life easier on
yourself:

  • Make a bin directory for your program files and scripts.

  • Organize non-executable files in appropriate directories, and make as
    many directories as you like. Examples include separate directories for
    images, documents, projects, downloaded files, spreadsheets, personal
    files, and so on.

  • Make directories private with the chmod 700 dirname command.

  • Give your files sensible names, such as Complaint to the prime minister
    050302 rather than letter1.



7.1.2. Make space

On some systems, the quota system may force you to clean up from time to
time, or the physical limits of your hard disk may force you to make more
space without running any monitoring programs. This section discusses a
number of ways, besides using the rm command, to reclaim disk space.

Run the quota -v command to see how much space is left.



7.1.2.1. Emptying files

Sometimes the content of a file doesn't interest you, but you need the file
name as a marker (for instance, you just need the timestamp of a file, a
reminder that the file was there or should be there some time in the future).
Redirecting the output of a null command is how this is done in the Bourne
and Bash shells:

andy:~> cat wishlist > placeholder

andy:~> ls -la placeholder
-rw-rw-r--    1 andy   andy       200 Jun 12 13:34 placeholder

andy:~>  > placeholder

andy:~> ls -la placeholder

-rw-rw-r--    1 andy   andy         0 Jun 12 13:35 placeholder

The process of reducing an existing file to a file with the same name that is
0 bytes large is called truncating.

For creating a new empty file, the same effect is obtained with the touch
command. On an existing file, touch will only update the timestamp. See the
Info pages on touch for more details.

To "almost" empty a file, use the tail command. Suppose user andy's wishlist
becomes rather long because he always adds stuff at the end but never deletes
the things he actually gets. Now he only wants to keep the last five items:

andy:~> tail -5 wishlist > newlist

andy:~> cat newlist > wishlist

andy:~> rm newlist



7.1.2.2. More about log files

Some Linux programs insist on writing all sorts of output in a log file.
Usually there are options to only log errors, or to log a minimal amount of
information, for example setting the debugging level of the program. But even
then, you might not care about the log file. Here are some ways to get rid of
them or at least set some limits to their size:

  • Try removing the log file when the program is not running, if you are
    sure that you won't need it again. Some programs may even see, when
    restarted, that there is no log file and will therefore not log.

  • If you remove the log file and the program recreates it, read the
    documentation for this particular program in search for command options
    that avoid making log files.

  • Try making smaller log files by logging only the information that is
    relevant to you, or by logging only significant information.

  • Try replacing the log file with a symbolic link to /dev/null; if you're
    lucky the program won't complain. Don't do this with the log files of
    programs that run at system boot or programs that run from cron (see
    Chapter 4). These programs might replace the symbolic link with a small
    file that starts growing again.



7.1.2.3. Mail

Regularly clean out your mailbox, make sub-folders and automatic redirects
using procmail (see the Info pages) or the filters of your favorite mail
reading application. If you have a trash folder, clean it out on a regular
basis.

To redirect mail, use the .forward file in your home directory. The Linux
mail service looks for this file whenever it has to deliver local mail. The
content of the file defines what the mail system should do with your mail. It
can contain a single line holding a fully qualified E-mail address. In that
case the system will send all your mail to this address. For instance, when
renting space for a website, you might want to forward the mail destined for
the webmaster to your own account in order not to waste disk space. The
webmaster's .forward may look like this:

webmaster@www ~/> cat .forward
mike@pandora.be

Using mail forwarding is also useful to prevent yourself from having to check
several different mailboxes. You can make every address point to a central
and easily accessible account.

You can ask your system administrator to define a forward for you in the
local mail aliases file, like when an account is being closed but E-mail
remains active for a while.



7.1.2.4. Save space with a link

When several users need access to the same file or program, when the original
file name is too long or too difficult to remember, use a symbolic link
instead of a separate copy for each user or purpose.

Multiple symbolic links may have different names, e.g. a link may be called
monfichier in one user's directory, and mylink in another's. Multiple links
(different names) to the same file may also occur in the same directory. This
is often done in the /lib directory: when issuing the command

ls -l /lib

you will see that this directory is plenty of links pointing to the same
files. These are created so that programs searching for one name would not
get stuck, so they are pointed to the correct/current name of the libraries
they need.



7.1.2.5. Limit file sizes

The shell contains a built-in command to limit file sizes, ulimit, which can
also be used to display limitations on system resources:

cindy:~> ulimit -a
core file size (blocks)     0
data seg size (kbytes)      unlimited
file size (blocks)          unlimited
max locked memory (kbytes)  unlimited
max memory size (kbytes)    unlimited
open files                  1024
pipe size (512 bytes)       8
stack size (kbytes)         8192
cpu time (seconds)          unlimited
max user processes          512
virtual memory (kbytes)     unlimited

Cindy is not a developer and doesn't care about core dumps, which contain
debugging information on a program. If you do want core dumps, you can set
their size using the ulimit command. Read the Info pages on bash for a
detailed explanation.

Note Core file?
     A core file or core dump is sometimes generated when things go wrong
     with a program during its execution. The core file contains a copy of
     the system's memory, as it was at the time that the error occured.



7.1.2.6. Compressed files

Compressed files are useful because they take less space on your hard disk.
Another advantage is that it takes less bandwidth to send a compressed file
over your network. A lot of files, such as the man pages, are stored in a
compressed format on your system. Yet unpacking these to get a little bit of
information and then having to compress them again is rather time-consuming.
You don't want to unpack a man page, for instance, read about an option to a
command and then compress the man page again. Most people will probably
forget to clean up after they found the information they needed.

So we have tools that work on compressed files, by uncompressing them only in
memory. The actual compressed file stays on your disk as it is. Most systems
support zgrep, zcat, bzless and other members of the z-family to prevent
unnecessary decompressing/compressing actions. See your system's binary
directory and the Info pages.

See Chapter 9 for more on the actual compressing of files and examples on
making archives.



7.2. Your text environment

7.2.1. Environment variables

7.2.1.1. General

We already mentioned a couple of environment variables, such as PATH and
HOME. Until now, we only saw examples in which they serve a certain purpose
to the shell. But there are many other Linux utilities that need information
about you in order to do a good job.

What other information do programs need apart from paths and home
directories?

A lot of programs want to know about the kind of terminal you are using; this
information is stored in the TERM variable. In text mode, this will be the
linux terminal emulation, in graphical mode you are likely to use xterm. Lots
of programs want to know what your favorite editor is, in case they have to
start an editor in a subprocess. The shell you are using is stored in the
SHELL variable, the operating system type in OS and so on. A list of all
variables currently defined for your session can be viewed entering the
printenv command.

The environment variables are managed by the shell. As opposed to regular
shell variables, environment variables are inherited by any program you
start, including another shell. New processes are assigned a copy of these
variables, which they can read, modify and pass on in turn to their own child
processes.

There is nothing special about variable names, except that the common ones
are in upper case characters by convention. You may come up with any name you
want, although there are standard variables that are important enough to be
the same on every Linux system, such as PATH and HOME.



7.2.1.2. Exporting variables

An individual variable's content is usually displayed using the echo command,
as in these examples:

debby:~> echo $PATH
/usr/bin:/usr/sbin:/bin:/sbin:/usr/X11R6/bin:/usr/local/bin

debby:~> echo $MANPATH
/usr/man:/usr/share/man/:/usr/local/man:/usr/X11R6/man

If you want to change the content of a variable in a way that is useful to
other programs, you have to export the new value from your environment into
the environment that runs these programs. A common example is exporting the
PATH variable. You may declare it as follows, in order to be able to play
with the flight simulator software that is in /opt/FlightGear/bin:

debby:~> PATH=$PATH:/opt/FlightGear/bin

This instructs the shell to not only search programs in the current path,
$PATH, but also in the additional directory /opt/FlightGear/bin.

However, as long as the new value of the PATH variable is not known to the
environment, things will still not work:

debby:~> runfgfs
bash: runfgfs: command not found

Exporting variables is done using the shell built-in command export:

debby:~> export PATH

debby:~> runfgfs
--flight simulator starts--

In Bash, we normally do this in one elegant step:

export VARIABLE=value

The same technique is used for the MANPATH variable, that tells the man
command where to look for compressed man pages. If new software is added to
the system in new or unusual directories, the documentation for it will
probably also be in an unusual directory. If you want to read the man pages
for the new software, extend the MANPATH variable:

debby:~> export MANPATH=$MANPATH:/opt/FlightGear/man

debby:~> echo $MANPATH
/usr/man:/usr/share/man:/usr/local/man:/usr/X11R6/man:/opt/FlightGear/man

You can avoid retyping this command in every window you open by adding it to
one of your shell setup files, see Section 7.2.2.



7.2.1.3. Reserved variables

The following table gives an overview of the most common predefined
variables:

Table 7-1. Common environment variables


Variable name  Stored information                                         

DISPLAY        used by the X Window system to identify the display server 

DOMAIN         domain name                                                

EDITOR         stores your favorite line editor                           

HISTSIZE       size of the shell history file in number of lines          

HOME           path to your home directory                                

HOSTNAME       local host name                                            

INPUTRC        location of definition file for input devices such as      
               keyboard                                                   

LANG           preferred language                                         

LD_LIBRARY_PATHpaths to search for libraries                              

LOGNAME        login name                                                 

MAIL           location of your incoming mail folder                      

MANPATH        paths to search for man pages                              

OS             string describing the operating system                     

OSTYPE         more information about version etc.                        

PAGER          used by programs like man which need to know what to do in 
               case output is more than one terminal window.              

PATH           search paths for commands                                  

PS1            primary prompt                                             

PS2            secondary prompt                                           

PWD            present working directory                                  

SHELL          current shell                                              

TERM           terminal type                                              

UID            user ID                                                    

USER(NAME)     user name                                                  

VISUAL         your favorite full-screen editor                           

XENVIRONMENT   location of your personal settings for X behavior          

XFILESEARCHPATHpaths to search for graphical libraries                    


A lot of variables are not only predefined but also preset, using
configuration files. We discuss these in the next section.



7.2.2. Shell setup files

When entering the ls -al command to get a long listing of all files,
including the ones starting with a dot, in your home directory, you will see
one or more files starting with a . and ending in rc. For the case of bash,
this is .bashrc. This is the counterpart of the system-wide configuration
file /etc/bashrc.

When logging into an interactive login shell, login will do the
authentication, set the environment and start your shell. In the case of bash
, the next step is reading the general profile from /etc, if that file
exists. bash then looks for ~/.bash_profile, ~/.bash_login and ~/.profile, in
that order, and reads and executes commands from the first one that exists
and is readable. If none exists, /etc/bashrc is applied.

When a login shell exits, bash reads and executes commands from the file ~
/.bash_logout, if it exists.

This procedure is explained in detail in the login and bash man pages.



7.2.3. A typical set of setup files

7.2.3.1. /etc/profile example

Let's look at some of these config files. First /etc/profile is read, in
which important variables such as PATH, USER and HOSTNAME are set:

debby:~> cat /etc/profile
# /etc/profile

# System wide environment and startup programs, for login setup
# Functions and aliases go in /etc/bashrc


# Path manipulation
if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/sbin" ; then
    PATH=/sbin:$PATH
fi

if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/usr/sbin" ; then
    PATH=/usr/sbin:$PATH
fi

if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/usr/local/sbin"
    then
    PATH=/usr/local/sbin:$PATH
fi

if ! echo $PATH | /bin/grep -q "/usr/X11R6/bin" ; then
    PATH="$PATH:/usr/X11R6/bin"
fi

These lines check the path to set: if root opens a shell (user ID 0), it is
checked that /sbin, /usr/sbin and /usr/local/sbin are in the path. If not,
they are added. It is checked for everyone that /usr/X11R6/bin is in the
path.

# No core files by default
ulimit -S -c 0 > /dev/null 2>&1

All trash goes to /dev/null if the user doesn't change this setting.

USER=`id -un`
LOGNAME=$USER
MAIL="/var/spool/mail/$USER"

HOSTNAME=`/bin/hostname`
HISTSIZE=1000

Here general variables are assigned their proper values.

if [ -z "$INPUTRC" -a ! -f "$HOME/.inputrc" ]; then
    INPUTRC=/etc/inputrc
fi

If the variable INPUTRC is not set, and there is no .inputrc in the user's
home directory, then the default input control file is loaded.

export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE INPUTRC

All variables are exported, so that they are available to other programs
requesting information about your environment.



7.2.3.2. The profile.d directory

for i in /etc/profile.d/*.sh ; do
    if [ -r $i ]; then
        . $i
    fi
done
unset i

All readable shell scripts from the /etc/profile.d directory are read and
executed. These do things like enabling color-ls, aliasing vi to vim, setting
locales etc. The temporary variable i is unset to prevent it from disturbing
shell behavior later on.



7.2.3.3. .bash_profile example

Then bash looks for a .bash_profile in the user's home directory:

debby:~> cat .bash_profile 
#################################################################
#                                                               #
#   .bash_profile file                                          #
#                                                               #
#   Executed from the bash shell when you log in.               #
#                                                               #
#################################################################

source ~/.bashrc
source ~/.bash_login

This very straight forward file instructs your shell to first read ~/.bashrc
and then ~/.bash_login. You will encounter the source built-in shell command
regularly when working in a shell environment: it is used to apply
configuration changes to the current environment.



7.2.3.4. .bash_login example

The ~/.bash_login file defines default file protection by setting the umask
value, see Section 3.4.2.2. The ~/.bashrc file is used to define a bunch of
user-specific aliases and functions and personal environment variables. It
first reads /etc/bashrc, which describes the default prompt (PS1) and the
default umask value. After that, you can add your own settings. If no ~
/.bashrc exists, /etc/bashrc is read by default.



7.2.3.5. /etc/bashrc example

Your /etc/bashrc file might look like this:

debby:~> cat /etc/bashrc
# /etc/bashrc

# System wide functions and aliases
# Environment stuff goes in /etc/profile

# by default, we want this to get set.
# Even for non-interactive, non-login shells.
if [ `id -gn` = `id -un` -a `id -u` -gt 99 ]; then
        umask 002
else
        umask 022
fi

These lines set the umask value. Then, depending on the type of shell, the
prompt is set:

# are we an interactive shell?
if [ "$PS1" ]; then
  if [ -x /usr/bin/tput ]; then
    if [ "x`tput kbs`" != "x" ]; then
# We can't do this with "dumb" terminal
      stty erase `tput kbs`
    elif [ -x /usr/bin/wc ]; then
      if [ "`tput kbs|wc -c `" -gt 0 ]; then
# We can't do this with "dumb" terminal
        stty erase `tput kbs`
      fi
    fi
  fi
  case $TERM in
        xterm*)
        if [ -e /etc/sysconfig/bash-prompt-xterm ]; then
                PROMPT_COMMAND=/etc/sysconfig/bash-prompt-xterm
        else
   PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME%%.*}:\
${PWD/$HOME/~}\007"'
        fi
    ;;
        *)
   [ -e /etc/sysconfig/bash-prompt-default ] && PROMPT_COMMAND=\
/etc/sysconfig/bash-prompt-default
            ;;
    esac
    [ "$PS1" = "\\s-\\v\\\$ " ] && PS1="[\u@\h \W]\\$ "

    if [ "x$SHLVL" != "x1" ]; then # We're not a login shell
        for i in /etc/profile.d/*.sh; do
            if [ -x $i ]; then
                . $i
            fi
        done
    fi
fi



7.2.3.6. .bash_logout example

Upon logout, the commands in ~/.bash_logout are executed, which can for
instance clear the terminal, so that you have a clean window upon logging out
of a remote session, or upon leaving the system console:

debby:~> cat .bash_logout
# ~/.bash_logout

clear

Let's take a closer look at how these scripts work in the next section. Keep
info bash close at hand.



7.2.4. The Bash prompt

7.2.4.1. Introduction

 The Bash prompt can do much more than displaying such simple information as
your user name, the name of your machine and some indication about the
present working directory. We can add other information such as the current
date and time, number of connected users etc.

Before we begin, however, we will save our current prompt in another
environment variable:

[jerry@nowhere jerry]$ MYPROMPT=$PS1

[jerry@nowhere jerry]$ echo $MYPROMPT
[\u@\h \W]\$

[jerry@nowhere jerry]$

When we change the prompt now, for example by issuing the command PS1="->",
we can always get our original prompt back with the command PS1=$MYPROMPT.
You will, of course, also get it back when you reconnect, as long as you just
fiddle with the prompt on the command line and avoid putting it in a shell
configuration file.



7.2.4.2. Some examples

In order to understand these prompts and the escape sequences used, we refer
to the Bash Info or man pages.

  • export PS1="[\t \j] "

    Displays time of day and number of running jobs

  • export PS1="[\d][\u@\h \w] : "

    Displays date, user name, host name and current working directory. Note
    that \W displays only base names of the present working directory.

  • export PS1="{\!} "

    Displays history number for each command.

  • export PS1="\[\033[1;35m\]\u@\h\[\033[0m\] "

    Displays user@host in pink.

  • export PS1="\[\033[1;35m\]\u\[\033[0m\] \[\033[1;34m\]\w\[\033[0m\] "

    Sets the user name in pink and the present working directory in blue.

  • export PS1="\[\033[1;44m\]$USER is in \w\[\033[0m\] "

    Prompt for people who have difficulties seeing the difference between the
    prompt and what they type.

  • export PS1="\[\033[4;34m\]\u@\h \w \[\033[0m\]"

    Underlined prompt.

  • export PS1="\[\033[7;34m\]\u@\h \w \[\033[0m\] "

    White characters on a blue background.

  • export PS1="\[\033[3;35m\]\u@\h \w \[\033[0m\]\a"

    Pink prompt in a lighter font that alerts you when your commands have
    finished.

  • export PS1=...

Variables are exported so the subsequently executed commands will also know
about the environment. The prompt configuration line that you want is best
put in your shell configuration file, ~/.bashrc.

If you want, prompts can execute shell scripts and behave different under
different conditions. You can even have the prompt play a tune every time you
issue a command, although this gets boring pretty soon. More information can
be found in the [http://www.tldp.org/HOWTO/Bash-Prompt-HOWTO/] Bash-Prompt
HOWTO.



7.2.5. Shell scripts

7.2.5.1. What are scripts?

A shell script is, as we saw in the shell configuration examples, a text file
containing shell commands. When such a file is used as the first non-option
argument when invoking Bash, and neither the -c nor -s option is supplied,
Bash reads and executes commands from the file, then exits. This mode of
operation creates a non-interactive shell. When Bash runs a shell script, it
sets the special parameter 0 to the name of the file, rather than the name of
the shell, and the positional parameters (everything following the name of
the script) are set to the remaining arguments, if any are given. If no
additional arguments are supplied, the positional parameters are unset.

A shell script may be made executable by using the chmod command to turn on
the execute bit. When Bash finds such a file while searching the PATH for a
command, it spawns a sub-shell to execute it. In other words, executing

filename ARGUMENTS

is equivalent to executing

bash filename ARGUMENTS

if "filename" is an executable shell script. This sub-shell reinitializes
itself, so that the effect is as if a new shell had been invoked to interpret
the script, with the exception that the locations of commands remembered by
the parent (see hash in the Info pages) are retained by the child.

Most versions of UNIX make this a part of the operating system's command
execution mechanism. If the first line of a script begins with the two
characters "#!", the remainder of the line specifies an interpreter for the
program. Thus, you can specify bash, awk, perl or some other interpreter or
shell and write the rest of the script file in that language.

The arguments to the interpreter consist of a single optional argument
following the interpreter name on the first line of the script file, followed
by the name of the script file, followed by the rest of the arguments. Bash
will perform this action on operating systems that do not handle it
themselves.

Bash scripts often begin with

#! /bin/bash

(assuming that Bash has been installed in /bin), since this ensures that Bash
will be used to interpret the script, even if it is executed under another
shell.



7.2.5.2. Some simple examples

A very simple script consisting of only one command, that says hello to the
user executing it:

[jerry@nowhere ~] cat hello.sh
#!/bin/bash
echo "Hello $USER"

The script actually consists of only one command, echo, which uses the value
of ($) the USER environment variable to print a string customized to the user
issuing the command.

Another one-liner, used for displaying connected users:

#!/bin/bash
who | cut -d " " -f 1 | sort -u

Here is a script consisting of some more lines, that I use to make backup
copies of all files in a directory. The script first makes a list of all the
files in the current directory and puts it in the variable LIST. Then it sets
the name of the copy for each file, and then it copies the file. For each
file, a message is printed:

tille:~> cat bin/makebackupfiles.sh
#!/bin/bash
# make copies of all files in a directory
LIST=`ls`
for i in $LIST; do
        ORIG=$i
        DEST=$i.old
        cp $ORIG $DEST
        echo "copied $i"
done

Just entering a line like mv * *.old won't work, as you will notice when
trying this on a set of test files. An echo command was added in order to
display some activity. echo's are generally useful when a script won't work:
insert one after each doubted step and you will find the error in no time.

The /etc/rc.d/init.d directory contains loads of examples. Let's look at this
script that controls the fictive ICanSeeYou server:

#!/bin/sh
# description: ICanSeeYou allows you to see networked people

# process name: ICanSeeYou
# pidfile: /var/run/ICanSeeYou/ICanSeeYou.pid
# config: /etc/ICanSeeYou.cfg

# Source function library.
. /etc/rc.d/init.d/functions

# See how (with which arguments) we were called.
case "$1" in
        start)
                echo -n "Starting ICanSeeYou: "
                daemon ICanSeeYou
                echo
                touch /var/lock/subsys/ICanSeeYou
                ;;
        stop)
                echo -n "Shutting down ICanSeeYou: "
                killproc ICanSeeYou
                echo
                rm -f /var/lock/subsys/ICanSeeYou
                rm -f /var/run/ICanSeeYou/ICanSeeYou.pid
                ;;
        status)
                status ICanSeeYou
                ;;
        restart)
                $0 stop
                $0 start
                ;;
        *)
                echo "Usage: $0 {start|stop|restart|status}"
                exit 1
esac

exit 0

First, with the . command (dot) a set of shell functions, used by almost all
shell scripts in /etc/rc.d/init.d, is loaded. Then a case command is issued,
which defines 4 different ways the script can execute. An example might be
ICanSeeYou start. The decision of which case to apply is made by reading the
(first) argument to the script, with the expression $1.

When no compliant input is given, the default case, marked with an asterisk,
is applied, upon which the script gives an error message. The case list is
ended with the esac statement. In the start case the server program is
started as a daemon, and a process ID and lock are assigned. In the stop
case, the server process is traced down and stopped, and the lock and the PID
are removed. Options, such as the daemon option, and functions like killproc,
are defined in the /etc/rc.d/init.d/functions file. This setup is specific to
the distribution used in this example. The initscripts on your system might
use other functions, defined in other files, or none at all.

Upon success, the script returns an exit code of zero to its parent.

 This script is a fine example of using functions, which make the script
easier to read and the work done faster. Note that they use sh instead of
bash, to make them useful on a wider range of systems. On a Linux system,
calling bash as sh results in the shell running in POSIX-compliant mode.

The bash man pages contain more information about combining commands, for-
and while-loops and regular expressions, as well as examples. A
comprehensible Bash course for system administrators and power users, with
exercises, from the same author as this Introduction to Linux guide, is at
[http://tille.garrels.be/training/bash/] http://tille.garrels.be/training/
bash/. Detailed description of Bash features and applications is in the
reference guide [http://tldp.org/LDP/abs/html/index.html] Advanced Bash
Scripting.



7.3. The graphical environment

7.3.1. Introduction

The average user may not care too much about his login settings, but Linux
offers a wide variety of flashy window and desktop managers for use under X,
the graphical environment. The use and configuration of window managers and
desktops is straightforward and may even resemble the standard MS Windows,
Apple or UNIX CDE environment, although many Linux users prefer flashier
desktops and fancier window managers. We won't discuss the user specific
configuration here. Just experiment and read the documentation using the
built-in Help functions these managers provide and you will get along fine.

We will, however, take a closer look at the underlying system.



7.3.2. The X Window System

The X Window System is a network-transparent window system which runs on a
wide range of computing and graphics machines. X Window System servers run on
computers with bitmap displays. The X server distributes user input to and
accepts output requests from several client programs through a variety of
different interprocess communication channels. Although the most common case
is for the client programs to be running on the same machine as the server,
clients can be run transparently from other machines (including machines with
different architectures and operating systems) as well. We will learn how to
do this in Chapter 10 on networking and remote applications.

X supports overlapping hierarchical sub-windows and text and graphics
operations, on both monochrome and color displays. The number of X client
programs that use the X server is quite large. Some of the programs provided
in the core X Consortium distribution include:

  • xterm: a terminal emulator

  • twm: a minimalistic window manager

  • xdm: a display manager

  • xconsole: a console redirect program

  • bitmap: a bitmap editor

  • xauth, xhost and iceauth: access control programs

  • xset, xmodmap and many others: user preference setting programs

  • xclock: a clock

  • xlsfonts and others: a font displayer, utilities for listing information
    about fonts, windows and displays

  • xfs: a font server

  • ...

We refer again to the man pages of these commands for detailed information.
More explanations on available functions can be found in the Xlib - C
language X Interface manual that comes with your X distribution, the X Window
System Protocol specification, and the various manuals and documentation of X
toolkits. The /usr/share/doc directory contains references to these documents
and many others.

Many other utilities, window managers, games, toolkits and gadgets are
included as user-contributed software in the X Consortium distribution, or
are available using anonymous FTP on the Internet. Good places to start are
[http://www.x.org] http://www.x.org and [http://www.xfree.org] http://
www.xfree.org.

Furthermore, all your graphical applications, such as your browser, your
E-mail program, your image viewing programs, sound playing tools and so on,
are all clients to your X server. Note that in normal operation, that is in
graphical mode, X clients and the X server on Linux run on the same machine.



7.3.2.1. Display names

From the user's perspective, every X server has a display name in the form
of:

hostname:displaynumber.screennumber

This information is used by the application to determine how it should
connect to the X server and which screen it should use by default (on
displays with multiple monitors):

  • hostname: The host name specifies the name of the client machine to which
    the display is physically connected. If the host name is not given, the
    most efficient way of communicating to a server on the same machine will
    be used.

  • displaynumber: The phrase "display" is usually used to refer to a
    collection of monitors that share a common key board and pointer (mouse,
    tablet, etc.). Most workstations tend to only have one keyboard, and
    therefore, only one display. Larger, multi-user systems, however,
    frequently have several displays so that more than one person can be
    doing graphics work at once. To avoid confusion, each display on a
    machine is assigned a display number (beginning at 0) when the X server
    for that display is started. The display number must always be given in a
    display name.

  • screen number: Some displays share a single keyboard and pointer among
    two or more monitors. Since each monitor has its own set of windows, each
    screen is assigned a screen number (beginning at 0) when the X server for
    that display is started. If the screen number is not given, screen 0 will
    be used.

On POSIX systems, the default display name is stored in your DISPLAY
environment variable. This variable is set automatically by the xterm
terminal emulator. However, when you log into another machine on a network,
you might need to set DISPLAY by hand to point to your display, see Section
10.4.3.2.

More information can be found in the X man pages.



7.3.2.2. Window and desktop managers

The layout of windows on the screen is controlled by special programs called
window managers. Although many window managers will honor geometry
specifications as given, others may choose to ignore them (requiring the user
to explicitly draw the window's region on the screen with the pointer, for
example).

Since window managers are regular (albeit complex) client programs, a variety
of different user interfaces can be built. The X Consortium distribution
comes with a window manager named twm, but most users prefer something more
fancy when system resources permit. Sawfish and Enlightenment are popular
examples which allow each user to have a desktop according to mood and style.

A desktop manager makes use of one window manager or another for arranging
your graphical desktop in a convenient way, with menubars, drop-down menus,
informative messages, a clock, a program manager, a file manager and so on.
Among the most popular desktop managers are Gnome and KDE, which both run on
almost any Linux distribution and many other UNIX systems.

Tip KDE applications in Gnome/Gnome applications in KDE
    You don't need to start your desktop in KDE in order to be able to run
    KDE applications. If you have the KDE libraries installed (the kdelibs
    package), you can run these applications from the Gnome menus or start
    them from a Gnome terminal.
 
    Running Gnome applications in a KDE environment is a bit more tricky,
    because there is no single set of base-libraries in Gnome. However, the
    dependencies and thus extra packages you might have to install will
    become clear when running or installing such an application.



7.3.3. X server configuration

The X distribution that used to come with Linux, XFree86, uses the
configuration file XF86Config for its initial setup. This file configures
your video card and is searched for in a number of locations, although it is
usually in /etc/X11.

If you see that the file /etc/X11/XF86Config is present on your system, a
full description can be found in the Info or man pages about XF86Config.

Because of licensing issues with XFree86, newer systems usually come with the
X.Org distribution of the X server and tools. The main configuration file
here is xorg.conf, usually also in /etc/X11. The file consists of a number of
sections that may occur in any order. The sections contain information about
your monitor, your video adaptor, the screen configuration, your keyboard
etcetera. As a user, you needn't worry too much about what is in this file,
since everything is normally determined at the time the system is installed.

Should you need to change graphical server settings, however, you can run the
configuration tools or edit the configuration files that set up the
infrastructure to use the XFree86 server. See the man pages for more
information; your distribution might have its own tools. Since
misconfiguration may result in unreadable garbage in graphical mode, you may
want to make a backup copy of the configuration file before attempting to
change it, just to be on the safe side.



7.4. Region specific settings

7.4.1. Keyboard setup

Setting the keyboard layout is done using the loadkeys command for text
consoles. Use your local X configuration tool or edit the Keyboard section in
XF86Config manually to configure the layout for graphical mode. The
XkbdLayout is the one you want to set:

    XkbLayout       "us"

This is the default. Change it to your local settings by replacing the quoted
value with any of the names listed in the subdirectories of your keymaps
directory. If you can't find the keymaps, try displaying their location on
your system issuing the command

locate keymaps

It is possible to combine layout settings, like in this example:

Xkblayout      "us,ru"

Make a backup of the /etc/X11/XF86Config file before editing it! You will
need to use the root account to do this.

Log out and reconnect in order to reload X settings.

The Gnome Keyboard Applet enables real-time switching between layouts; no
special pemissions are needed for using this program. KDE has a similar tool
for switching between keyboard layouts.



7.4.2. Fonts

Use the setfont tool to load fonts in text mode. Most systems come with a
standard inputrc file which enables combining of characters, such as the
French " " (meta characters). The system admin should then add the line

export INPUTRC="/etc/inputrc"

to the /etc/bashrc file.



7.4.3. Date and time zone

Setting time information is usually done at installation time. After that, it
can be kept up to date using an NTP (Network Time Protocol) client. Most
Linux systems run ntpd by default:

debby:~> ps -ef | grep ntpd
ntp      24678     1  0  2002 ?        00:00:33 ntpd -U ntp

You can run ntpdate manually to set the time, on condition that you can reach
a time server. The ntpd daemon should not be running when you adjust the time
using ntpdate. Use a time server as argument to the command:

root@box:~# ntpdate 10.2.5.200
26 Oct 14:35:42 ntpdate[20364]: adjust time server 10.2.5.200 offset
 -0.008049 sec

See your system manual and the documentation that comes with the NTP package.
Most desktop managers include tools to set the system time, providing that
you have access to the system administrator's account.

For setting the time zone correct, you can use tzconfig or timezone commands.
Timezone information is usually set during the installation of your machine.
Many systems have distribution-specific tools to configure it, see your
system documentation.



7.4.4. Language

If you'd rather get your messages from the system in Dutch or French, you may
want to set the LANG and LANGUAGE environment variables, thus enabling locale
support for the desired language and eventually the fonts related to
character conventions in that language.

With most graphical login systems, such as gdm or kdm, you have the
possibility to configure these language settings before logging in.

Note that on most systems, the default tends to be en_US.UTF-8 these days.
This is not a problem, because systems where this is the default, will also
come with all the programs supporting this encoding. Thus, vi can edit all
the files on your system, cat won't behave strange and so on.

Trouble starts when you connect to an older system not supporting this font
encoding, or when you open a UTF-8 encoded file on a system supporting only
1-byte character fonts. The recode utility might come in handy to convert
files from one character set to another. Read the man pages for an overview
of features and usage. Another solution might be to temporarily work with
another encoding definition, by setting the LANG environment variable:

debby:~> acroread /var/tmp/51434s.pdf
Warning: charset "UTF-8" not supported, using "ISO8859-1".
Aborted

debby:~> set | grep UTF
LANG=en_US.UTF-8

debby:~> export LANG=en_US

debby:~> acroread /var/tmp/51434s.pdf
<--new window opens-->

Refer to the [http://www.mozilla.org/] Mozilla web site for guidance on how
to get Firefox in your language. The [http://www.openoffice.org/]
OpenOffice.org web site has information on localization of your
OpenOffice.org suite.



7.4.5. Country-specific Information

The [http://www.tldp.org/HOWTO/HOWTO-INDEX/howtos.html] list of HOWTOs
contains references to Bangla, Belarusian, Chinese, Esperanto, Finnish,
Francophone, Hebrew, Hellenic, Latvian, Polish, Portugese, Serbian, Slovak,
Slovenian, Spanish, Thai and Turkish localization instructions.



7.5. Installing new software

7.5.1. General

Most people are surprised to see that they have a running, usable computer
after installing Linux; most distributions contain ample support for video
and network cards, monitors and other external devices, so there is usually
no need to install extra drivers. Also common tools such as office suites,
web browsers, E-mail and other network client programs are included in the
main distributions. Even so, an initial installation might not meet your
requirements.

If you just can't find what you need, maybe it is not installed on your
system. It may also be that you have the required software, but it does not
do what it is supposed to do. Remember that Linux moves fast, and software
improves on a daily basis. Don't waste your time troubleshooting problems
that might already be resolved.

You can update your system or add packages to it at any time you want. Most
software comes in packages. Extra software may be found on your installation
CDs or on the Internet. The website of your Linux distribution is a good
place to start looking for additional software and contains instructions
about how to install it on your type of Linux, see Appendix A. Always read
the documentation that comes with new software, and any installation
guidelines the package might contain. All software comes with a README file,
which you are very strongly advised to read.



7.5.2. Package formats

7.5.2.1. RPM packages

7.5.2.1.1. What is RPM?

RPM, the RedHat Package Manager, is a powerful package manager that you can
use to install, update and remove packages. It allows you to search for
packages and keeps track of the files that come with each package. A system
is built-in so that you can verify the authenticity of packages downloaded
from the Internet. Advanced users can build their own packages with RPM.

An RPM package consists of an archive of files and meta-data used to install
and erase the archive files. The meta-data includes helper scripts, file
attributes, and descriptive information about the package. Packages come in
two varieties: binary packages, used to encapsulate software to be installed,
and source packages, containing the source code and recipe necessary to
produce binary packages.

Many other distributions support RPM packages, among the popular ones RedHat
Enterprise Linux, Mandriva (former Mandrake), Fedora Core and SuSE Linux.
Apart from the advice for your distribution, you will want to read man rpm.



7.5.2.1.2. RPM examples

Most packages are simply installed with the upgrade option, -U, whether the
package is already installed or not. The RPM package contains a complete
version of the program, which overwrites existing versions or installs as a
new package. The typical usage is as follows:

rpm -Uvh /path/to/rpm-package(s)

The -v option generates more verbose output, and -h makes rpm print a
progress bar:

[root@jupiter tmp]# rpm -Uvh totem-0.99.5-1.fr.i386.rpm
Preparing...                ########################################### [100%]
   1:totem                  ########################################### [100%]
[root@jupiter tmp]#

New kernel packages, however, are installed with the install option -i, which
does not overwrite existing version(s) of the package. That way, you will
still be able to boot your system with the old kernel if the new one does not
work.

You can also use rpm to check whether a package is installed on your system:

[david@jupiter ~] rpm -qa | grep vim
vim-minimal-6.1-29
vim-X11-6.1-29
vim-enhanced-6.1-29
vim-common-6.1-29

Or you can find out which package contains a certain file or executable:

[david@jupiter ~] rpm -qf /etc/profile
setup-2.5.25-1

[david@jupiter ~] which cat
cat is /bin/cat

[david@jupiter ~] rpm -qf /bin/cat
coreutils-4.5.3-19

Note that you need not have access to administrative privileges in order to
use rpm to query the RPM database. You only need to be root when adding,
modifying or deleting packages.

Below is one last example, demonstrating how to uninstall a package using rpm
:

[root@jupiter root]# rpm -e totem
[root@jupiter root]#

Note that uninstalling is not that verbose by default, it is normal that you
don't see much happening. When in doubt, use rpm -qa again to verify that the
package has been removed.

RPM can do much more than the couple of basic functions we discussed in this
introduction; the [http://www.tldp.org/HOWTO/RPM-HOWTO/index.html] RPM HOWTO
contains further references.



7.5.2.2. DEB (.deb) packages

7.5.2.2.1. What are Debian packages?

This package format is the default on Debian GNU/Linux, where dselect, and,
nowadays more common, aptitude, is the standard tool for managing the
packages. It is used to select packages that you want to install or upgrade,
but it will also run during the installation of a Debian system and help you
to define the access method to use, to list available packages and to
configure packages.

The [http://debian.org] Debian web site contains all information you need,
including a "dselect Documentation for Beginners".

According to the latest news, the Debian package format is becoming more and
more popular. At the time of this writing, 5 of the top-10 distributions use
it. Also apt-get (see Section 7.5.3.2 is becoming extremely popular, also on
non-DEB systems.



7.5.2.2.2. Examples with DEB tools

Checking whether a package is installed is done using the dpkg command. For
instance, if you want to know which version of the Gallery software is
installed on your machine:

nghtwsh@gorefest:~$ dpkg -l *gallery*
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-files/Unpacked/Failed-config/Half-installed
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad)
||/ Name           Version        Description
+++-==============-==============-============================================
ii  gallery        1.5-1sarge2    a web-based photo album written in php

The "ii" prefix means the package is installed. Should you see "un" as a
prefix, that means that the package is known in the list that your computer
keeps, but that it is not installed.

Searching which package a file belongs to is done using the -S to dpkg:

nghtwsh@gorefest:~$ dpkg -S /bin/cat
coreutils: /bin/cat

More information can be found in the Info pages for dpkg.



7.5.2.3. Source packages

The largest part of Linux programs is Free/Open Source, so source packages
are available for these programs. Source files are needed for compiling your
own program version. Sources for a program can be downloaded from its web
site, often as a compressed tarball (program-version.tar.gz or similar). For
RPM-based distributions, the source is often provided in the
program-version.src.rpm. Debian, and most distributions based on it, provide
themselves the adapted source which can be obtained using apt-get source.

Specific requirements, dependencies and installation instructions are
provided in the README file. You will probably need a C compiler, gcc. This
GNU C compiler is included in most Linux systems and is ported to many other
platforms.



7.5.3. Automating package management and updates

7.5.3.1. General remarks

The first thing you do after installing a new system is applying updates;
this applies to all operating systems and Linux is not different.

The updates for most Linux systems can usually be found on a nearby site
mirroring your distribution. Lists of sites offering this service can be
found at your distribution's web site, see Appendix A.

Updates should be applied regularly, daily if possible - but every couple of
weeks would be a reasonable start. You really should try to have the most
recent version of your distribution, since Linux changes constantly. As we
said before, new features, improvements and bug fixes are supplied at a
steady rhythm, and sometimes important security problems are addressed.

The good news is that most Linux distributions provide tools so that you
don't have to upgrade tens of packages daily by hand. The following sections
give an overview of package manager managers. There is much more to this
subject, even regular updates of source packages is manageable automatically;
we only list the most commonly known systems. Always refer to the
documentation for your specific distribution for advised procedures.



7.5.3.2. APT

The Advanced Package Tool is a management system for software packages. The
command line tool for handling packages is apt-get, which comes with an
excellent man page describing how to install and update packages and how to
upgrade singular packages or your entire distribution. APT has its roots in
the Debian GNU/Linux distribution, where it is the default manager for the
Debian packages. APT has been ported to work with RPM packages as well. The
main advantage of APT is that it is free and flexible to use. It will allow
you to set up systems similar to the distribution specific (and in some cases
commercial) ones listed in the next sections.

Generally, when first using apt-get, you will need to get an index of the
available packages. This is done using the command

apt-get update

After that, you can use apt-get to upgrade your system:

apt-get upgrade

Do this often, it's an easy way to keep your system up-to-date and thus safe.

Apart from this general usage, apt-get is also very fast for installing
individual packages. This is how it works:

[david@jupiter ~] su - -c "apt-get install xsnow"
Password:
Reading Package Lists... Done
Building Dependency Tree... Done
The following NEW packages will be installed:
  xsnow
0 packages upgraded, 1 newly installed, 0 removed and 3 not upgraded.
Need to get 33.6kB of archives.
After unpacking 104kB of additional disk space will be used.
Get:1 http://ayo.freshrpms.net redhat/9/i386/os xsnow 1.42-10 [33.6kB]
Fetched 33.6kB in 0s (106kB/s)
Executing RPM (-Uvh)...
Preparing...                ########################################### [100%]
   1:xsnow                  ########################################### [100%]

Note the -c option to the su command, which indicates to the root shell to
only execute this command, and then return to the user's environment. This
way, you cannot forget to quit the root account.

If there are any dependencies on other packages, apt-get will download and
install these supporting packages.

More information can be found in the [http://www.debian.org/doc/user-manuals#
apt-howto] APT HOWTO.



7.5.3.3. Systems using RPM packages

Update Agent, which originally only supported RedHat RPM packages, is now
ported to a wider set of software, including non-RedHat repositories. This
tool provides a complete system for updating the RPM packages on a RedHat or
Fedora Core system. On the command line, type up2date to update your system.
On the desktop, by default a small icon is activated, telleng you whether or
not there are updates available for your system.

Yellowdog's Updater Modified (yum) is another tool that recently became more
popular. It is an interactive but automated update program for installing,
updating or removing RPM packages on a system. It is the tool of choice on
Fedora systems.

On SuSE Linux, everything is done with YaST, Yet another Setup Tool, which
supports a wide variety of system administration tasks, among which updating
RPM packages. Starting from SuSE Linux 7.1 you can also upgrade using a web
interface and YOU, Yast Online Update.

Mandrake Linux and Mandriva provide so-called URPMI tools, a set of wrapper
programs that make installing new software easier for the user. These tools
combine with RPMDrake and MandrakeUpdate to provide everything needed for
smooth install and uninstall of software packages. MandrakeOnline offers an
extended range of services and can automatically notify administrators when
updates are available for your particular Mandrake system. See man urpmi,
among others, for more info.

Also the KDE and Gnome desktop suites have their own (graphical) versions of
package managers.



7.5.4. Upgrading your kernel

Most Linux installations are fine if you periodically upgrade your
distribution. The upgrade procedure will install a new kernel when needed and
make all necessary changes to your system. You should only compile or install
a new kernel manually if you need kernel features that are not supported by
the default kernel included in your Linux distribution.

Whether compiling your own optimized kernel or using a pre-compiled kernel
package, install it in co-existence with the old kernel until you are sure
that everything works according to plan.

Then create a dual boot system that will allow you to choose which kernel to
boot by updating your boot loader configuration file grub.conf. This is a
simple example:

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making config changes.
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, e.g.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/hde8
#          initrd /initrd-version.img
#boot=/dev/hde
default=0
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
title Red Hat Linux new (2.4.9-31)
        root (hd0,0)
        kernel /vmlinuz-2.4.9-31 ro root=/dev/hde8
        initrd /initrd-2.4.9-31.img
title old-kernel
        root (hd0,0)
        kernel /vmlinuz-2.4.9-21 ro root=/dev/hde8
        initrd /initrd-2.4.9-21.img

After the new kernel has proven to work, you may remove the lines for the old
one from the GRUB config file, although it is best to wait a couple of days
just to be sure.



7.5.5. Installing extra packages from the installation CDs

7.5.5.1. Mounting a CD

This is basically done in the same way as installing packages manually,
except that you have to append the file system of the CD to your machine's
file system to make it accessible. On most systems, this will be done
automatically upon insertion of a CD in the drive because the automount
daemon is started up at boot time. If your CD is not made available
automatically, issue the mount command in a terminal window. Depending on
your actual system configuration, a line similar to this one will usually do
the trick:

mount /dev/cdrom /mnt/cdrom

On some systems, only root can mount removable media; this depends on the
configuration.

For automation purposes, the CD drive usually has an entry in /etc/fstab,
which lists the file systems and their mount points, that make up your file
system tree. This is such a line:

[david@jupiter ~] grep cdrom /etc/fstab
/dev/cdrom      /mnt/cdrom      iso9660         noauto,owner,ro 0 0

This indicates that the system will understand the command mount /mnt/cdrom.
The noauto option means that on this system, CDs are not mounted at boot
time.

You may even try to right click on the CD icon on your desktop to mount the
CD if your file manager doesn't do it for you. You can check whether it
worked issuing the mount command with no arguments:

[david@jupiter ~] mount | grep cdrom
/dev/cdrom on /mnt/cdrom type iso9660 (ro,nosuid,nodev)



7.5.5.2. Using the CD

After mounting the CD, you can change directories, usually to the mount point
/mnt/cdrom, where you can access the content of the CD-ROM. Use the same
commands for dealing with files and directories as you would use for files on
the hard disk.



7.5.5.3. Ejecting the CD

In order to get the CD out of the drive after you've finished using it, the
file system on the CD should be unused. Even being in one of the
subdirectories of the mount point, /mnt/cdrom in our example, will be
considered as "using the file system", so you should get out of there. Do
this for instance by typing cd with no arguments, which will put you back in
your home directory. After that, you can either use the command

umount /mnt/cdrom

or

eject cdrom

Warning Blocked drives
        NEVER force the drive. The trick with the paperclip is a bad idea,
        because this will eventually expunge the CD, but your system will
        think the CD is still there because normal procedures were not
        followed. Chances are likely that you will have to reboot to get the
        system back in a consistent state.

        If you keep getting "device busy" messages, check first that all
        shell sessions have left the CD file system and that no graphical
        applications are using it anymore. When in doubt, use the lsof tool
        to trace down the process(es) still using the CD resource.



7.6. Summary

When everything has its place, that means already half the work is done.

While keeping order is important, it is equally important to feel at home in
your environment, whether text or graphical. The text environment is
controlled through the shell setup files. The graphical environment is
primarily dependent on the X server configuration, on which a number of other
applications are built, such as window and desktop managers and graphical
applications, each with their own config files. You should read the system
and program specific documentation to find out about how to configure them.

Regional settings such as keyboard setup, installing appropriate fonts and
language support are best done at installation time.

Software is managed either automatically or manually using a package system.

The following commands were introduced in this chapter:

Table 7-2. New commands in chapter 7: Making yourself at home


Command  Meaning                                                      

aptitude Manage packages Debian-style.                                

automountautomatically include newly inserted file systems.           

dpkg     Debian package manager.                                      

dselect  Manage packages Debian-style.                                

loadkeys Load keyboard configuration.                                 

lsof     Identify processes.                                          

mount    Include a new file system into the existing file system tree.

ntpdate  Set the system time and date using a time server.            

quota    Display information about allowed disk space usage.          

recode   Convert files to another character set.                      

rpm      Manage RPM packages.                                         

setfont  Choose a font.                                               

timezone Set the timezone.                                            

tzconfig Set the timezone.                                            

ulimit   Set or display resource limits.                              

up2date  Manage RPM packages.                                         

urpmi    Manage RPM packages.                                         

yum      Manage RPM packages.                                         



7.7. Exercises

7.7.1. Shell environment

  • Print out your environment settings. Which variable may be used to store
    the CPU type of your machine?

  • Make a script that can say something on the lines of "hello, world." Give
    it appropriate permissions so it can be run. Test your script.

  • Create a directory in your home directory and move the script to the new
    directory. Permanently add this new directory to your search path. Test
    that the script can be executed without giving a path to its actual
    location.

  • Create subdirectories in your home directory to store various files, for
    instance a directory music to keep audio files, a directory documents for
    your notes, and so on. And use them!

  • Create a personalized prompt.

  • Display limits on resource usage. Can you change them?

  • Try to read compressed man pages without decompressing them first.

  • Make an alias lll which actually executes ls -la.

  • Why does the command tail testfile > testfile not work?

  • Mount a data CD, such as your Linux installation CD, and have a look
    around. Don't forget to unmount when you don't need it anymore.

  • The script from Section 7.2.5.2 is not perfect. It generates errors for
    files that are directories. Adapt the script so that it only selects
    plain files for copying. Use find to make the selection. Do not forget to
    make the script executable before you try to run it.



7.7.2. Graphical environment

  • Try all the mouse buttons in different regions (terminal, background,
    task bar).

  • Explore the menus.

  • Customize your terminal window.

  • Use the mouse buttons to copy and paste text from one terminal to
    another.

  • Find out how to configure your window manager; try different workspaces
    (virtual screens).

  • Add an applet, such as a load monitor, to the task bar.

  • Apply a different theme.

  • Enable the so-called sloppy focus - this is when a window is activated by
    just moving the mouse over it, so that you do not need to click the
    window in order to be able to use it.

  • Switch to a different window manager.

  • Log out and select a different session type, like KDE if you were using
    Gnome before. Repeat the previous steps.



Chapter 8. Printers and printing

    In this chapter we will learn more about printers and printing files.
    After reading this part, you will be able to:

     

      □ Format documents

      □ Preview documents before sending them to the printer

      □ Choose a good printer that works with your Linux system

      □ Print files and check on printer status

      □ Troubleshoot printing problems

      □ Find necessary documentation to install a printer



8.1. Printing files

8.1.1. Command line printing

8.1.1.1. Getting the file to the printer

Printing from within an application is very easy, selecting the Print option
from the menu.

From the command line, use the lp or lpr command.

lp file(s)

lpr file(s)

These commands can read from a pipe, so you can print the output of commands
using

command | lp

There are many options available to tune the page layout, the number of
copies, the printer that you want to print to if you have more than one
available, paper size, one-side or double-sided printing if your printer
supports this feature, margins and so on. Read the man pages for a complete
overview.



8.1.1.2. Status of your print jobs

Once the file is accepted in the print queue, an identification number for
the print job is assigned:

davy:~> lp /etc/profile
request id is blob-253 (1 file(s))

To view (query) the print queue, use the lpq or lpstat command. When entered
without arguments, it displays the contents of the default print queue.

davy:~> lpq
blob is ready and printing
Rank    Owner   Job     File(s)         Total Size
active  davy    253     profile         1024 bytes
davy:~> lpstat
blob-253        davy    1024    Tue 25 Jul 2006 10:20_01 AM CEST



8.1.1.3. Status of your printer

Which is the default printer on a system that has access to multiple
printers?

lpstat -d

davy:~> lpstat -d
system default destination: blob

What is the status of my printer(s)?

lpstat -p

davy:~> lpstat -p
printer blob now printing blob-253. enabled since Jan 01 18:01



8.1.1.4. Removing jobs from the print queue

If you don't like what you see from the status commands, use lprm or cancel
to delete jobs.

davy:~> lprm 253

In the graphical environment, you may see a popup window telling you that the
job has been canceled.

In larger environments, lpc may be used to control multiple printers. See the
Info or man pages on each command.

There are many GUI print tools used as a front-end to lp, and most graphical
applications have a print function that uses lp. See the built-in Help
functions and program specific documentation for more.

Note Why are there two commands for every task related to printing?
     Printing on UNIX and alikes has a long history. There used to be two
     rather different approaches: the BSD-style printing and the
     SystemV-style printing. For compatibility, Linux with CUPS supports the
     commands from both styles. Also note that lp does not behave exactly
     like lpr, lpq has somewhat different options than lpstat and lprm is
     almost, but not quite, like cancel. Which one you use is not important,
     just pick the commands that you are comfortable with, or that you may
     know from previous experiences with UNIX-like systems.



8.1.2. Formatting

8.1.2.1. Tools and languages

If we want to get something sensible out of the printer, files should be
formatted first. Apart from an abundance of formatting software, Linux comes
with the basic UNIX formatting tools and languages.

Modern Linux systems support direct printing, without any formatting by the
user, of a range of file types: text, PDF, PostScript and several image
formats like PNG, JPEG, BMP and GIF.

For those file formats that do need formatting, Linux comes with a lot of
formatting tools, such as the pdf2ps, fax2ps and a2ps commands, that convert
other formats to PostScript. These commands can create files that can then be
used on other systems that don't have all the conversion tools installed.

Apart from these command line tools there are a lot of graphical word
processing programs. Several complete office suites are available, many are
free. These do the formatting automatically upon submission of a print job.
Just to name a few: OpenOffice.org, KOffice, AbiWord, WordPerfect, etc.

The following are common languages in a printing context:

  • groff: GNU version of the UNIX roff command. It is a front-end to the
    groff document formatting system. Normally it runs the troff command and
    a post-processor appropriate for the selected device. It allows
    generation of PostScript files.

  • TeX and the macro package LaTeX: one of the most widely used markup
    languages on UNIX systems. Usually invoked as tex, it formats files and
    outputs a corresponding device-independent representation of the typeset
    document.

    Technical works are still frequently written in LaTeX because of its
    support for mathematic formulas, although efforts are being made at
    [http://www.w3.org] W3C (the World Wide Web Consortium) to include this
    feature in other applications.

  • SGML and XML: Free parsers are available for UNIX and Linux. XML is the
    next generation SGML, it forms the basis for DocBook XML, a document
    system (this book is written in XML, for instance).

Tip Printing documentation
    The man pages contain pre-formatted troff data which has to be formatted
    before it can roll out of your printer. Printing is done using the -t
    option to the man command:

    man -t command > man-command.ps

    Then print the PostScript file. If a default print destination is
    configured for your system/account, you can just issue the command man -t
    command to send the formatted page to the printer directly.



8.1.2.2. Previewing formatted files

Anything that you can send to the printer can normally be sent to the screen
as well. Depending on the file format, you can use one of these commands:

  • PostScript files: with the gv (GhostView) command.

  • TeX dvi files: with xdvi, or with KDE's kdvi.

  • PDF files: xpdf, kpdf, gpdf or Adobe's viewer, acroread, which is also
    available for free but is not free software. Adobe's reader supports PDF
    1.6, the others only support PDF versions up to 1.5. The version of a PDF
    file can be determined using the file command.

  • From within applications, such as Firefox or OpenOffice, you can usually
    select Print Preview from one of the menus.



8.2. The server side

8.2.1. General

Until a couple of years ago, the choice for Linux users was simple: everyone
ran the same old LPD from BSD's Net-2 code. Then LPRng became more popular,
but nowadays most modern Linux distributions use [http://www.cups.org] CUPS,
the Common UNIX Printing System. CUPS is an implementation of the Internet
Printing Protocol (IPP), an HTTP-like RFC standard replacement protocol for
the venerable (and clunky) LPD protocol. CUPS is distributed under the GNU
Public License. CUPS is also the default print system on MacOS X.



8.2.2. Graphical printer configuration

Most distributions come with a GUI for configuring networked and local
(parallel port or USB) printers. They let you choose the printer type from a
list and allow easy testing. You don't have to bother about syntax and
location of configuration files. Check your system documentation before you
attempt installing your printer.

CUPS can also be configured using a web interface that runs on port 631 on
your computer. To check if this feature is enabled, try browsing to
[localhost:631/help] localhost:631/help or [localhost:631/] localhost:631/.



8.2.3. Buying a printer for Linux

As more and more printer vendors make drivers for CUPS available, CUPS will
allow easy connection with almost any printer that you can plug into a
serial, parallel, or USB port, plus any printer on the network. CUPS will
ensure a uniform presentation to you and your applications of all different
types of printers.

Printers that only come with a Win9x driver could be problematic if they have
no other support. Check with [http://linuxprinting.org/] http://
linuxprinting.org/ when in doubt.

In the past, your best choice would have been a printer with native
PostScript support in the firmware, since nearly all UNIX or Linux software
producing printable output, produces it in PostScript, the publishing
industry's printer control language of choice. PostScript printers are
usually a bit more expensive, but it is a device-independent, open
programming language and you're always 100% sure that they will work. These
days, however, the importance of this rule of thumb is dwindling.



8.3. Print problems

In this section, we will discuss what you can do as a user when something
goes wrong. We won't discuss any problems that have to do with the
daemon-part of the printing service, as that is a task for system
administrators.



8.3.1. Wrong file

If you print the wrong file, the job may be canceled using the command lprm 
jobID, where jobID is in the form printername-printjobnumber (get it from
information displayed by lpq or lpstat). This will work when other jobs are
waiting to be printed in this printer's queue. However, you have to be really
quick if you are the only one using this printer, since jobs are usually
spooled and send to the printer in only seconds. Once they arrive on the
printer, it is too late to remove jobs using Linux tools.

What you can try in those cases, or in cases where the wrong print driver is
configured and only rubbish comes out of the printer, is power off the
printer. However, that might not be the best course of action, as you might
cause paper jams and other irregularities.



8.3.2. My print hasn't come out

Use the lpq command and see if you can spot your job:

elly:~> lpq
Printer: lp@blob
 Queue: 2 printable jobs
 Server: pid 29998 active
 Unspooler: pid 29999 active
 Status: waiting for subserver to exit at 09:43:20.699
 Rank   Owner/ID             Class Job Files          Size Time
1      elly@blob+997           A   997 (STDIN)         129 09:42:54
2      elly@blob+22            A    22 /etc/profile    917 09:43:20

Lots of printers have web interfaces these days, which can display status
information by typing the printer's IP address in your web browser:

Figure 8-1. Printer Status through web interface

[brother-we]

Note CUPS web interface versus printer web interface
     Note that this is not the CUPS web interface and only works for printers
     supporting this feature. Check the documentation of your printer.

If your job ID is not there and not on the printer, contact your system
administrator. If your job ID is listed in the output, check that the printer
is currently printing. If so, just wait, your job will get done in due time.

If the printer is not printing, check that it has paper, check the physical
connections to both electricity and data network. If that's okay, the printer
may need restarting. Ask your system admin for advice.

In the case of a network printer, try printing from another host. If the
printer is reachable from your own host (see Chapter 10 for the ping
utility), you may try to put the formatted file on it, like file.ps in case
of a PostScript printer, using an FTP client. If that works, your print
system is misconfigured. If it doesn't work, maybe the printer doesn't
understand the format you are feeding it.

The [http://www.linuxprinting.org] GNU/Linux Printing site contains more tips
and tricks.



8.4. Summary

The Linux print service comes with a set of printing tools based on the
standard UNIX LPD tools, whether it be the SystemV or BSD implementation.
Below is a list of print-related commands.

Table 8-1. New commands in chapter 8: Printing


Command       Meaning                   

lpr or lp     Print file                

lpq or lpstat Query print queue         

lprm or cancelRemove print job          

acroread      PDF viewer                

groff         Formatting tool           

gv            PostScript viewer         

printconf     Configure printers        

xdvi          DVI viewer                

xpdf          PDF viewer                

*2ps          Convert file to PostScript



8.5. Exercises

Configuring and testing printers involves being in the possession of one, and
having access to the root account. If so, you may try:

  • Installing the printer using the GUI on your system.

  • Printing a test page using the GUI.

  • Printing a test page using the lp command.

  • Print from within an application, for example Mozilla or OpenOffice, by
    choosing File->Print from the menu.

  • Disconnect the printer from the network or the local machine/
    print-server. What happens when you try to print something?

The following exercises can be done without printer or root access.

  • Try to make PostScript files from different source files, (e.g. HTML,
    PDF, man pages). Test the results with the gv viewer.

  • Check that the print daemon is running.

  • Print the files anyway. What happens?

  • Make a PostScript file using Mozilla. Test it with gv.

  • Convert it to PDF format. Test with xpdf.

  • How would you go about printing a GIF file from the command line?

  • Use a2ps to print the /etc/profile file to an output file. Test again
    with gv. What happens if you don't specify an output file?



Chapter 9. Fundamental Backup Techniques

    Accidents will happen sooner or later. In this chapter, we'll discuss how
    to get data to a safe place using other hosts, floppy disks, CD-ROMs and
    tapes. We will also discuss the most popular compressing and archiving
    commands.

    Upon completion of this chapter, you will know how to:

     

      □ Make, query and unpack file archives

      □ Handle floppy disks and make a boot disk for your system

      □ Write CD-ROMs

      □ Make incremental backups

      □ Create Java archives

      □ Find documentation to use other backup devices and programs

      □ Encrypt your data



9.1. Introduction

Although Linux is one of the safest operating systems in existence, and even
if it is designed to keep on going, data can get lost. Data loss is most
often the consequence of user errors, but occasionally a system fault, such
as a power or disk failure, is the cause, so it's always a good idea to keep
an extra copy of sensitive and/or important data.



9.1.1. Preparing your data

9.1.1.1. Archiving with tar

In most cases, we will first collect all the data to back up in a single
archive file, which we will compress later on. The process of archiving
involves concatenating all listed files and taking out unnecessary blanks. In
Linux, this is commonly done with the tar command. tar was originally
designed to archive data on tapes, but it can also make archives, known as
tarballs.

tar has many options, the most important ones are cited below:

  • -v: verbose

  • -t: test, shows content of a tarball

  • -x: extract archive

  • -c: create archive

  • -f archivedevice: use archivedevice as source/destination for the
    tarball, the device defaults to the first tape device (usually /dev/st0
    or something similar)

  • -j: filter through bzip2, see Section 9.1.1.2

It is common to leave out the dash-prefix with tar options, as you can see
from the examples below.

Note Use GNU tar for compatibility
     The archives made with a proprietary tar version on one system, may be
     incompatible with tar on another proprietary system. This may cause much
     headaches, such as if the archive needs to be recovered on a system that
     doesn't exist anymore. Use the GNU tar version on all systems to prevent
     your system admin from bursting into tears. Linux always uses GNU tar.
     When working on other UNIX machines, enter tar --help to find out which
     version you are using. Contact your system admin if you don't see the
     word GNU somewhere.

In the example below, an archive is created and unpacked.

gaby:~> ls images/
me+tux.jpg  nimf.jpg

gaby:~> tar cvf images-in-a-dir.tar images/
images/
images/nimf.jpg
images/me+tux.jpg

gaby:~> cd images

gaby:~/images> tar cvf images-without-a-dir.tar *.jpg
me+tux.jpg
nimf.jpg

gaby:~/images> cd

gaby:~> ls */*.tar
images/images-without-a-dir.tar

gaby:~> ls *.tar
images-in-a-dir.tar

gaby:~> tar xvf images-in-a-dir.tar 
images/
images/nimf.jpg
images/me+tux.jpg

gaby:~> tar tvf images/images-without-dir.tar 
-rw-r--r-- gaby/gaby  42888 1999-06-30 20:52:25 me+tux.jpg
-rw-r--r-- gaby/gaby   7578 2000-01-26 12:58:46 nimf.jpg

gaby:~> tar xvf images/images-without-a-dir.tar 
me+tux.jpg
nimf.jpg

gaby:~> ls *.jpg
me+tux.jpg  nimf.jpg

This example also illustrates the difference between a tarred directory and a
bunch of tarred files. It is advisable to only compress directories, so files
don't get spread all over when unpacking the tarball (which may be on another
system, where you may not know which files were already there and which are
the ones from the archive).

When a tape drive is connected to your machine and configured by your system
administrator, the file names ending in .tar are replaced with the tape
device name, for example:

tar cvf /dev/tape mail/

The directory mail and all the files it contains are compressed into a file
that is written on the tape immediately. A content listing is displayed
because we used the verbose option.



9.1.1.2. Incremental backups with tar

The tar tool supports the creation of incremental backups, using the -N
option. With this option, you can specify a date, and tar will check
modification time of all specified files against this date. If files are
changed more recent than date, they will be included in the backup. The
example below uses the timestamp on a previous archive as the date value.
First, the initial archive is created and the timestamp on the initial backup
file is shown. Then a new file is created, upon which we take a new backup,
containing only this new file:

jimmy:~> tar cvpf /var/tmp/javaproggies.tar java/*.java
java/btw.java
java/error.java
java/hello.java
java/income2.java
java/income.java
java/inputdevice.java
java/input.java
java/master.java
java/method1.java
java/mood.java
java/moodywaitress.java
java/test3.java
java/TestOne.java
java/TestTwo.java
java/Vehicle.java

jimmy:~> ls -l /var/tmp/javaproggies.tar
-rw-rw-r-- 1 jimmy   jimmy   10240 Jan 21 11:58 /var/tmp/javaproggies.tar

jimmy:~> touch java/newprog.java

jimmy:~> tar -N /var/tmp/javaproggies.tar \
-cvp /var/tmp/incremental1-javaproggies.tar java/*.java 2> /dev/null
java/newprog.java

jimmy:~> cd /var/tmp/

jimmy:~> tar xvf incremental1-javaproggies.tar
java/newprog.java

Standard errors are redirected to /dev/null. If you don't do this, tar will
print a message for each unchanged file, telling you it won't be dumped.

This way of working has the disadvantage that it looks at timestamps on
files. Say that you download an archive into the directory containing your
backups, and the archive contains files that have been created two years ago.
When checking the timestamps of those files against the timestamp on the
initial archive, the new files will actually seem old to tar, and will not be
included in an incremental backup made using the -N option.

A better choice would be the -g option, which will create a list of files to
backup. When making incremental backups, files are checked against this list.
This is how it works:

jimmy:~> tar cvpf work-20030121.tar -g snapshot-20030121 work/
work/
work/file1
work/file2
work/file3

jimmy:~> file snapshot-20030121
snapshot-20030121: ASCII text

The next day, user jimmy works on file3 a bit more, and creates file4. At the
end of the day, he makes a new backup:

jimmy:~> tar cvpf work-20030122.tar -g snapshot-20030121 work/
work/
work/file3
work/file4

These are some very simple examples, but you could also use this kind of
command in a cronjob (see Section 4.4.4), which specifies for instance a
snapshot file for the weekly backup and one for the daily backup. Snapshot
files should be replaced when taking full backups, in that case.

More information can be found in the tar documentation.

Tip The real stuff
    As you could probably notice, tar is OK when we are talking about a
    simple directory, a set of files that belongs together. There are tools
    that are easier to manage, however, when you want to archive entire
    partitions or disks or larger projects. We just explain about tar here
    because it is a very popular tool for distributing archives. It will
    happen quite often that you need to install a software that comes in a
    so-called "compressed tarball". See Section 9.3 for an easier way to
    perform regular backups.



9.1.1.3. Compressing and unpacking with gzip or bzip2

 Data, including tarballs, can be compressed using zip tools. The gzip
command will add the suffix .gz to the file name and remove the original
file.

jimmy:~> ls -la | grep tar
-rw-rw-r-- 1 jimmy  jimmy    61440 Jun  6 14:08 images-without-dir.tar

jimmy:~> gzip images-without-dir.tar

jimmy:~> ls -la images-without-dir.tar.gz 
-rw-rw-r-- 1 jimmy  jimmy    50562 Jun  6 14:08 images-without-dir.tar.gz

Uncompress gzipped files with the -d option.

bzip2 works in a similar way, but uses an improved compression algorithm,
thus creating smaller files. See the bzip2 info pages for more.

Linux software packages are often distributed in a gzipped tarball. The
sensible thing to do after unpacking that kind of archives is find the README
and read it. It will generally contain guidelines to installing the package.

The GNU tar command is aware of gzipped files. Use the command

tar zxvf file.tar.gz

for unzipping and untarring .tar.gz or .tgz files. Use

tar jxvf file.tar.bz2

for unpacking tar archives that were compressed with bzip2.



9.1.1.4. Java archives

The GNU project provides us with the jar tool for creating Java archives. It
is a Java application that combines multiple files into a single JAR archive
file. While also being a general purpose archiving and compression tool,
based on ZIP and the ZLIB compression format, jar was mainly designed to
facilitate the packing of Java code, applets and/or applications in a single
file. When combined in a single archive, the components of a Java
application, can be downloaded much faster.

Unlike tar, jar compresses by default, independent from other tools - because
it is basically the Java version of zip. In addition, it allows individual
entries in an archive to be signed by the author, so that origins can be
authenticated.

The syntax is almost identical as for the tar command, we refer to info jar
for specific differences.

Note tar, jar and symbolic links
     One noteworthy feature not really mentioned in the standard
     documentation is that jar will follow symbolic links. Data to which
     these links are pointing will be included in the archive. The default in
     tar is to only backup the symbolic link, but this behavior can be
     changed using the -h to tar.



9.1.1.5. Transporting your data

Saving copies of your data on another host is a simple but accurate way of
making backups. See Chapter 10 for more information on scp, ftp and more.

In the next section we'll discuss local backup devices.



9.2. Moving your data to a backup device

9.2.1. Making a copy on a floppy disk

9.2.1.1. Formatting the floppy

On most Linux systems, users have access to the floppy disk device. The name
of the device may vary depending on the size and number of floppy drives,
contact your system admin if you are unsure. On some systems, there will
likely be a link /dev/floppy pointing to the right device, probably /dev/fd0
(the auto-detecting floppy device) or /dev/fd0H1440 (set for 1,44MB
floppies).

 fdformat is the low-level floppy disk formatting tool. It has the device
name of the floppy disk as an option. fdformat will display an error when the
floppy is write-protected.

emma:~> fdformat /dev/fd0H1440
Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB.
Formatting ... done
Verifying ... done
emma:~>

The mformat command (from the mtools package) is used to create
DOS-compatible floppies which can then be accessed using the mcopy, mdir and
other m-commands.

Graphical tools are also available.

Figure 9-1. Floppy formatter

[formatflop]

After the floppy is formatted, it can be mounted into the file system and
accessed as a normal, be it small, directory, usually via the /mnt/floppy
entry.

Should you need it, install the mkbootdisk utility, which makes a floppy from
which the current system can boot.



9.2.1.2. Using the dd command to dump data

The dd command can be used to put data on a disk, or get it off again,
depending on the given input and output devices. An example:

gaby:~> dd if=images-without-dir.tar.gz of=/dev/fd0H1440
98+1 records in
98+1 records out

gaby~> dd if=/dev/fd0H1440 of=/var/tmp/images.tar.gz
2880+0 records in
2880+0 records out

gaby:~> ls /var/tmp/images*
/var/tmp/images.tar.gz


Note that the dumping is done on an unmounted device. Floppies created using
this method will not be mountable in the file system, but it is of course the
way to go for creating boot or rescue disks. For more information on the
possibilities of dd, read the man pages.

This tool is part of the GNU coreutils package.

Note Dumping disks
     The dd command can also be used to make a raw dump of an entire hard
     disk.



9.2.2. Making a copy with a CD-writer

On some systems users are allowed to use the CD-writer device. Your data will
need to be formatted first. Use the mkisofs command to do this in the
directory containing the files you want to backup. Check with df that enough
disk space is available, because a new file about the same size as the entire
current directory will be created:

[rose@blob recordables] df -h .
Filesystem            Size  Used Avail Use% Mounted on
/dev/hde5              19G   15G  3.2G  82% /home

[rose@blob recordables] du -h -s .
325M    .

[rose@blob recordables] mkisofs -J -r -o cd.iso .
<--snap-->
making a lot of conversions
<--/snap-->
98.95% done, estimate finish Fri Apr  5 13:54:25 2002
Total translation table size: 0
Total rockridge attributes bytes: 35971
Total directory bytes: 94208
Path table size(bytes): 452
Max brk space used 37e84
166768 extents written (325 Mb)

The -J and -r options are used to make the CD-ROM mountable on different
systems, see the man pages for more. After that, the CD can be created using
the cdrecord tool with appropriate options:

[rose@blob recordables] cdrecord -dev 0,0,0 -speed=8 cd.iso
Cdrecord 1.10 (i686-pc-linux-gnu) (C) 1995-2001 Joerg Schilling
scsidev: '0,0,0'
scsibus: 0 target: 0 lun: 0
Linux sg driver version: 3.1.20
Using libscg version 'schily-0.5'
Device type    : Removable CD-ROM
Version        : 0
Response Format: 1
Vendor_info    : 'HP      '
Identification : 'CD-Writer+ 8100 '
Revision       : '1.0g'
Device seems to be: Generic mmc CD-RW.
Using generic SCSI-3/mmc CD-R driver (mmc_cdr).
Driver flags   : SWABAUDIO
Starting to write CD/DVD at speed 4 in write mode for single session.
Last chance to quit, starting real write in 0 seconds.
Operation starts.

Depending on your CD-writer, you now have the time to smoke^H^H^H^H^H eat a
healthy piece of fruit and/or get a cup of coffee. Upon finishing the job,
you will get a confirmation message:

Track 01: Total bytes read/written: 341540864/341540864
          (166768 sectors).

There are some graphical tools available to make it easier on you. One of the
popular ones is xcdroast, which is freely available from [http://
www.xcdroast.org/] the X-CD-Roast web site and is included on most systems
and in the GNU directory. Both the KDE and Gnome desktop managers have
facilities to make your own CDs.



9.2.3. Backups on/from jazz drives, USB devices and other removables

These devices are usually mounted into the file system. After the mount
procedure, they are accessed as normal directories, so you can use the
standard commands for manipulating files.

In the example below, images are copied from a USB camera to the hard disk:

robin:~> mount /mnt/camera

robin:~> mount | grep camera
/dev/sda1 on /mnt/camera type vfat (rw,nosuid,nodev)

If the camera is the only USB storage device that you ever connect to your
system, this is safe. But keep in mind that USB devices are assigned entries
in /dev as they are connected to the system. Thus, if you first connect a USB
stick to your system, it will be on the /dev/sda entry, and if you connect
your camera after that, it will be assigned to /dev/sdb - provided that you
do not have any SCSI disks, which are also on /dev/sd*. On newer systems,
since kernel 2.6, a hotplug system called HAL (Hardware Abstraction Layer)
ensures that users don't have to deal with this burden. If you want to check
where your device is, type dmesg after inserting it.

You can now copy the files:

robin:~> cp -R /mnt/camera/* images/

robin:~> umount /mnt/camera

Likewise, a jazz drive may be mounted on /mnt/jazz.

Appropriate lines should be added in /etc/modules.conf and /etc/fstab to make
this work. Refer to specific hardware HOWTOs for more information. On systems
with a 2.6.x kernel or higher, you may also want to check the man pages for
modprobe and modprobe.conf.



9.2.4. Backing up data using a tape device

This is done using tar (see above). The mt tool is used for controlling the
magnetic tape device, like /dev/st0. Entire books have been written about
tape backup, therefore, refer to our reading-list in Appendix B for more
information. Keep in mind that databases might need other backup procedures
because of their architecture.

The appropriate backup commands are usually put in one of the cron
directories in order to have them executed on a regular basis. In larger
environments, the freely available [http://www.amanda.org] Amanda backup
suite or a commercial solution may be implemented to back up multiple
machines. Working with tapes, however, is a system administration task beyond
the scope of this document.



9.2.5. Tools from your distribution

Most Linux distributions offer their own tools for making your life easy. A
short overview:

  • SuSE: YaST now includes expanded backup and restore modules.

  • RedHat: the File Roller tool provides visual management of (compressed)
    archives. They seem to be in favour of the X-CD-Roast tool for moving
    backups to an external device.

  • Mandrake: X-CD-Roast.

  • Most distributions come with the BSD dump and restore utilities for
    making backups of ext2 and ext3 file systems. This tool can write to a
    variety of devices and literally dumps the file(s) or file system bit per
    bit onto the specified device. Like dd, this allows for backing up
    special file types such as the ones in /dev.



9.3. Using rsync

9.3.1. Introduction

The rsync program is a fast and flexible tool for remote backup. It is common
on UNIX and UNIX-like systems, easy to configure and use in scripts. While
the r in rsync stands for "remote", you do not need to take this all too
literally. Your "remote" device might just as well be a USB storage device or
another partition on your hard disk, you do not need to have two separated
machines.



9.3.2. An example: rsync to a USB storage device

As discussed in Section 3.1.2.3, we will first have to mount the device.
Possibly, this should be done as root:

root@theserver# mkdir /mnt/usbstore

root@theserver# mount -t vfat /dev/sda1 /mnt/usbstore

Note Userfriendly
     More and more distributions give access to removable devices for
     non-prilileged users and mount USB devices, CD-ROMs and other removable
     devices automatically.

Note that this guideline requires USB support to be installed on your system.
See [http://www.linux-usb.org/USB-guide/] the USB Guide for help if this does
not work. Check with dmesg that /dev/sda1 is indeed the device to mount.

Then you can start the actual backup, for instance of the /home/karl
directory:

karl@theserver:~> rsync -avz /home/karl/ /mnt/usbstore

As usual, refer to the man pages for more.



9.4. Encryption

9.4.1. General remarks

9.4.1.1. Why should you encrypt data?

Encryption is synonym to secrecy. In the context of backups, encryption can
be very useful, for instance if you need to leave your backed up data in a
place where you can not control access, such as the server of your provider.

Apart from that, encryption can be applied to E-mails as well: normally, mail
is not encrypted and it is often sent in the open over the netwerk or the
Internet. If your message contains sensitive information, better encrypt it.



9.4.1.2. GNU Privacy Guard

On Linux systems you will find GnuPG, the GNU Privacy Guard, which is a suite
of programs that are compatible with the PGP (Pretty Good Privacy) tools that
are commercially available.

In this guide we will only discuss the very simple usage of the encryption
tools and show what you will need in order to generate an encryption key and
use it to encrypt data for yourself, which you can then safely store in a
public place. More advanced usage directions can be found in the man pages of
the various commands.



9.4.2. Generate a key

Before you can start encrypting your data, you need to create a pair of keys.
The pair consists of a private and a public key. You can send the public key
to correspondents, who can use it to encrypt data for you, which you decrypt
with your private key. You always keep the private key, never share it with
somebody else, or they will be able to decrypt data that is only destined for
you. Just to make sure that no accidents happen, the private key is protected
with a password. The key pair is created using this command:

willy@ubuntu:~$ gpg --key-gen
gpg (GnuPG) 1.4.2.2; Copyright (C) 2005 Free Software Foundation, Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions.  See the file COPYING for details.

gpg: directory `/home/willy.gnupg' created
gpg: new configuration file `/home/willy/.gnupg/gpg.conf' created
gpg: WARNING: options in `/home/willy/.gnupg/gpg.conf' are not yet
 active during this run
gpg: keyring `/home/willy/.gnupg/secring.gpg' created
gpg: keyring `/home/willy/.gnupg/pubring.gpg' created
Please select what kind of key you want:
    (1) DSA and Elgamal (default)
    (2) DSA (sign only)
    (5) RSA (sign only)
Your selection? 1
DSA keypair will have 1024 bits.
ELG-E keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n month
      <n>y = key expires in n years
Key is valid for? (0) 0
Key does not expire at all
Is this correct? (y/N) y

You need a user ID to identify your key; the software constructs the
user ID from the Real Name, Comment and Email Address in this form:
    "Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>"

Real name: Willy De Wandel
Email address: wdw@mvg.vl
Comment: Willem
You selected this USER-ID:
    "Willy De Wandel (Willem) <wdw@mvg.vl>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.

Passphrase:

Now enetr your password. This can be a phrase, the longer, the better, the
only condition is that you should be able to remember it at all times. For
verification, you need to enter the same phrase again.

Now the key pair is generated by a program that spawns random numbers and
that is, among other factors, fed with the activity data of the system. So it
is a good idea to start some programs now, to move the mouse cursor or to
type some random characters in a terminal window. That way, the chances to
generate a number that contains lots of different digits will be much bigger
and the key will be more difficult to crack.



9.4.3. About your key

When your key has been created, you will get a message about the fingerprint.
This is a sequence of 40 hexadecimal numbers, which is so long that it is
very, very hard to generate the same key twice, on any computer. You can be
rather sure that this is a unique sequence. The short form of this key
consists of your name, followed by the last 8 hexadecimal numbers.

You can get information about your key as follows:

willy@ubuntu:~$ gpg --list-keys
/home/willy/.gnupg/pubring.gpg
------------------------------
pub     1024D/BF5C3DBB 2006-08-08
uid                    Willy De Wandel (Willem) <wdw@mvg.vl>
sub     4096g/A3449CF7 2006-08-08

The key ID of this key is "BF5C3DBB". You can send your key ID and your name
to a key server, so that other people can get this info about you and use it
to encrypt data for you. Alternatively, you can send your public key directly
to the people who need it. The public part of your key is the long series of
numbers that you see when using the --export option to the gpg command:

gpg --export -a

However, as far is this guide is concerned, we assume that you only need your
key in order to encrypt and decrypt data for yourself. Read the gpg man pages
if you want to know more.



9.4.4. Encrypt data

Now you can encrypt a .tar archive or a compressed archive, prior to saving
it to a backup medium or transporting it to the backup server. Use the gpg
command like this:

gpg -e -r (part of) uid archive

The -e option tells gpg to encrypt, the -r option indicates who to encrypt
for. Keep in mind that only only the user name(s) following this -r option
will be able to decrypt the data again. An example:

willy@ubuntu:~$ gpg -e -r Willy /var/tmp/home-willy-20060808.tar



9.4.5. Decrypting files

Using the -d option, you can decrypt files that have been encrypted for you.
The data will scroll over your screen, but an encrypted copy will remain on
disk. So for file formats other than plain text, you will want to save the
decrypted data, so that you can view them with the appropriate program. This
is done using the -o option to the gpg command:

willy@ubuntu:~$ gpg -d -o /var/tmp/home-willy-decrypt.tar /var/tmp/home-willy-20060808.tar.gpg

You need a passphrase to unlock the secret key for
user: "Willy De Wandel (Willem) <wdw@mvg.vl>"
4096 ELG-E key, ID A3449CF7, created 2006-08-08 (main key ID BF5C3DBB)

gpg: encrypted with 4096-bit ELG-E key, ID A3449CF7, created 2006-08-08
        "Willy De Wandel (Willem) <wdw@mvg.vl>"

Warning No password = no data
        If you can not remember your password, the data is lost. Not even the
        system administrator will be able to decrypt the data. That is why a
        copy of important keys is sometimes kept in a sealed vault in a bank.



9.5. Summary

Here's a list of the commands involving file backup:

Table 9-1. New commands in chapter 9: Backup


Command   Meaning                                                         

bzip2     A block-sorting file compressor.                                

cdrecord  Record audio or data Compact Disks from a master.               

dd        Convert and copy a file                                         

fdformat  Low-level formats a floppy disk.                                

gpg       Encrypt and decrypt data.                                       

gzip      Compress or expand files.                                       

mcopy     Copy MSDOS files to/from UNIX.                                  

mdir      Display an MSDOS directory.                                     

mformat   Add an MSDOS file system to a low-level formatted floppy disk.  

mkbootdiskCreates a stand-alone boot floppy for the running system.       

mount     Mount a file system (integrate it with the current file system  
          by connecting it to a mount point).                             

rsync     Synchronize directories.                                        

tar       Tape archiving utility, also used for making archives on disk   
          instead of on tape.                                             

umount    Unmount file systems.                                           



9.6. Exercises

  • Make a backup copy of your home directory in /var/tmp using the tar
    command. Then further compress the file using gzip or bzip2. Make it a
    clean tarred file, one that doesn't make a mess when unpacking.

  • Format a floppy and put some files from your home directory on it. Switch
    floppies with another trainee and recover his/her floppy in your home
    directory.

  • DOS format the floppy. Use the mtools to put and delete files on it.

  • What happens to an unformatted floppy when you want to mount it into the
    file system?

  • If you have any USB storage, try to put a file on it.

  • Using rsync, make a copy of your home directory to another local or
    remote file system.

  • When leaving files on a network server, it's best to encrypt them. Make a
    tar archive of your home directory and encrypt it.



Chapter 10. Networking

    When it comes to networking, Linux is your operating system of choice,
    not only because networking is tightly integrated with the OS itself and
    a wide variety of free tools and applications are available, but for the
    robustness under heavy loads that can only be achieved after years of
    debugging and testing in an Open Source project.

    Bookshelves full of information have been written about Linux and
    networking, but we will try to give an overview in this chapter. After
    completing this, you will know more about

     

      □ Supported networking protocols

      □ Network configuration files

      □ Commands for configuring and probing the network

      □ Daemons and client programs enabling different network applications

      □ File sharing and printing

      □ Remote execution of commands and applications

      □ Basic network interconnection

      □ Secure execution of remote applications

      □ Firewalls and intrusion detection



10.1. Networking Overview

10.1.1. The OSI Model

A protocol is, simply put, a set of rules for communication.

In order to get data over the network, for instance an E-mail from your
computer to some computer at the other end of the world, lots of different
hard- and software needs to work together.

All these pieces of hardware and the different software programs speak
different languages. Imagine your E-mail program: it is able to talk to the
computer operating system, through a specific protocol, but it is not able to
talk to the computer hardware. We need a special program in the operating
system that performs this function. In turn, the computer needs to be able to
communicate with the telephone line or other Internet hookup method. And
behind the scenes, network connection hardware needs to be able to
communicate in order to pass your E-mail from one appliance to the other, all
the way to the destination computer.

All these different types of communication protocols are classified in 7
layers, which are known as the Open Systems Interconnection Reference Model,
the OSI Model for short. For easy understanding, this model is reduced to a
4-layer protocol description, as described in the table below:

Table 10-1. The simplified OSI Model


Layer name          Layer Protocols          

Application layer   HTTP, DNS, SMTP, POP, ...

Transport layer     TCP, UDP                 

Network layer       IP, IPv6                 

Network access layerPPP, PPPoE, Ethernet     


Each layer can only use the functionality of the layer below; each layer can
only export functionality to the layer above. In other words: layers
communicate only with adjacent layers. Let's take the example of your E-mail
message again: you enter it through the application layer. In your computer,
it travels down the transport and network layer. Your computer puts it on the
network through the network access layer. That is also the layer that will
move the message around the world. At the destination, the receiving computer
will accept the message through it's own network layer, and will display it
to the recepient using the transport and application layer.

Note It's really much more complicated
     The above and following sections are included because you will come
     across some networking terms sooner or later; they will give you some
     starting points, should you want to find out about the details.



10.1.2. Some popular networking protocols

Linux supports many different networking protocols. We list only the most
important:



10.1.2.1. TCP/IP

The Transport Control Protocol and the Internet Protocol are the two most
popular ways of communicating on the Internet. A lot of applications, such as
your browser and E-mail program, are built on top of this protocol suite.

Very simply put, IP provides a solution for sending packets of information
from one machine to another, while TCP ensures that the packets are arranged
in streams, so that packets from different applications don't get mixed up,
and that the packets are sent and received in the correct order.

A good starting point for learning more about TCP and IP is in the following
documents:

  • man 7 ip: Describes the IPv4 protocol implementation on Linux (version 4
    currently being the most wide-spread edition of the IP protocol).

  • man 7 tcp: Implementation of the TCP protocol.

  • RFC793, RFC1122, RFC2001 for TCP, and RFC791, RFC1122 and RFC1112 for IP.

    The [http://www.ietf.org/rtf/] Request For Comments documents contain the
    descriptions of networking standards, protocols, applications and
    implementation. These documents are managed by the Internet Engineering
    Task Force, an international community concerned with the smooth
    operation of the Internet and the evolution and development of the
    Internet architecture.

    Your ISP usually has an RFC archive available, or you can browse the RFCs
    via [http://www.ietf.org/rfc.html] http://www.ietf.org/rfc.html.



10.1.2.2. TCP/IPv6

Nobody expected the Internet to grow as fast as it does. IP proved to have
quite some disadvantages when a really large number of computers is in a
network, the most important being the availability of unique addresses to
assign to each machine participating. Thus, IP version 6 was deviced to meet
the needs of today's Internet.

Unfortunately, not all applications and services support IPv6, yet. A
migration is currently being set in motion in many environments that can
benefit from an upgrade to IPv6. For some applications, the old protocol is
still used, for applications that have been reworked the new version is
already active. So when checking your network configuration, sometimes it
might be a bit confusing since all kinds of measures can be taken to hide one
protocol from the other so as the two don't mix up connections.

More information can be found in the following documents:

  • man 7 ipv6: the Linux IPv6 protocol implementation.

  • RFC1883 describing the IPv6 protocol.



10.1.2.3. PPP, SLIP, PLIP, PPPOE

The Linux kernel has built-in support for PPP (Point-to-Point-Protocol), SLIP
(Serial Line IP), PLIP (Parallel Line IP) and PPPP Over EThernet. PPP is the
most popular way individual users access their ISP (Internet Service
Provider), although in densely populated areas it is often being replaced by
PPPOE, the protocol used for ADSL (Asymmetric Digital Subscriber Line)
connections.

Most Linux distributions provide easy-to-use tools for setting up an Internet
connection. The only thing you basically need is a username and password to
connect to your Internet Service Provider (ISP), and a telephone number in
the case of PPP. These data are entered in the graphical configuration tool,
which will likely also allow for starting and stopping the connection to your
provider.



10.1.2.4. ISDN

The Linux kernel has built-in ISDN capabilities. Isdn4linux controls ISDN PC
cards and can emulate a modem with the Hayes command set ("AT" commands). The
possibilities range from simply using a terminal program to full connection
to the Internet.

Check your system documentation.



10.1.2.5. AppleTalk

Appletalk is the name of Apple's internetworking stack. It allows a
peer-to-peer network model which provides basic functionality such as file
and printer sharing. Each machine can simultaneously act as a client and a
server, and the software and hardware necessary are included with every Apple
computer.

Linux provides full AppleTalk networking. Netatalk is a kernel-level
implementation of the AppleTalk Protocol Suite, originally for BSD-derived
systems. It includes support for routing AppleTalk, serving UNIX and AFS file
systems using AppleShare and serving UNIX printers and accessing AppleTalk
printers.



10.1.2.6. SMB/NMB

For compatibility with MS Windows environments, the Samba suite, including
support for the NMB and SMB protocols, can be installed on any UNIX-like
system. The Server Message Block protocol (also called Session Message Block,
NetBIOS or LanManager protocol) is used on MS Windows 3.11, NT, 95/98, 2K and
XP to share disks and printers.

The basic functions of the Samba suite are: sharing Linux drives with Windows
machines, accessing SMB shares from Linux machines, sharing Linux printers
with Windows machines and sharing Windows printers with Linux machines.

Most Linux distributions provide a samba package, which does most of the
server setup and starts up smbd, the Samba server, and nmbd, the netbios name
server, at boot time by default. Samba can be configured graphically, via a
web interface or via the command line and text configuration files. The
daemons make a Linux machine appear as an MS Windows host in an MS Windows My
Network Places/Network Neighbourhood window; a share from a Linux machine
will be indistinguishable from a share on any other host in an MS Windows
environment.

More information can be found at the following locations:

  • man smb.conf: describes the format of the main Samba configuration file.

  • The [http://www.samba.org/] Samba Project Documentation (or check your
    local samba.org mirror) contains an easy to read installation and testing
    guide, which also explains how to configure your Samba server as a
    Primary Domain Controller. All the man pages are also available here.



10.1.2.7. Miscellaneous protocols

Linux also has support for Amateur Radio, WAN internetworking (X25, Frame
Relay, ATM), InfraRed and other wireless connections, but since these
protocols usually require special hardware, we won't discuss them in this
document.



10.2. Network configuration and information

10.2.1. Configuration of network interfaces

All the big, userfriendly Linux distributions come with various graphical
tools, allowing for easy setup of the computer in a local network, for
connecting it to an Internet Service Provider or for wireless access. These
tools can be started up from the command line or from a menu:

  • Ubuntu configuration is done selecting System->Administration->
    Networking.

  • RedHat Linux comes with redhat-config-network, which has both a graphical
    and a text mode interface.

  • Suse's YAST or YAST2 is an all-in-one configuration tool.

  • Mandrake/Mandriva comes with a Network and Internet Configuration Wizard,
    which is preferably started up from Mandrake's Control Center.

  • On Gnome systems: gnome-network-preferences.

  • On KDE systems: knetworkconf.

Your system documentation provides plenty of advice and information about
availability and use of tools.

Information that you will need to provide:

  • For connecting to the local network, for instance with your home
    computers, or at work: hostname, domainname and IP address. If you want
    to set up your own network, best do some more reading first. At work,
    this information is likely to be given to your computer automatically
    when you boot it up. When in doubt, it is better not to specify any
    information than making it up.

  • For connecting to the Internet: username and password for your ISP,
    telephone number when using a modem. Your ISP usually automatically
    assigns you an IP address and all the other things necessary for your
    Internet applications to work.



10.2.2. Network configuration files

The graphical helper tools edit a specific set of network configuration
files, using a couple of basic commands. The exact names of the configuration
files and their location in the file system is largely dependent on your
Linux distribution and version. However, a couple of network configuration
files are common on all UNIX systems:



10.2.2.1. /etc/hosts

The /etc/hosts file always contains the localhost IP address, 127.0.0.1,
which is used for interprocess communication. Never remove this line!
Sometimes contains addresses of additional hosts, which can be contacted
without using an external naming service such as DNS (the Domain Name
Server).

A sample hosts file for a small home network:

# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1       localhost.localdomain   localhost
192.168.52.10   tux.mylan.com           tux
192.168.52.11   winxp.mylan.com         winxp

Read more in man hosts.



10.2.2.2. /etc/resolv.conf

The /etc/resolv.conf file configures access to a DNS server, see Section
10.3.7. This file contains your domain name and the name server(s) to
contact:

search mylan.com
nameserver 193.134.20.4

Read more in the resolv.conf man page.



10.2.2.3. /etc/nsswitch.conf

The /etc/nsswitch.conf file defines the order in which to contact different
name services. For Internet use, it is important that dns shows up in the
"hosts" line:

[bob@tux ~] grep hosts /etc/nsswitch.conf
hosts:  files dns

This instructs your computer to look up hostnames and IP addresses first in
the /etc/hosts file, and to contact the DNS server if a given host does not
occur in the local hosts file. Other possible name services to contact are
LDAP, NIS and NIS+.

More in man nsswitch.conf.



10.2.3. Network configuration commands

10.2.3.1. The ip command

The distribution-specific scripts and graphical tools are front-ends to ip
(or ifconfig and route on older systems) to display and configure the
kernel's networking configuration.

The ip command is used for assigning IP addresses to interfaces, for setting
up routes to the Internet and to other networks, for displaying TCP/IP
configurations etcetera.

The following commands show IP address and routing information:

benny@home benny> ip addr show
1: lo: <LOOPBACK,UP> mtu 16436 qdisc noqueue
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 brd 127.255.255.255 scope host lo
    inet6 ::1/128 scope host
2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 100
    link/ether 00:50:bf:7e:54:9a brd ff:ff:ff:ff:ff:ff
    inet 192.168.42.15/24 brd 192.168.42.255 scope global eth0
    inet6 fe80::250:bfff:fe7e:549a/10 scope link

benny@home benny> ip route show
192.168.42.0/24 dev eth0  scope link
127.0.0.0/8 dev lo  scope link
default via 192.168.42.1 dev eth0


Things to note:

  • two network interfaces, even on a system that has only one network
    interface card: "lo" is the local loop, used for internal network
    communication; "eth0" is a common name for a real interface. Do not ever
    change the local loop configuration, or your machine will start
    mallfunctioning! Wireless interfaces are usually defined as "wlan0";
    modem interfaces as "ppp0", but there might be other names as well.

  • IP addresses, marked with "inet": the local loop always has 127.0.0.1,
    the physical interface can have any other combination.

  • The hardware address of your interface, which might be required as part
    of the authentication procedure to connect to a network, is marked with
    "ether". The local loop has 6 pairs of all zeros, the physical loop has 6
    pairs of hexadecimal characters, of which the first 3 pairs are
    vendor-specific.



10.2.3.2. The ifconfig command

While ip is the most novel way to configure a Linux system, ifconfig is still
very popular. Use it without option for displaying network interface
information:

els@asus:~$ /sbin/ifconfig
eth0      Link encap:Ethernet  HWaddr 00:50:70:31:2C:14
          inet addr:60.138.67.31  Bcast:66.255.255.255  Mask:255.255.255.192
          inet6 addr: fe80::250:70ff:fe31:2c14/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:31977764 errors:0 dropped:0 overruns:0 frame:0
          TX packets:51896866 errors:0 dropped:0 overruns:0 carrier:0
          collisions:802207 txqueuelen:1000
          RX bytes:2806974916 (2.6 GiB)  TX bytes:2874632613 (2.6 GiB)
          Interrupt:11 Base address:0xec00
                                                                                lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:765762 errors:0 dropped:0 overruns:0 frame:0
          TX packets:765762 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:624214573 (595.2 MiB)  TX bytes:624214573 (595.2 MiB)

Here, too, we note the most important aspects of the interface configuration:

  • The IP address is marked with "inet addr".

  • The hardware address follows the "HWaddr" tag.

Both ifconfig and ip display more detailed configuration information and a
number of statistics about each interface and, maybe most important, whether
it is "UP" and "RUNNING".



10.2.3.3. PCMCIA commands

On your laptop which you usually connect to the company network using the
onboard Ethernet connection, but which you are now to configure for dial-in
at home or in a hotel, you might need to activate the PCMCIA card. This is
done using the cardctl control utility, or the pccardctl on newer
distributions.

A usage example:

cardctl insert

Now the card can be configured, either using the graphical or the command
line interface. Prior to taking the card out, use this command:

cardctl eject

However, a good distribution should provide PCMCIA support in the network
configuration tools, preventing users from having to execute PCMCIA commands
manually.



10.2.3.4. More information

Further discussion of network configuration is out of the scope of this
document. Your primary source for extra information is the man pages for the
services you want to set up. Additional reading:

  • [http://www.tldp.org/HOWTO/Modem-HOWTO.html] The Modem-HOWTO: Help with
    selecting, connecting, configuring, trouble-shooting, and understanding
    analog modems for a PC.

  • [http://www.tldp.org/HOWTO/HOWTO-INDEX/networking.html#NETGENERAL] LDP
    HOWTO Index, section 4.4: categorized list of HOWTOs about general
    networking, protocols, dial-up, DNS, VPNs, bridging, routinfg, security
    and more.

  • Most systems have a version of the ip-cref file (locate it using the
    locate command); the PS format of this file is viewable with for instance
    gv.



10.2.4. Network interface names

On a Linux machine, the device name lo or the local loop is linked with the
internal 127.0.0.1 address. The computer will have a hard time making your
applications work if this device is not present; it is always there, even on
computers which are not networked.

The first ethernet device, eth0 in the case of a standard network interface
card, points to your local LAN IP address. Normal client machines only have
one network interface card. Routers, connecting networks together, have one
network device for each network they serve.

If you use a modem to connect to the Internet, your network device will
probably be named ppp0.

There are many more names, for instance for Virtual Private Network
interfaces (VPNs), and multiple interfaces can be active simultaneously, so
that the output of the ifconfig or ip commands might become quite extensive
when no options are used. Even multiple interfaces of the same type can be
active. In that case, they are numbered sequentially: the first will get the
number 0, the second will get a suffix of 1, the third will get 2, and so on.
This is the case on many application servers, on machines which have a
failover configuration, on routers, firewalls and many more.



10.2.5. Checking the host configuration with netstat

Apart from the ip command for displaying the network configuration, there's
the common netstat command which has a lot of options and is generally useful
on any UNIX system.

Routing information can be displayed with the -nr option to the netstat
command:

bob:~> netstat -nr
Kernel IP routing table
Destination  Gateway      Genmask       Flags MSS Window irtt Iface
192.168.42.0 0.0.0.0      255.255.255.0 U      40 0         0 eth0
127.0.0.0    0.0.0.0      255.0.0.0     U      40 0         0 lo
0.0.0.0      192.168.42.1 0.0.0.0       UG     40 0         0 eth0

This is a typical client machine in an IP network. It only has one network
device, eth0. The lo interface is the local loop.

Note The modern way
     The novel way to get this info from your system is by using the ip
     command:

     ip route show

When this machine tries to contact a host that is on another network than its
own, indicated by the line starting with 0.0.0.0, it will send the connection
requests to the machine (router) with IP address 192.168.42.1, and it will
use its primary interface, eth0, to do this.

Hosts that are on the same network, the line starting with 192.168.42.0, will
also be contacted through the primary network interface, but no router is
necessary, the data are just put on the network.

Machines can have much more complicated routing tables than this one, with
lots of different "Destination-Gateway" pairs to connect to different
networks. If you have the occasion to connect to an application server, for
instance at work, it is most educating to check the routing information.



10.2.6. Other hosts

An impressive amount of tools is focused on network management and remote
administration of Linux machines. Your local Linux software mirror will offer
plenty of those. It would lead us too far to discuss them in this document,
so please refer to the program-specific documentation.

We will only discuss some common UNIX/Linux text tools in this section.



10.2.6.1. The host command

To display information on hosts or domains, use the host command:

[emmy@pc10 emmy]$ host www.eunet.be
www.eunet.be. has address 193.74.208.177

[emmy@pc10 emmy]$ host -t any eunet.be
eunet.be. SOA dns.eunet.be. hostmaster.Belgium.EU.net.
  2002021300 28800 7200 604800 86400
eunet.be. mail is handled by 50 pophost.eunet.be.
eunet.be. name server ns.EU.net.
eunet.be. name server dns.eunet.be.

Similar information can be displayed using the dig command, which gives
additional information about how records are stored in the name server.



10.2.6.2. The ping command

To check if a host is alive, use ping. If your system is configured to send
more than one packet, interrupt ping with the Ctrl+C key combination:

[emmy@pc10 emmy]$ ping a.host.be 
PING a.host.be (1.2.8.3) from 80.20.84.26: 56(84) bytes of data.
64 bytes from a.host.be(1.2.8.3):icmp_seq=0 ttl=244 time=99.977msec
--- a.host.be ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/mdev = 99.977/99.977/99.977/0.000 ms



10.2.6.3. The traceroute command

To check the route that packets follow to a network host, use the traceroute
command:

[emmy@pc10 emmy]$ /usr/sbin/traceroute www.eunet.be
traceroute to www.eunet.be(193.74.208.177),30 hops max,38b packets
1 blob (10.0.0.1)
       0.297ms  0.257ms  0.174ms
2 adsl-65.myprovider.be (217.136.111.1)
       12.120ms 13.058ms 13.009ms
3 194.78.255.177 (194.78.255.177)
       13.845ms 14.308ms 12.756ms
4 gigabitethernet2-2.intl2.gam.brussels.skynet.be (195.238.2.226)
       13.123ms 13.164ms 12.527ms
5 pecbru2.car.belbone.be (194.78.255.118)
       16.336ms 13.889ms 13.028ms
6 ser-2-1-110-ias-be-vil-ar01.kpnbelgium.be (194.119.224.9)
       14.602ms 15.546ms 15.959ms
7 unknown-195-207-939.eunet.be (195.207.93.49)
       16.514ms 17.661ms 18.889ms
8 S0-1-0.Leuven.Belgium.EU.net (195.207.129.1)
       22.714ms 19.193ms 18.432ms
9 dukat.Belgium.EU.net (193.74.208.178) 22.758ms * 25.263ms

On some systems, traceroute has been renamed to tracepath.



10.2.6.4. The whois command

Specific domain name information can be queried using the whois command, as
is explained by many whois servers, like the one below:

[emmy@pc10 emmy]$ whois cnn.com
[whois.crsnic.net]

Whois Server Version 1.3

        $<--snap server message-->

   Domain Name: CNN.COM
   Registrar: NETWORK SOLUTIONS, INC.
   Whois Server: whois.networksolutions.com
   Referral URL: http://www.networksolutions.com
   Name Server: TWDNS-01.NS.AOL.COM
   Name Server: TWDNS-02.NS.AOL.COM
   Name Server: TWDNS-03.NS.AOL.COM
   Name Server: TWDNS-04.NS.AOL.COM
   Updated Date: 12-mar-2002
>>> Last update of whois database: Fri, 5 Apr 2002 05:04:55 EST <<<

The Registry database contains ONLY .COM, .NET, .ORG, .EDU domains
and Registrars.

[whois.networksolutions.com]

        $<--snap server message-->

Registrant:
Turner Broadcasting (CNN-DOM)
   1 CNN Center
   Atlanta, GA 30303

   Domain Name: CNN.COM

   Administrative Contact:
        $<--snap contactinfo-->
   Technical Contact:
        $<--snap contactinfo-->
   Billing Contact:
        $<--snap contactinfo-->
   Record last updated on 12-Mar-2002.
   Record expires on 23-Sep-2009.
   Record created on 22-Sep-1993.
   Database last updated on 4-Apr-2002 20:10:00 EST.

   Domain servers in listed order:

   TWDNS-01.NS.AOL.COM          149.174.213.151
   TWDNS-02.NS.AOL.COM          152.163.239.216
   TWDNS-03.NS.AOL.COM          205.188.146.88
   TWDNS-04.NS.AOL.COM          64.12.147.120

For other domain names than .com, .net, .org and .edu, you might need to
specify the whois server, such as this one for .be domains:

whois domain.be@whois.dns.be



10.3. Internet/Intranet applications

The Linux system is a great platform for offering networking services. In
this section, we will try to give an overview of most common network servers
and applications.



10.3.1. Server types

10.3.1.1. Standalone server

Offering a service to users can be approached in two ways. A daemon or
service can run in standalone mode, or it can be dependent on another service
to be activated.

Network services that are heavily and/or continuously used, usually run in
the standalone mode: they are independent program daemons that are always
running. They are most likely started up at system boot time, and they wait
for requests on the specific connection points or ports for which they are
set up to listen. When a request comes, it is processed, and the listening
continues until the next request. A web server is a typical example: you want
it to be available 24 hours a day, and if it is too busy it should create
more listening instances to serve simultaneous users. Other examples are the
large software archives such as [http://sourceforge.net] Sourceforge or
[http://tucows.com] your Tucows mirror, which must handle thousands of FTP
requests per day.

An example of a standalone network service on your home computer might be the
named (name daemon), a caching name server. Standalone services have their
own processes running, you can check any time using ps:

bob:~> ps auxw | grep named
named   908  0.0  1.0 14876 5108 ?   S  Mar14  0:07 named -u named

However, there are some services that you can use on your PC, even if there
is no server process running for that services. Examples could be the FTP
service, the secure copy service or the finger service. Those services have
the Internet Daemon (inetd) listening in their place.



10.3.1.2. (x)inetd

On your home PC, things are usually a bit calmer. You may have a small
network, for instance, and you may have to transfer files from one PC to
another from time to time, using FTP or Samba (for connectivity with MS
Windows machines). In those cases, starting all the services which you only
need occasionally and having them run all the time would be a waste of
resources. So in smaller setups, you will find the necessary daemons
dependent on a central program, that listen on all the ports of the services
for which it is responsible.

This super-server, the Internet services daemon, is started up at system
initialization time. There are two common implementations: inetd and xinetd
(the extended Internet services daemon). One or the other is usually running
on every Linux system:

bob:~> ps -ef | grep inet
root  926   1 0 Mar14 ?   00:00:00 xinetd-ipv6 -stayalive -reuse \
-pidfile /var/run/xinetd.pid

The services for which the Internet daemon is responsible, are listed in its
configuration file, /etc/inetd.conf, for inetd, and in the directory /etc/
xinetd.d for xinetd. Commonly managed services include file share and print
services, SSH, FTP, telnet, the Samba configuration daemon, talk and time
services.

As soon as a connection request is received, the central server will start an
instance of the required server. Thus, in the example below, when user bob
starts an FTP session to the local host, an FTP daemon is running as long as
the session is active:

bob:~> ps auxw | grep ftp
bob     793  0.1  0.2  3960 1076 pts/6    S    16:44   0:00 ncftp localhost
ftp     794  0.7  0.5  5588 2608 ?        SN   16:44   0:00 ftpd:
localhost.localdomain: anonymous/bob@his.server.com: IDLE

Of course, the same happens when you open connections to remote hosts: either
a daemon answers directly, or a remote (x)inetd starts the service you need
and stops it when you quit.



10.3.2. Mail

10.3.2.1. Servers

Sendmail is the standard mail server program or Mail Transport Agent for UNIX
platforms. It is robust, scalable, and when properly configured with
appropriate hardware, handles thousands of users without blinking. More
information about how to configure Sendmail is included with the sendmail and
sendmail-cf packages, you may want to read the README and README.cf files in
/usr/share/doc/sendmail. The man sendmail and man aliases are also useful.

Qmail is another mail server, gaining popularity because it claims to be more
secure than Sendmail. While Sendmail is a monolithic program, Qmail consists
of smaller interacting program parts that can be better secured. Postfix is
another mail server which is gaining popularity.

These servers handle mailing lists, filtering, virus scanning and much more.
Free and commercial scanners are available for use with Linux. Examples of
mailing list software are Mailman, Listserv, Majordomo and EZmlm. See the web
page of your favorite virus scanner for information on Linux client and
server support. Amavis and Spamassassin are free implementations of a virus
scanner and a spam scanner.



10.3.2.2. Remote mail servers

The most popular protocols to access mail remotely are POP3 and IMAP4. IMAP
and POP both allow offline operation, remote access to new mail and they both
rely on an SMTP server to send mail.

While POP is a simple protocol, easy to implement and supported by almost any
mail client, IMAP is to be preferred because:

  • It can manipulate persistent message status flags.

  • It can store as well as fetch mail messages.

  • It can access and manage multiple mailboxes.

  • It supports concurrent updates and shared mailboxes.

  • It is also suitable for accessing Usenet messages and other documents.

  • IMAP works both on-line and off-line.

  • it is optimized for on-line performance, especially over low-speed links.



10.3.2.3. Mail user-agents

There are plenty of both text and graphical E-mail clients, we'll just name a
few of the common ones. Pick your favorite.

The UNIX mail command has been around for years, even before networking
existed. It is a simple interface to send messages and small files to other
users, who can then save the message, redirect it, reply to it etcetera.

While it is not commonly used as a client anymore, the mail program is still
useful, for example to mail the output of a command to somebody:

mail <future.employer@whereIwant2work.com> < cv.txt

The elm mail reader is a much needed improvement to mail, and so is pine
(Pine Is Not ELM). The mutt mail reader is even more recent and offers
features like threading.

For those users who prefer a graphical interface to their mail (and a tennis
elbow or a mouse arm), there are hundreds of options. The most popular for
new users are Mozilla Mail/Thunderbird, which has easy anti-spam configuring
options, and Evolution, the MS Outlook clone. Kmail is popular among KDE
users.

Figure 10-1. Evolution mail and news reader

[evolution]

There are also tens of web mail applications available, such as Squirrelmail,
Yahoo! mail, gmail from Google and Hotmail.

An overview is available via the [http://www.tldp.org/HOWTO/Mail-User-HOWTO/
index.html] Linux Mail User HOWTO.

Most Linux distributions include fetchmail, a mail-retrieval and forwarding
utility. It fetches mail from remote mail servers (POP, IMAP and some others)
and forwards it to your local delivery system. You can then handle the
retrieved mail using normal mail clients. It can be run in daemon mode to
repeatedly poll one or more systems at a specified interval. Information and
usage examples can be found in the Info pages; the directory /usr/share/doc/
fetchmail[-<version>] contains a full list of features and a FAQ for
beginners.

The procmail filter can be used for filtering incoming mail, to create
mailing lists, to pre-process mail, to selectively forward mail and more. The
accompanying formail program, among others, enables generation of
auto-replies and splitting up mailboxes. Procmail has been around for years
on UNIX and Linux machines and is a very robust system, designed to work even
in the worst circumstances. More information may be found in the /usr/share/
doc/procmail[-<version>] directory and in the man pages.

Note A note on E-mail Etiquette
     Some people these days seem to think that an E-mail message shouldn't be
     too formal. That depends, of course. If you are writing to someone you
     don't know, best to keep some distance, just like you would do in a
     traditional letter. And don't forget: people you don't know might be
     male or female...



10.3.3. Web

10.3.3.1. The Apache Web Server

Apache is by far the most popular web server, used on more than half of all
Internet web servers. Most Linux distributions include Apache. Apache's
advantages include its modular design, SSL support, stability and speed.
Given the appropriate hardware and configuration it can support the highest
loads.

On Linux systems, the server configuration is usually done in the /etc/httpd
directory. The most important configuration file is httpd.conf; it is rather
self-explanatory. Should you need help, you can find it in the httpd man page
or on the [http://www.apache.org] Apache website.



10.3.3.2. Web browsers

A number of web browsers, both free and commercial, exist for the Linux
platform. Netscape Navigator as the only decent option has long been a thing
of the past, as Mozilla/Firefox offers a competitive alternative running on
many other operating systems, like MS Windows and MacOS X as well.

Amaya is the W3C browser. Opera is a commercial browser, compact and fast.
Many desktop managers offer web browsing features in their file manager, like
nautilus.

Among the popular text based browsers are lynx and links. You may need to
define proxy servers in your shell, by setting the appropriate variables.
Text browsers are fast and handy when no graphical environment is available,
such as when used in scripts.



10.3.3.3. Proxy servers

10.3.3.3.1. What is a proxy server?

Companies and organizations often want their users to use a proxy server.
Especially in environments with lots of users, a proxy server can enable
faster downloads of web pages. The proxy server stores web pages. When a user
asks for a web page that has already been requested previously, the proxy
server will give that page to the user directly, so that s/he does not need
to get it from the Internet, which would take longer. Of course, measures can
be taken so that the proxy server does a quick check and always serves the
most recent version of a page. In some environments, usage of the proxy
server is compulsory, in other environments you may have the choice whether
or not to use it.



10.3.3.3.2. Proxy configuration

If you have the proxy server name and port, it should be rather obvious to
feed that information into your browser. However, many (command line)
applications depend on the variables http_proxy and ftp_proxy for correct
functioning. For your convenience, you might want to add a line like the
following to your ~/.bashrc:

export http_proxy=http://username:password@proxy_server_name:port_number

For instance:

export http_proxy=http://willy:Appelsi3ntj3@proxy:80

If you do not need to give a username and password, simply leave out
everything before the "@" sign, this sign included.



10.3.4. File Transfer Protocol

10.3.4.1. FTP servers

On a Linux system, an FTP server is typically run from xinetd, using the
WU-ftpd server, although the FTP server may be configured as a stand-alone
server on systems with heavy FTP traffic. See the exercises.

Other FTP servers include among others vsftpd, Ncftpd and Proftpd.

Most Linux distributions contain the anonftp package, which sets up an
anonymous FTP server tree and accompanying configuration files.



10.3.4.2. FTP clients

Most Linux distributions include ncftp, an improved version of the common
UNIX ftp command, which you may also know from the Windows command line. The
ncftp program offers extra features such as a nicer and more comprehensible
user interface, file name completion, append and resume functions,
bookmarking, session management and more:

thomas:~> ncftp blob
NcFTP 3.0.3 (April 15, 2001) by Mike Gleason (ncftp@ncftp.com).
Connecting to blob...
blob.some.net FTP server (Version wu-2.6.1-20) ready.
Logging in...
Guest login ok, access restrictions apply.
Logged in to blob.
ncftp / > help
Commands may be abbreviated.  'help showall' shows hidden and
unsupported commands.
'help <command>' gives a brief description of <command>.

ascii      cat      help      lpage     open     quote    site
bgget      cd       jobs      lpwd      page     rename   type
bgput      chmod    lcd       lrename   pdir     rhelp    umask
bgstart    close    lchmod    lrm       pls      rm       version
binary     debug    lls       lrmdir    put      rmdir
bookmark   dir      lmkdir    ls        pwd      set
bookmarks  get      lookup    mkdir     quit     show
ncftp / >


Excellent help with lot of examples can be found in the man pages. And again,
a number of GUI applications are available.

Warning FTP is insecure!
        Don't use the File Transfer Protocol for non-anonymous login unless
        you know what you are doing. Your user name and password might be
        captured by malevolent fellow network users! Use secure FTP instead;
        the sftp program comes with the Secure SHell suite, see Section
        10.4.4.4.



10.3.5. Chatting and conferencing

Various clients and systems are available in each distribution, replacing the
old-style IRC text-based chat. A short and incomplete list of the most
popular programs:

  • gaim: multi-protocol instant messaging client for Linux, Windows and Mac,
    compatible with MSN Messenger, ICQ, IRC and much more; see the Info pages
    or [http://gaim.sourceforge.net] the Gaim site for more.

  • xchat: IRC client for the X window system:

    Figure 10-2. X-Chat

    [xchat]

    The home page is at [http://sourceforge.net/projects/xchat/] SourceForge.

  • aMSN: an MSN clone.

  • Konversation, kopete, KVIrc and many other K-tools from the KDE suite.

  • gnomemeeting: videoconferencing program for UNIX (now Ekiga).

  • jabber: Open Source Instant Messenging platform, compatible with ICQ,
    AIM, Yahoo, MSN, IRC, SMTP and much more.

  • psi: jabber client, see [http://psi.affinix.com/] the PSI Jabber Client
    Homepage.

  • skype: program for making telephone-like calls over the Internet to other
    Skype users, see [http://www.skype.com] http://www.skype.com for more
    info. Skype is free but not open.

  • Gizmo: a free (but not open) phone for your computer, see [http://
    www.gizmoproject.com] http://www.gizmoproject.com.



10.3.6. News services

Running a Usenet server involves a lot of expertise and fine-tuning, so refer
to the [http://www.isc.org] INN homepage for more information.

There are a couple of interesting newsgroups in the comp.* hierarchy, which
can be accessed using a variety of text and graphical clients. A lot of mail
clients support newsgroup browsing as well, check your program or see your
local Open Source software mirror for text clients such as tin, slrnn and
mutt, or download Mozilla or one of a number of other graphical clients.

[http://deja.com] Deja.com keeps a searchable archive of all newsgroups,
powered by Google. This is a very powerful instrument for getting help:
chances are very high that somebody has encountered your problem, found a
solution and posted it in one of the newsgroups.



10.3.7. The Domain Name System

All these applications need DNS services to match IP addresses to host names
and vice versa. A DNS server does not know all the IP addresses in the world,
but networks with other DNS servers which it can query to find an unknown
address. Most UNIX systems can run named, which is part of the BIND (Berkeley
Internet Name Domain) package distributed by the Internet Software
Consortium. It can run as a stand-alone caching nameserver, which is often
done on Linux systems in order to speed up network access.

Your main client configuration file is /etc/resolv.conf, which determines the
order in which Domain Name Servers are contacted:

search somewhere.org
nameserver 192.168.42.1
nameserver 193.74.208.137

More information can be found in the Info pages on named, in the /usr/share/
doc/bind[-<version>] files and on the [http://www.isc.org/products/BIND] Bind
project homepage. The [http://www.tldp.org/HOWTO/DNS-HOWTO.html] DNS HOWTO
covers the use of BIND as a DNS server.



10.3.8. DHCP

DHCP is the Dynamic Host Configuration Protocol, which is gradually replacing
good old bootp in larger environments. It is used to control vital networking
parameters such as IP addresses and name servers of hosts. DHCP is backward
compatible with bootp. For configuring the server, you will need to read the
HOWTO.

DHCP client machines will usually be configured using a GUI that configures
the dhcpcd, the DHCP client daemon. Check your system documentation if you
need to configure your machine as a DHCP client.



10.3.9. Authentication services

10.3.9.1. Traditional

Traditionally, users are authenticated locally, using the information stored
in /etc/passwd and /etc/shadow on each system. But even when using a network
service for authenticating, the local files will always be present to
configure system accounts for administrative use, such as the root account,
the daemon accounts and often accounts for additional programs and purposes.

These files are often the first candidates for being examined by hackers, so
make sure the permissions and ownerships are strictly set as should be:

bob:~> ls -l /etc/passwd /etc/shadow
-rw-r--r--    1 root     root         1803 Mar 10 13:08 /etc/passwd
-r--------    1 root     root         1116 Mar 10 13:08 /etc/shadow



10.3.9.2. PAM

Linux can use PAM, the Pluggable Authentication Module, a flexible method of
UNIX authentication. Advantages of PAM:

  • A common authentication scheme that can be used with a wide variety of
    applications.

  • PAM can be implemented with various applications without having to
    recompile the applications to specifically support PAM.

  • Great flexibility and control over authentication for the administrator
    and application developer.

  • Application developers do not need to develop their program to use a
    particular authentication scheme. Instead, they can focus purely on the
    details of their program.

The directory /etc/pam.d contains the PAM configuration files (used to be /
etc/pam.conf). Each application or service has its own file. Each line in the
file has four elements:

  • Module:

      □ auth: provides the actual authentication (perhaps asking for and
        checking a password) and sets credentials, such as group membership
        or Kerberos tickets.

      □ account: checks to make sure that access is allowed for the user (the
        account has not expired, the user is allowed to log in at this time
        of day, and so on).

      □ password: used to set passwords.

      □ session: used after a user has been authenticated. This module
        performs additional tasks which are needed to allow access (for
        example, mounting the user's home directory or making their mailbox
        available).

    The order in which modules are stacked, so that multiple modules can be
    used, is very important.

  • Control Flags: tell PAM which actions to take upon failure or success.
    Values can be required, requisite, sufficient or optional.

  • Module Path: path to the pluggable module to be used, usually in /lib/
    security.

  • Arguments: information for the modules

Shadow password files are automatically detected by PAM.

More information can be found in the pam man pages or at [http://
www.kernel.org/pub/linux/libs/pam] the Linux-PAM project homepage.



10.3.9.3. LDAP

The Lightweight Directory Access Protocol is a client-server system for
accessing global or local directory services over a network. On Linux, the
OpenLDAP implementation is used. It includes slapd, a stand-alone server;
slurpd, a stand-alone LDAP replication server; libraries implementing the
LDAP protocol and a series of utilities, tools and sample clients.

The main benefit of using LDAP is the consolidation of certain types of
information within your organization. For example, all of the different lists
of users within your organization can be merged into one LDAP directory. This
directory can be queried by any LDAP-enabled applications that need this
information. It can also be accessed by users who need directory information.

 Other LDAP or X.500 Lite benefits include its ease of implementation
(compared to X.500) and its well-defined Application Programming Interface
(API), which means that the number of LDAP-enabled applications and LDAP
gateways should increase in the future.

 On the negative side, if you want to use LDAP, you will need LDAP-enabled
applications or the ability to use LDAP gateways. While LDAP usage should
only increase, currently there are not very many LDAP-enabled applications
available for Linux. Also, while LDAP does support some access control, it
does not possess as many security features as X.500.

Since LDAP is an open and configurable protocol, it can be used to store
almost any type of information relating to a particular organizational
structure. Common examples are mail address lookups, central authentication
in combination with PAM, telephone directories and machine configuration
databases.

See your system specific information and the man pages for related commands
such as ldapmodify and ldapsearch for details. More information can be found
in the [http://www.tldp.org/HOWTO/LDAP-HOWTO.html] LDAP Linux HOWTO, which
discusses installation, configuration, running and maintenance of an LDAP
server on Linux. The author of this Introduction to Linux document also wrote
an [http://tille.garrels.be/training/ldap/] LDAP Operations HOWTO, describing
the basics everyone should know about when dealing with LDAP management,
operations and integration of services.



10.4. Remote execution of applications

10.4.1. Introduction

There are a couple of different ways to execute commands or run programs on a
remote machine and have the output, be it text or graphics, sent to your
workstation. The connections can be secure or insecure. While it is of course
advised to use secure connections instead of transporting your password over
the network unencrypted, we will discuss some practical applications of the
older (unsafe) mechanisms, as they are still useful in a modern networked
environment, such as for troubleshooting or running exotic programs.



10.4.2. Rsh, rlogin and telnet

The rlogin and rsh commands for remote login and remote execution of commands
are inherited from UNIX. While seldom used because they are blatantly
insecure, they still come with almost every Linux distribution for backward
compatibility with UNIX programs.

Telnet, on the other hand, is still commonly used, often by system and
network administrators. Telnet is one of the most powerful tools for remote
access to files and remote administration, allowing connections from anywhere
on the Internet. Combined with an X server, remote graphical applications can
be displayed locally. There is no difference between working on the local
machine and using the remote machine.

Because the entire connection is unencrypted, allowing telnet connections
involves taking high security risks. For normal remote execution of programs,
Secure SHell or ssh is advised. We will discuss the secure method later in
this section.

However, telnet is still used in many cases. Below are some examples in which
a mail server and a web server are tested for replies:

Checking that a mail server works:

[jimmy@blob ~] telnet mailserver 25
Trying 192.168.42.1...
Connected to mailserver.
Escape character is '^]'.
220 m1.some.net ESMTP Sendmail 8.11.6/8.11.6; 200302281626
ehlo some.net
250-m1.some.net Hello blob.some.net [10.0.0.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-SIZE
250-DSN
250-ONEX
250-ETRN
250-XUSR
250 HELP
mail from: jimmy@some.net
250 2.1.0 jimmy@some.net... Sender ok
rcpt to: davy@some.net
250 2.1.5 davy@some.net... Recipient ok
data
354 Enter mail, end with "." on a line by itself
test
.
250 2.0.0 g2MA1R619237 Message accepted for delivery
quit
221 2.0.0 m1.some.net closing connection
Connection closed by foreign host.

Checking that a web server answers to basic requests:

[jimmy@blob ~] telnet www.some.net 80
Trying 64.39.151.23...
Connected to www.some.net.
Escape character is '^]'.
HEAD / ;HTTP/1.1

HTTP/1.1 200 OK
Date: Fri, 22 Mar 2002 10:05:14 GMT
Server: Apache/1.3.22 (UNIX) (Red-Hat/Linux)
 mod_ssl/2.8.5 OpenSSL/0.9.6
 DAV/1.0.2 PHP/4.0.6 mod_perl/1.24_01
Last-Modified: Fri, 04 Jan 2002 08:21:00 GMT
ETag: "70061-68-3c3565ec"
Accept-Ranges: bytes
Content-Length: 104
Connection: close
Content-Type: text/html

Connection closed by foreign host.

[jimmy@blob ~]

This is perfectly safe, because you never have to give a username and/or
password for getting the data you want, so nobody can snoop that important
information off the cable.



10.4.3. The X Window System

10.4.3.1. X features

As we already explained in Chapter 7 (see Section 7.3.3), the X Window system
comes with an X server which serves graphics to clients that need a display.

It is important to realize the distinction between the X server and the X
client application(s). The X server controls the display directly and is
responsible for all input and output via keyboard, mouse and display. The X
client, on the other hand, does not access the input and output devices
directly. It communicates with the X server which handles input and output.
It is the X client which does the real work, like computing values, running
applications and so forth. The X server only opens windows to handle input
and output for the specified client.

In normal operation (graphical mode), every Linux workstation is an X server
to itself, even if it only runs client applications. All the applications you
are running (for example, Gimp, a terminal window, your browser, your office
application, your CD playing tool, and so on) are clients to your X server.
Server and client are running on the same machine in this case.

This client/server nature of the X system makes it an ideal environment for
remote execution of applications and programs. Because the process is
actually being executed on the remote machine, very little CPU power is
needed on the local host. Such machines, purely acting as servers for X, are
called X terminals and were once very popular. More information may be found
in the [http://www.tldp.org/HOWTO/Remote-X-Apps.html] Remote X applications
mini-HOWTO.



10.4.3.2. Telnet and X

If you would want to use telnet to display graphical applications running on
a remote machine, you first need to give the remote machine access to your
display (to your X server!) using the xhost command, by typing a command
similar to the one below in a terminal window on your local machine:

davy:~> xhost +remote.machine.com

After that, connect to the remote host and tell it to display graphics on the
local machine by setting the environment variable DISPLAY:

[davy@remote ~] export DISPLAY="local.host.com:0.0"

After completing this step, any application started in this terminal window
will be displayed on your local desktop, using remote resources for
computing, but your local graphical resources (your X server) for displaying
the application.

This procedure assumes that you have some sort of X server (XFree86, X.org,
Exceed, Cygwin) already set up on the machine where you want to display
images. The architecture and operating system of the client machine are not
important as long as they allow you to run an X server on it.

Mind that displaying a terminal window from the remote machine is also
considered to be a display of an image.



10.4.4. The SSH suite

10.4.4.1. Introduction

Most UNIX and Linux systems now run Secure SHell in order to leave out the
security risks that came with telnet. Most Linux systems will run a version
of OpenSSH, an Open Source implementation of the SSH protocol, providing
secure encrypted communications between untrusted hosts over an untrusted
network. In the standard setup X connections are automatically forwarded, but
arbitrary TCP/IP ports may also be forwarded using a secure channel.

The ssh client connects and logs into the specified host name. The user must
provide his identity to the remote machine as specified in the sshd_config
file, which can usually be found in /etc/ssh. The configuration file is
rather self-explanatory and by defaults enables most common features. Should
you need help, you can find it in the sshd man pages.

When the user's identity has been accepted by the server, the server either
executes the given command, or logs into the machine and gives the user a
normal shell on the remote machine. All communication with the remote command
or shell will be automatically encrypted.

The session terminates when the command or shell on the remote machine exits
and all X11 and TCP/IP connections have been closed.

When connecting to a host for the first time, using any of the programs that
are included in the SSH collection, you need to establish the authenticity of
that host and acknowledge that you want to connect:

lenny ~> ssh blob
The authenticity of host 'blob (10.0.0.1)' can't be established.
RSA fingerprint is 18:30:50:46:ac:98:3c:93:1a:56:35:09:8d:97:e3:1d.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'blob,192.168.30.2' (RSA) to the list of
known hosts.
Last login: Sat Dec 28 13:29:19 2002 from octarine
This space for rent.

lenny is in ~

It is important that you type "yes", in three characters, not just "y". This
edits your ~/.ssh/known_hosts file, see Section 10.4.4.3.

If you just want to check something on a remote machine and then get your
prompt back on the local host, you can give the commands that you want to
execute remotely as arguments to ssh:

lenny ~> ssh blob who
jenny@blob's password:
root     tty2         Jul 24 07:19
lena     tty3         Jul 23 22:24
lena     0:           Jul 25 22:03

lenny ~> uname -n
magrat.example.com



10.4.4.2. X11 and TCP forwarding

If the X11Forwarding entry is set to yes on the target machine and the user
is using X applications, the DISPLAY environment variable is set, the
connection to the X11 display is automatically forwarded to the remote side
in such a way that any X11 programs started from the shell will go through
the encrypted channel, and the connection to the real X server will be made
from the local machine. The user should not manually set DISPLAY. Forwarding
of X11 connections can be configured on the command line or in the sshd
configuration file.

The value for DISPLAY set by ssh will point to the server machine, but with a
display number greater than zero. This is normal, and happens because ssh
creates a proxy X server on the server machine (that runs the X client
application) for forwarding the connections over the encrypted channel.

This is all done automatically, so when you type in the name of a graphical
application, it is displayed on your local machine and not on the remote
host. We use xclock in the example, since it is a small program which is
generally installed and ideal for testing:

Figure 10-3. SSH X11 forwarding

[ssh_X11]

SSH will also automatically set up Xauthority data on the server machine. For
this purpose, it will generate a random authorization cookie, store it in
Xauthority on the server, and verify that any forwarded connections carry
this cookie and replace it by the real cookie when the connection is opened.
The real authentication cookie is never sent to the server machine (and no
cookies are sent in the plain).

Forwarding of arbitrary TCP/IP connections over the secure channel can be
specified either on the command line or in a configuration file.

Note The X server
     This procedure assumes that you have a running X server on the client
     where you want to display the application from the remote host. The
     client may be of different architecture and operating system than the
     remote host, as long as it can run an X server, such as Cygwin (which
     implements an X.org server for MS Windows clients and others) or Exceed,
     it should be possible to set up a remote connection with any Linux or
     UNIX machine.



10.4.4.3. Server authentication

The ssh client/server system automatically maintains and checks a database
containing identifications for all hosts it has ever been used with. Host
keys are stored in $HOME/.ssh/known_hosts in the user's home directory.
Additionally, the file /etc/ssh/ssh_known_hosts is automatically checked for
known hosts. Any new hosts are automatically added to the user's file. If a
host's identification ever changes, ssh warns about this and disables
password authentication to prevent a Trojan horse from getting the user's
password. Another purpose of this mechanism is to prevent man-in-the-middle
attacks which could otherwise be used to circumvent the encryption. In
environments where high security is needed, sshd can even be configured to
prevent logins to machines whose host keys have changed or are unknown.



10.4.4.4. Secure remote copying

The SSH suite provides scp as a secure alternative to the rcp command that
used to be popular when only rsh existed. scp uses ssh for data transfer,
uses the same authentication and provides the same security as ssh. Unlike
rcp, scp will ask for passwords or passphrases if they are needed for
authentication:

lenny /var/tmp> scp Schedule.sdc.gz blob:/var/tmp/
lenny@blob's password:
Schedule.sdc.gz  100% |*****************************| 100 KB 00:00

lenny /var/tmp>

Any file name may contain a host and user specification to indicate that the
file is to be copied to/from that host. Copies between two remote hosts are
permitted. See the Info pages for more information.

If you would rather use an FTP-like interface, use sftp:

lenny /var/tmp> sftp blob
Connecting to blob...
lenny@blob's password:

sftp> cd /var/tmp

sftp> get Sch*
Fetching /var/tmp/Schedule.sdc.gz to Schedule.sdc.gz

sftp> bye

lenny /var/tmp>

Note Secure copy or FTP GUIs
     Don't feel comfortable with the command line yet? Try Konqueror's
     capabilities for secure remote copy, or install Putty.



10.4.4.5. Authentication keys

The ssh-keygen command generates, manages and converts authentication keys
for ssh. It can create RSA keys for use by SSH protocol version 1 and RSA or
DSA keys for use by SSH protocol version 2.

Normally each user wishing to use SSH with RSA or DSA authentication runs
this once to create the authentication key in $HOME/.ssh/identity, id_dsa or
id_rsa. Additionally, the system administrator may use this to generate host
keys for the system.

Normally this program generates the key and asks for a file in which to store
the private key. The public key is stored in a file with the same name but
.pub appended. The program also asks for a passphrase. The passphrase may be
empty to indicate no passphrase (host keys must have an empty passphrase), or
it may be a string of arbitrary length.

There is no way to recover a lost passphrase. If the passphrase is lost or
forgotten, a new key must be generated and copied to the corresponding public
keys.

We will study SSH keys in the exercises. All information can be found in the
man or Info pages.



10.4.5. VNC

VNC or Virtual Network Computing is in fact a remote display system which
allows viewing a desktop environment not only on the local machine on which
it is running, but from anywhere on the Internet and from a wide variety of
machines and architectures, including MS Windows and several UNIX
distributions. You could, for example, run MS Word on a Windows NT machine
and display the output on your Linux desktop. VNC provides servers as well as
clients, so the opposite also works and it may thus be used to display Linux
programs on Windows clients. VNC is probably the easiest way to have X
connections on a PC. The following features make VNC different from a normal
X server or commercial implementations:

  • No state is stored at the viewer side: you can leave your desk and resume
    from another machine, continuing where you left. When you are running a
    PC X server, and the PC crashes or is restarted, all remote applications
    that you were running will die. With VNC, they keep on running.

  • It is small and simple, no installation needed, can be run from a floppy
    if needed.

  • Platform independent with the Java client, runs on virtually everything
    that supports X.

  • Sharable: one desktop may be displayed on multiple viewers.

  • Free.

More information can be found in the VNC client man pages (man vncviewer) or
on the [http://www.realvnc.com/] VNC website.



10.4.6. The rdesktop protocol

In order to ease management of MS Windows hosts, recent Linux distributions
support the Remote Desktop Protocol (RDP), which is implemented in the
rdesktop client. The protocol is used in a number of Microsoft products,
including Windows NT Terminal Server, Windows 2000 Server, Windows XP and
Windows 2003 Server.

Surprise your friends (or management) with the fullscreen mode, multiple
types of keyboard layouts and single application mode, just like the real
thing. The man rdesktop manual provides more information. The project's
homepage is at [http://www.rdesktop.org/] http://www.rdesktop.org/.



10.4.7. Cygwin

[http://www.cygwin.com] Cygwin provides substantial UNIX functionality on MS
Windows systems. Apart from providing UNIX command line tools and graphical
applications, it can also be used to display a Linux desktop on an MS Windows
machine, using remote X. From a Cygwin Bash shell, type the command

/usr/X11R6/bin/XWin.exe -query your_linux_machine_name_or_IP

The connection is by default denied. You need to change the X Display Manager
(XDM) configuration and possibly the X Font Server (XFS) configuration to
enable this type of connection, where you get a login screen on the remote
machine. Depending on your desktop manager (Gnome, KDE, other), you might
have to change some configurations there, too.

If you do not need to display the entire desktop, you can use SSH in Cygwin,
just like explained in Section 10.4.4. without all the fuss of editing
configuration files.



10.5. Security

10.5.1. Introduction

As soon as a computer is connected to the network, all kinds of abuse becomes
possible, be it a UNIX-based or any other system. Admittedly, mountains of
papers have been spilled on this subject and it would lead us too far to
discuss the subject of security in detail. There are, however, a couple of
fairly logical things even a novice user can do to obtain a very secure
system, because most break-ins are the result of ignorant or careless users.

Maybe you are asking yourself if this all applies to you, using your computer
at home or working at your office on a desktop in a fairly protected
environment. The questions you should be asking yourself, however, are more
on the lines of:

  • Do you want to be in control of your own system?

  • Do you want to (unwittingly) participate in criminal activities?

  • Do you want your equipment to be used by someone else?

  • Do you want to take risks on losing your Internet connection?

  • Do you want to rebuild your system every time it has been hacked?

  • Do you want to risk personal or other data loss?

Presuming you don't, we will quickly list the steps you can take to secure
your machine. Extended information can be found in the [http://www.tldp.org/
HOWTO/Security-HOWTO.html] Linux Security HOWTO.



10.5.2. Services

The goal is to run as few services as possible. If the number of ports that
are open for the outside world are kept to a minimum, this is all the better
to keep an overview. If services can't be turned off for the local network,
try to at least disable them for outside connections.

A rule of thumb is that if you don't recognize a particular service, you
probably won't need it anyway. Also keep in mind that some services are not
really meant to be used over the Internet. Don't rely on what should be
running, check which services are listening on what TCP ports using the
netstat command:

[elly@mars ~] netstat -l | grep tcp
tcp     0      0 *:32769                 *:*      LISTEN
tcp     0      0 *:32771                 *:*      LISTEN
tcp     0      0 *:printer               *:*      LISTEN
tcp     0      0 *:kerberos_master       *:*      LISTEN
tcp     0      0 *:sunrpc                *:*      LISTEN
tcp     0      0 *:6001                  *:*      LISTEN
tcp     0      0 *:785                   *:*      LISTEN
tcp     0      0 localhost.localdom:smtp *:*      LISTEN
tcp     0      0 *:ftp                   *:*      LISTEN
tcp     0      0 *:ssh                   *:*      LISTEN
tcp     0      0 ::1:x11-ssh-offset      *:*      LISTEN

Things to avoid:

  • exec, rlogin and rsh, and telnet just to be on the safe side.

  • X11 on server machines.

  • No lp if no printer is physically attached.

  • No MS Windows hosts in the network, no Samba required.

  • Don't allow FTP unless an FTP server is required.

  • Don't allow NFS and NIS over the Internet, disable all related services
    on a stand-alone installation.

  • Don't run an MTA if you're not actually on a mail server.

  • ...

Stop running services using the chkconfig command, the initscripts or by
editing the (x)inetd configuration files.



10.5.3. Update regularly

Its ability to adapt quickly in an ever changing environment is what makes
Linux thrive. But it also creates a possibility that security updates have
been released even while you are installing a brand new version, so the first
thing you should do (and this goes for about any OS you can think of) after
installing is getting the updates as soon as possible. After that, update all
the packages you use regularly.

Some updates may require new configuration files, and old files may be
replaced. Check the documentation, and ensure that everything runs normal
after updating.

Most Linux distributions provide mailing list services for security update
announcements, and tools for applying updates to the system. General Linux
only security issues are reported among others at [http://linuxsecurity.com]
Linuxsecurity.com.

Updating is an ongoing process, so it should be an almost daily habit.



10.5.4. Firewalls and access policies

10.5.4.1. What is a firewall?

In the previous section we already mentioned firewall capabilities in Linux.
While firewall administration is one of the tasks of your network admin, you
should know a couple of things about firewalls.

Firewall is a vague term that can mean anything that acts as a protective
barrier between us and the outside world, generally the Internet. A firewall
can be a dedicated system or a specific application that provides this
functionality. Or it can be a combination of components, including various
combinations of hardware and software. Firewalls are built from "rules" that
are used to define what is allowed to enter and/or exit a given system or
network.

After disabling unnecessary services, we now want to restrict accepted
services as to allow only the minimum required connections. A fine example is
working from home: only the specific connection between your office and your
home should be allowed, connections from other machines on the Internet
should be blocked.



10.5.4.2. Packet filters

The first line of defense is a packet filter, which can look inside IP
packets and make decisions based on the content. Most common is the Netfilter
package, providing the iptables command, a next generation packet filter for
Linux.

One of the most noteworthy enhancements in the newer kernels is the stateful
inspection feature, which not only tells what is inside a packet, but also
detects if a packet belongs or is related to a new or existing connection.

The Shoreline Firewall or Shorewall for short is a front-end for the standard
firewall functionality in Linux.

More information can be found at [http://www.netfilter.org/] the Netfilter/
iptables project page.



10.5.4.3. TCP wrappers

TCP wrapping provides much the same results as the packet filters, but works
differently. The wrapper actually accepts the connection attempt, then
examines configuration files and decides whether to accept or reject the
connection request. It controls connections at the application level rather
than at the network level.

TCP wrappers are typically used with xinetd to provide host name and
IP-address-based access control. In addition, these tools include logging and
utilization management capabilities that are easy to configure.

The advantages of TCP wrappers are that the connecting client is unaware that
wrappers are used, and that they operate separately from the applications
they protect.

The host based access is controlled in the hosts.allow and hosts.deny files.
More information can be found in the TCP wrapper documentation files in /usr/
share/doc/tcp_wrappers[-<version>/] or /usr/share/doc/tcp and in the man
pages for the host based access control files, which contain examples.



10.5.4.4. Proxies

Proxies can perform various duties, not all of which have much to do with
security. But the fact that they are an intermediary make proxies a good
place to enforce access control policies, limit direct connections through a
firewall, and control how the network behind the proxy looks to the Internet.

Usually in combination with a packet filter, but sometimes all by themselves,
proxies provide an extra level of control. More information can be found in
the [http://www.tldp.org/HOWTO/Firewall-HOWTO.html] Firewall HOWTO or on the
Squid website.



10.5.4.5. Access to individual applications

Some servers may have their own access control features. Common examples
include Samba, X Window, Bind, Apache and CUPS. For every service you want to
offer check which configuration files apply.



10.5.4.6. Log files

If anything, the UNIX way of logging all kinds of activities into all kinds
of files confirms that "it is doing something." Of course, log files should
be checked regularly, manually or automatically. Firewalls and other means of
access control tend to create huge amounts of log files, so the trick is to
try and only log abnormal activities.



10.5.5. Intrusion detection

Intrusion Detection Systems are designed to catch what might have gotten past
the firewall. They can either be designed to catch an active break-in attempt
in progress, or to detect a successful break-in after the fact. In the latter
case, it is too late to prevent any damage, but at least we have early
awareness of a problem. There are two basic types of IDS: those protecting
networks, and those protecting individual hosts.

For host based IDS, this is done with utilities that monitor the file system
for changes. System files that have changed in some way, but should not
change, are a dead give-away that something is amiss. Anyone who gets in and
gets root access will presumably make changes to the system somewhere. This
is usually the very first thing done, either so he can get back in through a
backdoor, or to launch an attack against someone else, in which case, he has
to change or add files to the system. Some systems come with the tripwire
monitoring system, which is documented at the [http://www.tripwire.org]
Tripwire Open Source Project website.

Network intrusion detection is handled by a system that sees all the traffic
that passes the firewall (not by portscanners, which advertise usable ports).
[http://www.snort.org] Snort is an Open Source example of such a program.
Whitehats.com features an open Intrusion detection database, [http://
www.whitehats.com] arachNIDS.



10.5.6. More tips

Some general things you should keep in mind:

  • Do not allow root logins. UNIX developers came up with the su over two
    decades ago for extra security.

  • Direct root access is always dangerous and susceptible to human errors,
    be it by allowing root login or by using the su - command. Rather than
    using su, it is even better to use sudo to only execute the command that
    you need extra permissions for, and to return afterwards to your own
    environment.

  • Take passwords seriously. Use shadow passwords. Change your passwords
    regularly.

  • Try to always use SSH or SSL. Avoid telnet, FTP and E-mail clients and
    other client programs which send unencrypted passwords over the network.
    Security is not only about securing your computer, it is also about
    securing your passwords.

  • Limit resources using quota and/or ulimit.

  • The mail for root should be delivered to, or at least read by, an actual
    person.

  • The [http://www.sans.org] SANS institute has more tips and tricks, sorted
    per distribution, with mailing list service.

  • Check the origin of new software, get it from a trusted place/site.
    Verify new packages before installing.

  • When using a non-permanent Internet connection, shut it down as soon as
    you don't need it anymore.

  • Run private services on odd ports instead of the ones expected by
    possible hackers.

  • Know your system. After a while, you can almost feel when something is
    happening.



10.5.7. Have I been hacked?

How can you tell? This is a checklist of suspicious events:

  • Mysterious open ports, strange processes.

  • System utilities (common commands) behaving strange.

  • Login problems.

  • Unexplained bandwidth usage.

  • Damaged or missing log files, syslog daemon behaving strange.

  • Interfaces in unusual modes.

  • Unexpectedly modified configuration files.

  • Strange entries in shell history files.

  • Unidentified temporary files.



10.5.8. Recovering from intrusion

In short, stay calm. Then take the following actions in this order:

  • Disconnect the machine from the network.

  • Try to find out as much as you can about how your security was breached.

  • Backup important non-system data. If possible, check these data against
    existing backups, made before the system was compromised, to ensure data
    integrity.

  • Re-install the system.

  • Use new passwords.

  • Restore from system and data backups.

  • Apply all available updates.

  • Re-examine the system: block off unnecessary services, check firewall
    rules and other access policies.

  • Reconnect.



10.6. Summary

Linux and networking go hand in hand. The Linux kernel has support for all
common and most uncommon network protocols. The standard UNIX networking
tools are provided in each distribution. Next to those, most distributions
offer tools for easy network installation and management.

Linux is well known as a stable platform for running various Internet
services, the amount of Internet software is endless. Like UNIX, Linux can be
just as well used and administered from a remote location, using one of
several solutions for remote execution of programs.

We briefly touched the subject of security. Linux is an ideal firewall
system, light and cheap, but can be used in several other network functions
such as routers and proxy servers.

Increasing network security is mainly done by applying frequent updates and
common sense.

Here is an overview of network related commands:

Table 10-2. New commands in chapter 10: Networking


Command           Meaning                                                 

ftp               Transfer files to another host (insecure).              

host              Get information about networked hosts.                  

ifconfig          Display IP address information.                         

ip                Display IP address information.                         

netstat           Display routing information and network statistics.     

ping              Send answer requests to other hosts.                    

rdesktop          Display and MS Windows desktop on your Linux system.    

route             Show routing information.                               

scp               Secure copy files to and from other hosts.              

sftp              Secure FTP files to and from other hosts.               

ssh               Make an encrypted connection to another host.           

ssh-keygen        Generate authentication keys for Secure SHell.          

telnet            Make an insecure connection to another hosts.           

tracepath/        Print the route that packets follow to another host.    
traceroute                                                                

whois             Get information abotu a domain name.                    

xclock            X Window clock application, handy for testing remote    
                  display.                                                

xhost             X Window access control tool.                           



10.7. Exercises

10.7.1. General networking

  • Display network information for your workstation: IP address, routes,
    name servers.

  • Suppose no DNS is available. What would you do to reach your neighbour's
    machine without typing the IP address all the time?

  • How would you permanently store proxy information for a text mode browser
    such as links?

  • Which name servers handle the redhat.com domain?

  • Send an E-mail to your local account. Try two different ways to send and
    read it. How can you check that it really arrived?

  • Does your machine accept anonymous FTP connections? How do you use the
    ncftp program to authenticate with your user name and password?

  • Does your machine run a web server? If not, make it do so. Check the log
    files!



10.7.2. Remote connections

  • From your local workstation, display a graphical application, such as
    xclock on your neighbour's screen. The necessary accounts will have to be
    set up. Use a secure connection!

  • Set up SSH keys so you can connect to your neighbour's machine without
    having to enter a password.

  • Make a backup copy of your home directory in /var/tmp on your neighbour's
    "backup server," using scp. Archive and compress before starting the data
    transfer! Connect to the remote host using ssh, unpack the backup, and
    put one file back on the original machine using sftp.



10.7.3. Security

  • Make a list of open (listening) ports on your machine.

  • Supposing you want to run a web server. Which services would you
    deactivate? How would you do that?

  • Install available updates.

  • How can you see who connected to your system?

  • Make a repetitive job that reminds you to change your password every
    month, and preferably the root password as well.



Chapter 11. Sound and Video

    This chapter addresses the following tasks (briefly, as the field of
    sound and video is very wide):

      □ Sound card configuration

      □ Playing CDs, copying CDs,

      □ Playing music files

      □ Volume control

      □ Video and television

      □ Recording sound



11.1. Audio Basics

11.1.1. Installation

Most likely, your system is already installed with audio drivers and the
configuration was done at installation time. Likewise, should you ever need
to replace your audio hardware, most systems provide tools that allow easy
setup and configuration of the device. Most currently available plug-and-play
sound cards should be recognized automatically. If you can hear the samples
that are played during configuration, just click OK and everything will be
set up for you.

If your card is not detected automatically, you may be presented with a list
of sound cards and/or of sound card properties from which to choose. After
that, you will have to provide the correct I/O port, IRQ and DMA settings.
Information about these settings can be found in your sound card
documentation. If you are on a dual boot system with MS Windows, this
information can be found in the Windows Control Panel as well.

Note If automatic sound card detection fails
     If your soundcard is not supported by default, you will need to apply
     other techniques. These are described in the [http://www.tldp.org/HOWTO/
     Sound-HOWTO/index.html] Linux Sound HOWTO.



11.1.2. Drivers and Architecture

There are generally two types of sound architecture: the older Open Sound
System or OSS, which works with every UNIX-like system, and the newer
Advanced Linux Sound Architecture or ALSA, that has better support for Linux,
as the name indicates. ALSA also has more features and allows for faster
driver development. We will focus here on the ALSA system.

Today, almost all mainstream audio chipsets are supported. Only some high-end
professional solutions and some cards developed by manufacturers refusing to
document their chipset specifications are unsupported. An overview of
supported devices can be found on the ALSA site at [http://
www.alsa-project.org/alsa-doc/index.php?vendor=All#matrix] http://
www.alsa-project.org/alsa-doc/index.php?vendor=All#matrix.

Configuring systems installed with ALSA is done using the alsaconf tool.
Additionally, distributions usually provide their own tools for configuring
the sound card; these tools might even integrate the old and the new way of
handling sound devices.



11.2. Sound and video playing

11.2.1. CD playing and copying

The cdp package comes with most distributions and provides cdp or cdplay, a
text-based CD player. Desktop managers usually include a graphical tool, such
as the gnome-cd player in Gnome, that can be started from a menu.

Be sure to understand the difference between an audio CD and a data CD. You
do not have to mount an audio CD into the file system in order to listen to
it. This is because the data on such a CD are not Linux file system data;
they are accessed and sent to the audio output channel directly, using a CD
player program. If your CD is a data CD containing .mp3 files, you will first
need to mount it into the file system, and then use one of the programs that
we discuss below in order to play the music. How to mount CDs into the file
system is explained in Section 7.5.5.

The cdparanoia tool from the package with the same name reads audio directly
as data from the CD, without analog conversions, and writes data to a file or
pipe in different formats, of which .wav is probably the most popular.
Various tools for conversion to other formats, formats, such as .mp3, come
with most distributions or are downloadable as separate packages. The GNU
project provides several CD playing, ripping and encoding tools, database
managers; see the [http://www.gnu.org/directory/audio/] Free Software
Directory, Audio section for detailed information.

Audio-CD creation is eased, among many others, with the kaudiocreator tool
from the KDE suite. It comes with clear information from the KDE Help Center.

CD burning is covered in general in Section 9.2.2.



11.2.2. Playing music files

11.2.2.1. mp3 files

The popular .mp3 format is widely supported on Linux machines. Most
distributions include multiple programs that can play these files. Among many
other applications, XMMS, which is presented in the screenshot below, is one
of the most wide-spread, partially because it has the same look and feel as
the Windows tool.

Figure 11-1. XMMS mp3 player

[xmms]

Also very popular for playing music are AmaroK, a KDE application that is
steadily gaining popularity, and MPlayer, which can also play movies.

Note Restrictions
     Some distributions don't allow you to play MP3's without modifying your
     configuration, this is due to license restrictions on the MP3 tools. You
     might need to install extra software to be able to play your music.

In text mode, you can use the mplayer command:

[tille@octarine ~]$ mplayer /opt/mp3/oriental/*.mp3
MPlayer 1.0pre7-RPM-3.4.2 (C) 2000-2005 MPlayer Team
CPU: Advanced Micro Devices Duron Spitfire (Family: 6, Stepping: 1)
Detected cache-line size is 64 bytes
CPUflags:  MMX: 1 MMX2: 1 3DNow: 1 3DNow2: 1 SSE: 0 SSE2: 0
Playing /opt/oldopt/mp3/oriental/Mazika_Diana-Krozon_Super-Star_Ensani-Ma-
Bansak.mp3.
Cache fill:  1.17% (98304 bytes)    Audio file detected.
Clip info:
Title: Ensani-Ma-Bansak.mp3
Artist: Diana-Krozon
Album: Super-Star
Year:
Comment:
Genre: Unknown
==========================================================================
Opening audio decoder: [mp3lib] MPEG layer-2, layer-3
mpg123: Can't rewind stream by 450 bits!
AUDIO: 44100 Hz, 2 ch, s16le, 160.0 kbit/11.34% (ratio: 20000->176400)
Selected audio codec: [mp3] afm:mp3lib (mp3lib MPEG layer-2, layer-3)
==========================================================================
Checking audio filter chain for 44100Hz/2ch/s16le -> 44100Hz/2ch/s16le...
AF_pre: 44100Hz/2ch/s16le
AO: [oss] 44100Hz 2ch s16le (2 bps)
Building audio filter chain for 44100Hz/2ch/s16le -> 44100Hz/2ch/s16le...
Video: no video
Starting playback...
A: 227.8 (03:23:.1) 1.8% 12%



11.2.2.2. Other formats

It would lead us too far to discuss all possible audio formats and ways to
play them. An (incomplete) overview of other common sound playing and
manipulating software:

  • Ogg Vorbis: Free audio format: see [http://www.gnu.org/directory/audio/
    ogg/] the GNU audio directory for tools - they might be included in your
    distribution as well. The format was developed because MP3 is patented.

  • Real audio and video: realplay from [http://real.com] RealNetworks.

  • SoX or Sound eXchange: actually a sound converter, comes with th e play
    program. Plays .wav, . ogg and various other formats, including raw
    binary formats.

  • Playmidi: a MIDI player, see the GNU directory.

  • AlsaPlayer: from the Advanced Linux Sound Architecture project, see the
    [http://www.alsaplayer.org] AlsaPlayer web site.

  • mplayer: plays just about anything, including mp3 files. More info on the
    [http://www.mplayerhq.hu/] MPlayerHQ website.

  • hxplay: supports RealAudio and RealVideo, mp3, mp4 audio, Flash, wav and
    more, see [http://www.helixdna.com] HelixDNA (not all components of this
    software are completely free).

  • rhythmbox: based on the GStreamer framework, can play everything
    supported in GStreamer, which claims to be able to play everything, see
    the [http://www.rhythmbox.org] Rhythmbox and [http://
    gstreamer.freedesktop.org] GStreamer sites.

Check your system documentation and man pages for particular tools and
detailed explanations on how to use them.

Note I don't have these applications on my system!
     A lot of the tools and applications discussed in the above sections are
     optional software. It is possible that such applications are not
     installed on your system by default, but that you can find them in your
     distribution as additional packages. It might also very well be that the
     application that you are looking for is not in your distribution at all.
     In that case, you need to download it from the application's web site.



11.2.2.3. Volume control

aumix and alsamixer are two common text tools for adjusting audio controls.
Use the arrow keys to toggle settings. The alsamixer has a graphical
interface when started from the Gnome menu or as gnome-alsamixer from the
command line. The kmix tool does the same in KDE.

Regardless of how you choose to listen to music or other sounds, remember
that there may be other people who may not be interested in hearing you or
your computer. Try to be courteous, especially in office environments. Use a
quality head-set, rather than the ones with the small ear pieces. This is
better for your ears and causes less distraction for your colleagues.



11.2.3. Recording

Various tools are again available that allow you to record voice and music.
For recording voice you can use arecord on the command line:

alexey@russia:~> arecord /var/tmp/myvoice.wav
Recording WAVE '/var/tmp/myvoice.wav' : Unsigned 8 bit, Rate 8000 Hz, Mono
Aborted by signal Interrups...

"Interrupt" means that the application has caught a Ctrl+C. Play the sample
using the simple play command.

This is a good test that you can execute prior to testing applications that
need voice input, like Voice over IP (VoIP). Keep in mind that the microphone
input should be activated. If you don't hear your own voice, check your sound
settings. It often happens that the microphone is muted or on verry low
volume. This can be easily adjusted using alsamixer or your
distribution-specific graphical interface to the sound system.

In KDE you can start the krec utility, Gnome provides the
gnome-sound-recorder.



11.3. Video playing, streams and television watching

Various players are available:

  • xine: a free video player

  • ogle: DVD player.

  • okle: KDE version of ogle

  • mplayer: Movie Player for Linux

  • totem: plays both audio and video files, audio CDs, VCD and DVD.

  • realplay: from RealNetworks.

  • hxplay: a Real alternative, see [http://helixcommunity.org] HelixDNA.

  • kaffeine: media player for KDE3.

Most likely, you will find one of these in your graphical menus.

Keep in mind that all codecs necessary for viewing different types of video
might not be on your system by default. You can get a long way downloading
W32codecs and libdvdcss.

The [http://www.tldp.org/] LDP released a document that is very appropriate
for this section. It is entitled [http://www.tldp.org/HOWTO/
DVD-Playback-HOWTO/index.html] DVD Playback HOWTO and describes the different
tools available for viewing movies on a system that has a DVD drive. It is a
fine addition to the [http://www.tldp.org/HOWTO/DVD-HOWTO.html] DVD HOWTO
that explains installation of the drive.

For watching TV there is choice of the following tools, among many others for
watching and capturing TV, video and other streams:

  • tvtime: great program with station management, interaction with teletext,
    film mode and [http://tvtime.sourceforge .net/] much more.

  • zapping: Gnome-specific TV viewer.

  • xawtv: X11 TV viewer.



11.4. Internet Telephony

11.4.1. What is it?

Internet telephony, or more common, Voice over IP (VoIP) or digital telephony
allows parties to exchange voice data flows over the network. The big
difference is that the data flows over a general purpose network, the
Internet, contrary to conventional telephony, that uses a dedicated network
of voice transmission lines. The two networks can be connected, however,
under special circumstances, but for now this is certainly not a standard. In
other words: it is very likely that you will not be able to call people who
are using a conventional telephone. If it is possible at all, it is likely
that you will need to pay for a subscription.

While there are currently various applications available for free download,
both free and proprietary, there are some major drawbacks to telephony over
the Internet. Most noticably, the system is unreliable, it can be slow or
there can be a lot of noise on the connection, and it can thus certainly not
be used to replace conventional telephony - think about emergency calls.
While some providers take their precautions, there is no guarantee that you
can reach the party that you want to call.

Most applications currently do not use encryption, so be aware that it is
potentially easy for someone to eavesdrop on your conversations. If security
is a concern for you, read the documentation that comes with your VoIP
client. Additionally, if you are using a firewall, it should be configured to
allow incoming connections from anywhere, so using VoIP also includes taking
risks on the level of site security.



11.4.2. What do you need?

11.4.2.1. Server Side

First of all, you need a provider offering the service. This service might
integrate traditional telephony and it might or might not be free. Among
others are [http://www.sipphone.com/] SIPphone, [http://www.vonage.com/]
Vonage, [http://www.lingo.com/] Lingo, [http://www.totaltalk.com/] AOL
TotalTalk and many locally accessible providers offering the so-called "full
phone service". Internet phone service only is offered by [http://
www.skype.com/] Skype, [http://www.sipbroker.com/] SIP Broker, [http://
www.google.com/talk/] Google and many others.

If you want to set up a server of your own, you might want to look into
[http://www.asterisk.org] Asterisk.



11.4.2.2. Client Side

On the client side, the applications that you can use depend on your network
configuration. If you have a direct Internet connection, there won't be any
problems, provided that you know on what server you can connect, and usually
that you also have a username and password to authenticate to the service.

If you are behind a firewall that does Network Address Translation (NAT),
however, some services might not work, as they will only see the IP address
of the firewall and not the address of your computer, which might well be
unroutable over the Internet, for instance when you are in a company network
and your IP address starts with 10., 192.168. or another non-routable subnet
prefix. This depends on the protocol that is used by the application.

Also, available bandwidth might be a blocking factor: some applications are
optimized for low bandwidth consumption, while others might require high
bandwidth connections. This depends on the codec that is used by the
application.

Among the most common applications are the Skype client, which has an
interface that reminds of instant messaging, and [http://www.counterpath.com
/] X-Lite, the free version of the XTen softphone, which looks like a mobile
telephone. However, while these programs are available for free download and
very popular, they are not free as in free speech: they use proprietary
protocols and/or are only available in binary packages, not in source format.

Free and open VoIP clients are for instance [http://www.gizmoproject.com/]
Gizmo, [http://www.linphone.org] Linphone, [http://www.gnomemeeting.org/]
GnomeMeeting and [http://www.wirlab.net/kphone/] KPhone.

Tip Client hardware
    While your computer, especially if it is a laptop PC, might have a
    built-in microphone, the result will be far better if you connect a
    headset. If you have the choice, opt for a USB headset, as it functions
    independently from existing audio hardware. Use alsamixer to configure
    input and output sound levels to your taste.

VoIP applications are definitely a booming market. Volunteers try to document
the current status at [http://www.voip-info.org/] http://www.voip-info.org/.



11.5. Summary

The GNU/Linux platform is fully multi-media enabled. A wide variety of
devices like sound cards, tv-cards, headsets, microphones, CD and DVD players
is supported. The list of applications is sheer endless, that is why we
needed to shortenthe list of new commands below and limit ourselves to
general audio commands.

Table 11-1. New commands in chapter 11: Audio


Command             Meaning                                     

alsaconf            Configure the ALSA sound system.            

alsamixer           Tune output levels of ALSA driver.          

arecord             Record a sound sample.                      

aumix               Audio mixer tool.                           

cdp                 Play an audio CD.                           

cdparanoia          Rip an audio CD.                            

cdplay              Play an audio CD.                           

gnome-alsamixer     Gnome ALSA front-end.                       

gnome-cd            Gnome front-end for playing audio CDs.      

gnome-sound-recorderGnome front-end for recording sound samples.

kaudiocreator       KDE front-end for creating audio CDs.       

kmix                KDE front-end for sound settings.           

krec                KDE front-end for recording sound samples.  

mplayer             Multi-media player.                         

play                Command line tool for playing sound samples.



11.6. Exercises

 1. From the Gnome or KDE menu, open your sound configuration panel. Make
    sure audio boxes or headset are connected to your system and find an
    output level that is comfortable for you. Make sure, when your system is
    ALSA-compatible, that you use the appropriate panel.

 2. If you have a microphone, try recording a sample of your own voice. Make
    sure the input volume is not too high, as this will result in
    high-pitched tones when you communicate with others, or in transfering
    background noise to the other party. On the command line, you might even
    try to use arecord and aplay for recording and playing sound.

 3. Locate sound files on your system and try to play them.

 4. Insert an audio CD and try to play it.

 5. Find a chat partner and configure a VoIP program. (You might need to
    install one first.)

 6. Can you listen to Internet radio?

 7. If you have a DVD player and a movie on a DVD disk, try to play it.



Appendix A. Where to go from here?

    This document gives an overview of useful books and sites.



A.1. Useful Books

A.1.1. General Linux

  • "Linux in a Nutshell" by Ellen Siever, Jessica P. Hackman, Stephen
    Spainhour, Stephen Figgins, O'Reilly UK, ISBN 0596000251

  • "Running Linux" by Matt Welsh, Matthias Kalle Dalheimer, Lar Kaufman,
    O'Reilly UK, ISBN 156592469X

  • "Linux Unleashed" by Tim Parker, Bill Ball, David Pitts, Sams, ISBN
    0672316889

  • "When You Can't Find Your System Administrator" by Linda Mui, O'Reilly
    UK, ISBN 1565921046

  • When you actually buy a distribution, it will contain a very decent user
    manual.



A.1.2. Editors

  • "Learning the Vi Editor" by Linda Lamb and Arnold Robbins, O'Reilly UK,
    ISBN 1565924266

  • "GNU Emacs Manual" by Richard M.Stallman, iUniverse.Com Inc., ISBN
    0595100333

  • "Learning GNU Emacs" by Debra Cameron, Bill Rosenblatt and Eric Raymond,
    O'Reilly UK, ISBN 1565921526

  • "Perl Cookbook" by Tom Christiansen and Nathan Torkington, O'Reilly UK,
    ISBN 1565922433



A.1.3. Shells

  • "Unix Shell Programming" by Stephen G.Kochan and Patrick H.Wood, Sams
    Publishing, ISBN 067248448X

  • "Learning the Bash Shell" by Cameron Newham and Bill Rosenblatt, O'Reilly
    UK, ISBN 1565923472

  • "The Complete Linux Shell Programming Training Course" by Ellie Quigley
    and Scott Hawkins, Prentice Hall PTR, ISBN 0130406767

  • "Linux and Unix Shell Programming" by David Tansley, Addison Wesley
    Publishing Company, ISBN 0201674726

  • "Unix C Shell Field Guide" by Gail and Paul Anderson, Prentice Hall, ISBN
    013937468X



A.1.4. X Window

  • "Gnome User's Guide" by the Gnome Community, iUniverse.Com Inc., ISBN
    0595132251

  • "KDE Bible" by Dave Nash, Hungry Minds Inc., ISBN 0764546929

  • "The Concise Guide to XFree86 for Linux" by Aron HSiao, Que, ISBN
    0789721821

  • "The New XFree86" by Bill Ball, Prima Publishing, ISBN 0761531521

  • "Beginning GTK+ and Gnome" by Peter Wright, Wrox Press, ISBN 1861003811

  • "KDE 2.0 Development" by David Sweet and Matthias Ettrich, Sams
    Publishing, ISBN 0672318911

  • "GTK+/Gnome Application Development" by Havoc Pennington, New Riders
    Publishing, ISBN 0735700788



A.1.5. Networking

  • "TCP/IP Illustrated, Volume I: The Protocols" by W. Richard Stevens,
    Addison-Wesley Professional Computing Series, ISBN 0-201-63346-9

  • "DNS and BIND" by Paul Albitz, Cricket Liu, Mike Loukides and Deborah
    Russell, O'Reilly & Associates, ISBN 0596001584

  • "The Concise Guide to DNS and BIND" by Nicolai Langfeldt, Que, ISBN
    0789722739

  • "Implementing LDAP" by Mark Wilcox, Wrox Press, ISBN 1861002211

  • "Understanding and deploying LDAP directory services" by Tim Howes and
    co., Sams, ISBN 0672323168

  • "Sendmail" by Brian Costales and Eric Allman, O'Reilly UK, ISBN
    1565922220

  • "Removing the Spam : Email Processing and Filtering" by Geoff Mulligan,
    Addison Wesley Publishing Company, ISBN 0201379570

  • "Managing IMAP" by Dianna & Kevin Mullet, O'Reilly UK, ISBN 059600012X



A.2. Useful sites

A.2.1. General information

  •  [http://www.tldp.org] The Linux documentation project: all docs,
    manpages, HOWTOs, FAQs

  •  [http://www.linuxquestions.org] LinuxQuestions.org: forum, downloads,
    docs and much more

  •  [http://www.google.com/linux] Google for Linux: the specialized search
    engine

  •  [http://groups.google.com] Google Groups: an archive of all newsgroup
    postings, including the comp.os.linux hierarchy

  •  [http://slashdot.org] Slashdot: daily news

  •  [http://www.oreilly.com] http://www.oreilly.com: books on Linux System
    and Network administration, Perl, Java, ...

  •  [http://www.posix.com/posix.html] POSIX: the standard

  •  [http://www.linuxhq.com] Linux HQ: Maintains a complete database of
    source, patches and documentation for various versions of the Linux
    kernel.



A.2.2. Architecture Specific References

  •  [http://www.alphalinux.org] AlphaLinux: Linux on Alpha architecture
    (e.g. Digital Workstation)

  •  [http://www.linux-mips.org] Linux-MIPS: Linux on MIPS (e.g. SGI Indy)

  •  [http://tldp.org/LDP/Mobile-Guide/html/index.html] Linux on the Road:
    Specific guidelines for installing and running Linux on laptops, PDAs,
    mobile phones and so on. Configuration files for various models.

  •  [http://www.mklinux.org] MkLinux: Linux on Apple



A.2.3. Distributions

  •  [http://fedora.redhat.com/] The Fedora Project: RedHat-sponsored
    community effort OS.

  •  [http://www.mandriva.com] Mandriva

  •  [http://www.ubuntu.com] Ubuntu

  •  [http://www.debian.org] Debian

  •  [http://www.turbolinux.com] TurboLinux

  •  [http://www.slackware.com] Slackware

  •  [http://www.suse.de] SuSE

  •  [http://www.linuxiso.org] LinuxISO.org: CD images for all distributions.

  •  [http://www.knoppix.org/] Knoppix: distribution that runs from a CD, you
    don't need to install anything for this one.

  •  [http://www.distrowatch.com/] DistroWatch.com: find a Linux that goes
    with your style.

  • ...



A.2.4. Software

  •  [http://freshmeat.net] Freshmeat: new software, software archives

  •  [http://www.openssh.org] OpenSSH: Secure SHell site

  •  [http://www.openoffice.org] OpenOffice: MS compatible Office Suite

  •  [http://www.kde.org] KDE: K Desktop site

  •  [http://www.gnu.org] GNU: GNU and GNU software

  •  [http://www.gnome.org] Gnome: The official Gnome site

  •  [http://www.rpmfind.net] RPM Find: all RPM packages

  •  [http://www.samba.org] Samba: MS Windows file and print services

  •  [http://www.openldap.org] Home of the OpenLDAP Project: OpenLDAP server/
    clients/utilities, FAQ and other documentation.

  •  [http://www.sendmail.org] Sendmail Homepage: A thorough technical
    discussion of Sendmail features, includes configuration examples.

  •  [http://netfilter.samba.org] Netfilter: contains assorted information
    about iptables: HOWTO, FAQ, guides, ...

  •  [http://www.gimp.org] Official GIMP website: All information about the
    GNU Image Manipulation Program.

  •  [http://sourceforge.net/] SourceForge.net: Open SOurce software
    development site.

  •  [http://www.vim.org/] vIm homepage



Appendix B. DOS versus Linux commands

    In this appendix, we matched DOS commands with their Linux equivalent.

As an extra means of orientation for new users with a Windows background, the
table below lists MS-DOS commands with their Linux counterparts. Keep in mind
that Linux commands usually have a number of options. Read the Info or man
pages on the command to find out more.

Table B-1. Overview of DOS/Linux commands


DOS commandsLinux command                  

<command> /?man <command> or command --help

cd          cd                             

chdir       pwd                            

cls         clear                          

copy        cp                             

date        date                           

del         rm                             

dir         ls                             

echo        echo                           

edit        vim (or other editor)          

exit        exit                           

fc          diff                           

find        grep                           

format      mke2fs or mformat              

mem         free                           

mkdir       mkdir                          

more        more or even less              

move        mv                             

ren         mv                             

time        date                           



Appendix C. Shell Features

    This document gives an overview of common shell features (the same in
    every shell flavour) and differing shell features (shell specific
    features).



C.1. Common features

The following features are standard in every shell. Note that the stop,
suspend, jobs, bg and fg commands are only available on systems that support
job control.

Table C-1. Common Shell Features


Command Meaning                                              

>       Redirect output                                      

>>      Append to file                                       

<       Redirect input                                       

<<      "Here" document (redirect input)                     

|       Pipe output                                          

&       Run process in background.                           

;       Separate commands on same line                       

*       Match any character(s) in filename                   

?       Match single character in filename                   

[ ]     Match any characters enclosed                        

( )     Execute in subshell                                  

` `     Substitute output of enclosed command                

" "     Partial quote (allows variable and command expansion)

' '     Full quote (no expansion)                            

\       Quote following character                            

$var    Use value for variable                               

$$      Process id                                           

$0      Command name                                         

$n      nth argument (n from 0 to 9)                         

$*      All arguments as a simple word                       

#       Begin comment                                        

bg      Background execution                                 

break   Break from loop statements                           

cd      Change directories                                   

continueResume a program loop                                

echo    Display output                                       

eval    Evaluate arguments                                   

exec    Execute a new shell                                  

fg      Foreground execution                                 

jobs    Show active jobs                                     

kill    Terminate running jobs                               

newgrp  Change to a new group                                

shift   Shift positional parameters                          

stop    Suspend a background job                             

suspend Suspend a foreground job                             

time    Time a command                                       

umask   Set or list file permissions                         

unset   Erase variable or function definitions               

wait    Wait for a background job to finish                  



C.2. Differing features

The table below shows major differences between the standard shell (sh),
Bourne Again SHell (bash), Korn shell (ksh) and the C shell (csh).

Note Shell compatibility
     Since the Bourne Again SHell is a superset of sh, all sh commands will
     also work in bash - but not vice versa. bash has many more features of
     its own, and, as the table below demonstrates, many features
     incorporated from other shells.

     Since the Turbo C shell is a superset of csh, all csh commands will work
     in tcsh, but not the other way round.

Table C-2. Differing Shell Features


sh      bash              ksh      csh     Meaning/Action              

$       $                 $        %       Default user prompt         

        >|                >|       >!      Force redirection           

> file  &> file or > file > file 2>>& file Redirect stdout and stderr  
2>&1    2>&1              &1               to file                     

        { }                        { }     Expand elements in list     

`command`command` or $    $        `commandSubstitute output of        
`       (command)         (command)`       enclosed command            

$HOME   $HOME             $HOME    $home   Home directory              

        ~                 ~        ~       Home directory symbol       

        ~+, ~-, dirs      ~+, ~-   =-, =N  Access directory stack      

var=    VAR=value         var=valueset var=Variable assignment         
value                              value                               

export  export VAR=value  export   setenv  Set environment variable    
var                       var=val  var val                             

        ${nnnn}           ${nn}            More than 9 arguments can be
                                           referenced                  

"$@"    "$@"              "$@"             All arguments as separate   
                                           words                       

$#      $#                $#       $#argv  Number of arguments         

$?      $?                $?       $status Exit status of the most     
                                           recently executed command   

$!      $!                $!               PID of most recently        
                                           backgrounded process        

$-      $-                $-               Current options             

. file  source file or .  . file   source  Read commands in file       
        file                       file                                

        alias x='y'       alias x=yalias x Name x stands for command y 
                                   y                                   

case    case              case     switch  Choose alternatives         
                                   or case                             

done    done              done     end     End a loop statement        

esac    esac              esac     endsw   End case or switch          

exit n  exit n            exit n   exit    Exit with a status          
                                   (expr)                              

for/do  for/do            for/do   foreach Loop through variables      

        set -f , set -o                    Ignore substitution         
        nullglob|dotglob|          noglob  characters for filename     
        nocaseglob|noglob                  generation                  

hash    hash              alias -t hashstatDisplay hashed commands     
                                           (tracked aliases)           

hash    hash cmds         alias -t rehash  Remember command locations  
cmds                      cmds                                         

hash -r hash -r                    unhash  Forget command locations    

        history           history  history List previous commands      

        ArrowUp+Enter or  r        !!      Redo previous command       
        !!                                                             

        !str              r str    !str    Redo last command that      
                                           starts with "str"           

                                   !cmd:s/xReplace "x" with "y" in most
        !cmd:s/x/y/       r x=y cmd/y/     recent command starting with
                                           "cmd", then execute.        

if [ $i if [ $i -eq 5 ]   if ((i==5if ($i==Sample condition test       
-eq 5 ]                   ))       5)                                  

fi      fi                fi       endif   End if statement            

ulimit  ulimit            ulimit   limit   Set resource limits         

pwd     pwd               pwd      dirs    Print working directory     

read    read              read     $<      Read from terminal          

trap 2  trap 2            trap 2   onintr  Ignore interrupts           

        unalias           unalias  unalias Remove aliases              

until   until             until            Begin until loop            

while/dowhile/do          while/do while   Begin while loop            


The Bourne Again SHell has many more features not listed here. This table is
just to give you an idea of how this shell incorporates all useful ideas from
other shells: there are no blanks in the column for bash. More information on
features found only in Bash can be retrieved from the Bash info pages, in the
"Bash Features" section.

More information:

You should at least read one manual, being the manual of your shell. The
preferred choice would be info bash, bash being the GNU shell and easiest for
beginners. Print it out and take it home, study it whenever you have 5
minutes.

See Appendix B if you are having difficulties to assimilate shell commands.

Glossary

    This section contains an alphabetical overview of commands discussed in
    this document.

A

a2ps

    Format files for printing on a PostScript printer, see Section 8.1.2.

acroread

    PDF viewer, see Section 8.1.2.2.

adduser

    Create a new user or update default new user information.

alias

    Create a shell alias for a command.

alsaconf

    Configure sound card using the ALSA driver, see Section 11.1.2.

alsamixer

    Tune ALSA sound device output, see Section 11.2.2.3.

anacron

    Execute commands periodically, does not assume continuously running
    machine.

apropos

    Search the whatis database for strings, see Section 2.3.3.2.

apt-get

    APT package handling utility, see Section 7.5.3.2.

arecord

    Record a sound sample, see Section 11.2.3.

aspell

    Spell checker.

at, atq, atrm

    Queue, examine or delete jobs for later execution, see Section 4.1.2.2
    and Section 4.4.3.

aumix

    Adjust audio mixer, see Section 11.2.2.3.

(g)awk

    Pattern scanning and processing language.

B

bash

    Bourne Again SHell, see Section 3.2.3.2 and Section 7.2.5.

batch

    Queue, examine or delete jobs for later execution, see Section 4.1.2.2.

bg

    Run a job in the background, see Section 4.1.2.1.

bitmap

    Bitmap editor and converter utilities for the X window System.

bzip2

    A block-sorting file compressor, see Section 9.1.1.3.

C

cardctl

    Manage PCMCIA cards, see Section 10.2.3.3.

cat

    Concatenate files and print to standard output, see Section 2.2 and
    Section 3.2.4.

cd

    Change directory, see Section 2.2.

cdp/cdplay

    An interactive text-mode program for controlling and playing audio CD
    Roms under Linux, see Section 11.2.1.

cdparanoia

    An audio CD reading utility which includes extra data verification
    features, see Section 11.2.1.

cdrecord

    Record a CD-R, see Section 9.2.2.

chattr

    Change file attributes.

chgrp

    Change group ownership, see Section 3.4.2.3.

chkconfig

    Update or query run level information for system services, see Section
    4.2.5.1.

chmod

    Change file access permissions, see Section 3.4.1, Section 3.4.2.1 and
    Section 3.4.2.4.

chown

    Change file owner and group, see Section 3.4.2.3.

compress

    Compress files.

cp

    Copy files and directories, see Section 3.3.2.

crontab

    Maintain crontab files, see Section 4.4.4.

csh

    Open a C shell, see Section 3.2.3.2.

cut

    Remove sections from each line of file(s), see Section 7.2.5.2.

D

date

    Print or set system date and time.

dd

    Convert and copy a file (disk dump), see Section 9.2.1.2.

df

    Report file system disk usage, see Section 3.1.2.3.

dhcpcd

    DHCP client daemon, see Section 10.3.8.

diff

    Find differences between two files.

dig

    Send domain name query packets to name servers, see Section 10.2.6.1.

dmesg

    Print or control the kernel ring buffer.

du

    Estimate file space usage.

dump

    Backup file system, see Section 9.2.5.

E

echo

    Display a line of text, see Section 3.2.1.

ediff

    Diff to English translator.

egrep

    Extended grep.

eject

    Unmount and eject removable media, see Section 7.5.5.2.

emacs

    Start the Emacs editor, see Section 6.1.2.1.

exec

    Invoke subprocess(es), see Section 4.1.5.1.

exit

    Exit current shell, see Section 2.2.

export

    Add function(s) to the shell environment, see Section 3.2.1, Section
    7.2.1.2 and Section 7.2.4.2.

F

fax2ps

    Convert a TIFF facsimile to PostScript, see Section 8.1.2.

fdformat

    Format floppy disk, see Section 9.2.1.1.

fdisk

    Partition table manipulator for Linux, see Section 3.1.2.2.

fetchmail

    Fetch mail from a POP, IMAP, ETRN or ODMR-capable server, see Section
    10.3.2.3.

fg

    Bring a job in the foreground, see Section 4.1.2.1.

file

    Determine file type, see Section 3.3.1.2.

find

    Find files, see Section 3.3.3.3.

firefox

    Web browser, see Section 10.3.3.2.

fork

    Create a new process, see Section 4.1.5.1.

formail

    Mail (re)formatter, see Section 10.3.2.3.

fortune

    Print a random, hopefully interesting adage.

ftp

    Transfer files (unsafe unless anonymous account is used!)services, see
    Section 10.3.4.2.

G

galeon

    Graphical web browser.

gdm

    Gnome Display Manager, see Section 4.2.4.

gedit

    GUI editor, see Section 6.3.3.3.

(min/a)getty

    Control console devices.

gimp

    Image manipulation program.

gpg

    Encrypt, check and decrypt files, see Section 9.4.1.2.

grep

    Print lines matching a pattern, see Section 3.3.3.4 and Section 5.3.1.

groff

    Emulate nroff command with groff, see Section 8.1.2.

grub

    The grub shell, see Section 4.2.3 and Section 7.5.4.

gv

    A PostScript and PDF viewer, see Section 8.1.2.2.

gvim

    Graphical version of the vIm editor, see Section 6.3.3.3.

gzip

    Compress or expand files, see Section 9.1.1.3.

H

halt

    Stop the system, see Section 4.2.6.

head

    Output the first part of files, see Section 3.3.4.3.

help

    Display help on a shell built-in command.

host

    DNS lookup utility, see Section 10.2.6.1.

httpd

    Apache hypertext transfer protocol server, see Section 10.2.3.1.

I

id

    Print real and effective UIDs and GIDs, see Section 3.4.1.

ifconfig

    Configure network interface or show configuration, see Section 10.1.2.3.

info

    Read Info documents, see Section 2.3.3.1.

init

    Process control initialization, see Section 4.1.5.1, Section 4.2.4 and
    Section 4.2.5.

insserv

    Manage init scripts, see Section 4.2.5.1.

iostat

    Display I/O statistics, see Section 4.3.5.4.

ip

    Display/change network interface status, see Section 10.1.2.3.

ipchains

    IP firewall administration, see Section 10.4.4.2.

iptables

    IP packet filter administration, see Section 10.4.4.2.

J

jar

    Java archive tool, see Section 9.1.1.4.

jobs

    List backgrounded tasks.

K

kdm

    Desktop manager for KDE, see Section 4.2.4.

kedit

    KDE graphical editor, see Section 6.3.3.3.

kill(all)

    Terminate process(es), see Section 4.1.2.1.

konqueror

    File manager, (help) browser, see Section 3.3.2.1.

ksh

    Open a Korn shell, see Section 3.2.3.2.

kwrite

    KDE graphical editor, see Section 6.3.3.3.

L

less

    more with features, see Section 3.3.4.2.

lilo

    Linux boot loader, see Section 4.2.

links

    Text mode WWW browser, see Section 10.2.3.2.

ln

    Make links between files, see Section 3.3.5.

loadkeys

    Load keyboard translation tables, see Section 7.4.1.

locate

    Find files, see Section 3.3.3.3 and Section 4.4.4.

logout

    Close current shell, see Section 2.1.3.

lp

    Send requests to the LP print service, see Section 8.1.

lpc

    Line printer control program, see Section 8.1.

lpq

    Print spool queue examination program, see Section 8.1.

lpr

    Offline print, see Section 8.1.

lprm

    Remove print requests, see Section 8.1.

ls

    List directory content, see Section 2.2, Section 3.1.1.2 and Section
    3.3.1.1.

lynx

    Text mode WWW browser, see Section 10.2.3.2.

M

mail

    Send and receive mail, see Section 10.3.2.3.

man

    Read man pages, see Section 2.3.2.

mc

    Midnight COmmander, file manager, see Section 3.3.2.1.

mcopy

    Copy MSDOS files to/from Unix.

mdir

    Display an MSDOS directory.

memusage

    Display memory usage, see Section 4.3.5.3.

memusagestat

    Display memory usage statistics, see Section 4.3.5.3.

mesg

    Control write access to your terminal, see Section 4.1.6.

mformat

    Add an MSDOS file system to a low-level formatted floppy disk, see
    Section 9.2.1.1.

mkbootdisk

    Creates a stand-alone boot floppy for the running system.

mkdir

    Create directory, see Section 3.3.2.

mkisofs

    Create a hybrid ISO9660 filesystem, see Section 9.2.2.

mplayer

    Movie player/encoder for Linux, see Section 11.2.2 and Section 11.3.

more

    Filter for displaying text one screen at the time, see Section 3.3.4.2.

mount

    Mount a file system or display information about mounted file systems,
    see Section 7.5.5.1.

mozilla

    Web browser, see Section 10.2.3.2.

mt

    Control magnetic tape drive operation.

mtr

    Network diagnostic tool.

mv

    Rename files, Section 3.3.2.

N

named

    Internet domain name server, see Section 10.3.7.

nautilus

    File manager, see Section 3.3.2.1.

ncftp

    Browser program for ftp services (insecure!), see Section 10.3.4.2.

netstat

    Print network connections, routing tables, interface statistics,
    masquerade connections, and multi-cast memberships, see Section 10.1.2.5
    and Section 10.4.2.

newgrp

    Log in to another group, see Section 3.4.2.2.

nfsstat

    Print statistics about networked file systems.

nice

    Run a program with modified scheduling priority, see Section 4.3.5.1.

nmap

    Network exploration tool and security scanner.

ntpd

    Network Time Protocol Daemon, see Section 7.4.3.

ntpdate

    Set the date and time via an NTP server, see Section 7.4.3.

ntsysv

    Simple interface for configuring run levels, see Section 4.2.5.1.

O

ogle

    DVD player with support for DVD menus, see Section 11.3.

P

passwd

    Change password, see Section 2.2 and Section 4.1.6.

pccardctl

    Manage PCMCIA cards, see Section 10.2.3.3.

pdf2ps

    Ghostscript PDF to PostScript translator, see Section 8.1.2.

perl

    Practical Extraction and Report Language.

pg

    Page through text output, see Section 3.3.4.2.

pgrep

    Look up processes based on name and other attributes, see Section 4.1.4.

ping

    Send echo request to a host, see Section 10.2.6.2.

play

    Play a sound sample, see Section 11.2.3.

pr

    Convert text files for printing.

printenv

    Print all or part of environment, see Section 7.2.1.

procmail

    Autonomous mail processor, see Section 10.3.2.3.

ps

    Report process status, see Section 4.1.4 and Section 4.3.5.4.

pstree

    Display a tree of processes, see Section 4.1.4.

pwd

    Print present working directory, see Section 2.2.

Q

quota

    Display disk usage and limits, see Section 3.2.3.3.

R

rcp

    Remote copy (unsafe!)

rdesktop

    Remote Desktop Protocol client, see Section 10.4.6.

reboot

    Stop the system, see Section 4.2.6.

recode

    Convert files to another character set, see Section 7.4.4.

renice

    Alter priority of a running process, see Section 4.3.5.1.

restore

    Restore backups made with dump, see Section 9.2.5.

rlogin

    Remote login (telnet, insecure!), see Section 10.4.2 and Section 10.5.2.

rm

    Remove a file, see Section 3.3.2.

rmdir

    Remove a directory, see Section 3.3.2.2.

roff

    A survey of the roff typesetting system, see Section 8.1.2.

rpm

    RPM Package Manager, see Section 7.5.2.1.

rsh

    Remote shell (insecure!), see Section 10.4.2.

rsync

    Synchronize two directories, see Section 9.3.

S

scp

    Secure remote copy, see Section 10.4.4.1.

screen

    Screen manager with VT100 emulation, see Section 4.1.2.1.

set

    Display, set or change variable.

setterm

    Set terminal attributes.

sftp

    Secure (encrypted) ftp, see and Section 10.4.4.1.

sh

    Open a standard shell, see Section 3.2.3.2.

shutdown

    Bring the system down, see Section 4.2.6.

sleep

    Wait for a given period, see Section 4.4.1.

slocate

    Security Enhanced version of the GNU Locate, see Section 3.3.3.3.

slrnn

    text mode Usenet client, see Section 10.2.6.

snort

    Network intrusion detection tool.

sort

    Sort lines of text files, see Section 5.3.2.

spell

    Spell checker, see Section 5.1.2.3.

ssh

    Secure shell, see Section 10.4.4.1.

ssh-keygen

    Authentication key generation, management and conversion, see Section
    10.4.4.5.

stty

    Change and print terminal line settings.

su

    Switch user, see Section 3.2.1, Section 7.5.3.2 and Section 10.4.6.

T

tac

    Concatenate and print files in reverse, see cat.

tail

    Output the last part of files, see Section 3.3.4.3.

talk

    Talk to a user.

tar

    Archiving utility, see Section 9.1.1.1.

tcsh

    Open a Turbo C shell, see Section 3.2.3.2.

telinit

    Process control initialization, see Section 4.2.5.

telnet

    User interface to the TELNET protocol (insecure!), see Section 10.4.2.

tex

    Text formatting and typesetting, see Section 8.1.2.

time

    Time a simple command or give resource usage, see Section 4.3.2.

tin

    News reading program, see Section 10.2.6.

top

    Display top CPU processes, see Section 4.1.4, Section 4.3.5.3 and Section
    4.3.5.4.

touch

    Change file timestamps, see Section 7.1.2.

traceroute

    Print the route packets take to network host, see Section 10.2.6.3.

tripwire

    A file integrity checker for UNIX systems, see Section 10.4.5.

troff

    Format documents, see Section 8.1.2.

tvime

    A high quality television application.

twm

    Tab Window Manager for the X Window System.

U

ulimit

    Controll resources, see Section 7.1.2.5.

umask

    Set user file creation mask, see Section 3.4.2.2.

umount

    Unmount a file system.

uncompress

    Decompress compressed files.

uniq

    Remove duplicate lines from a sorted file, see Section 5.3.2.

up2date

    Update RPM packages, see Section 7.5.3.3.

update

    Kernel daemon to flush dirty buffers back to disk.

update-rc.d

    Configure init scripts, see Section 4.2.5.1.

uptime

    Display system uptime and average load, see Section 4.1.4 and Section
    4.3.5.2.

urpmi

    Update RPM packages, see Section 7.5.3.3.

userdel

    Delete a user account and related files.

V

vi(m)

    Start the vi (improved) editor, see Section 6.1.2.2.

vimtutor

    The Vim tutor.

vmstat

    Report virtual memory statistics, see Section 4.3.5.4.

W

w

    Show who is logged on and what they are doing.

wall

    Send a message to everybody's terminal, see Section 4.1.6.

wc

    Print the number of bytes, words and lines in files, see Section 3.2.1.

which

    Shows the full path of (shell) commands, see Section 3.2.1 and Section
    3.3.3.2.

who

    Show who is logged on, see Section 4.1.6.

who am i

    Print effective user ID.

whois

    Query a whois or nicname database, see Section 10.2.6.4.

write

    Send a message to another user, see Section 4.1.6.

X

xargs

    Build and execute command lines from standard input, see Section 3.3.3.3.

xauth

    X authority file utility.

xawtv

    An X11 program for watching TV.

xcdroast

    Graphical front end to cdrecord, see Section 9.2.2.

xclock

    Analog/digital clock for X.

xconsole

    Monitor system console messages with X.

xdm

    X Display Manager with support for XDMCP, host chooser, see Section 4.2.4
    and Section 7.3.2.

xdvi

    DVI viewer, see Section 8.1.2.2.

xedit

    X Window graphical editor, see Section 6.3.3.3.

xfs

    X font server.

xhost

    Server access control program for X, see Section 10.4.3.2.

xine

    A free video player, see Section 11.3.

xinetd

    The extended Internet services daemon, see Section 10.3.1.2.

xload

    System load average display for X, see Section 4.3.5.6.

xlsfonts

    Server font list displayer for X.

xmms

    Audio player for X, see Section 11.2.2.1.

xpdf

    PDF viewer, see Section 8.1.2.2.

xterm

    Terminal emulator for X.

Y

yast

    System administration tool on Novell SuSE Linux.

yum

    Update RPM packages, see Section 7.5.3.3.

Z

zapping

    A TV viewer for the Gnome environment.

zcat

    Compress or expand files.

zgrep

    Search possibly compressed files for a regular expression.

zmore

    Filter for viewing compressed text.



Index