Linux Guide

1. Linux OS
2. File System
3. Processes
4. Programming Languages
5. XWindows
6. Gnome
7. Games
8. Networking
9. bash
10. Shell Programming

I. The Linux OS

a. History of Linux

Linux was developed in the late 1980s by Linus Torvalds, a Finnish student who could not afford a Unix machine. He developed Linux as a free clone of Unix, with an open source model of source code so that other poor students like him would be able to have an OS.

Over time, several several variants of Linux have developed, all of which are available for free download via their websites :

• Red Hat (http://www.redhat.com) : Beginners' linux.
• SuSE (http://www.suse.com) : For Business Users, includes several accounting packages.
• Slackware, (http://www.slackware.org) : The open-source slackware.
• Caldera (http://www.caldera.com)
• BSD (Berkely Software Distribution) (http://www.bsd.org, http://www.freebsd.org)

Several varieties of Unix exist, including IBM AIX, Sun Solaris OS, HP-UX, etc.

b. Advantages of Linux

• Free : Linux is available at no cost for downloading from the Internet. Compare that with Windows NT, which lists for $149 per workstation upgrade and $439 per server with 10 client licenses. [Information Week, January 24, 2000]
• Fewer Bugs : Open-Source Scrutiny leads to constant Peer Review and early elimination of bugs and robustness
• Stability: Linux OS is stable, greatly reducing the possibility of system crashes and server downtime. In a study by Gartner Datapro, Linux ranked higher than Windows NT and all commercial Unix OSs in customer satisfaction. [ Computer User, Dec. 2000 ]
• Scalable: It's inherently scalable, suitable for CPU-intensive database applications as well as straightforward Web serving.
• Flexible: It's flexible, allowing developers to obtain the optimum blend of hardware and software components to serve up an ad banner, or safeguard customer data.
• Industry Backing : Several big companies have started backing linux, including Dell, Compaq, and IBM. IBM may even create an open-source `Blue Linux'

c. OS Holy War

Linux and Unix dominate the server market :

O/S Server Market Share Windows NT 36 % Linux 24 % NetWare 19 % Unix 15 % Host/Server 3 % Other 3 % Table: Server Market 1999 (5.7 million shipments) IDC, IT Forecaster, August 8, 2000

As per ComputerUser.com, "Linux is the leading choice for Web deployment, powering 30 percent of all public Web sites, according to a study conducted last May [1999] by Netcraft, Inc.."

Google is powered by linux servers: "Google, based in Mountain View, Calif., relies on more that 4,000 Apache Web servers running Linux to drive its wildly successful search engine technology, incorporated into the Net initiatives of companies like Palm Inc., Cisco Systems, Hungry Minds.com, and the Washington Post."

"Free shall make you free" was the clarion call of the original Web, Torvalds and other open-source pioneers such as Richard Stallman of the GNU project and Jon "maddog" Hall--currently executive director of Linux International--heard that message, tirelessly promoting it in the burgeoning Web community.

Linux is thus outpacing Unix, and is arising as a serious competitor to Microsoft in the client desktop programming market. By comparison, NT user base is 2 - 4 million and the Mac user base is 22 million 50 million units of Windows 3.1 and 3.11 have been installed as per microsoft.com. Prior to Win 95 launch [ 1995 ], there was a user base of 50 million Win 3.x users

	O/S Client Market Share MicroSoft 87.7 % Linux 4.1 % Mac OS 5.0 % Other 3.8 % Table: Client/Desktop Market (98.8 million shipments) IDC, Press Release 07/17/00

Linux has been branded as the number one threat to Microsoft's dominance of the client market. An `OS Holy War' has thus broken out.

The OS Holy War on Rediff, http://www.rediff.com/computer/dgos.htm

d. Restricted User Priveleges

Linux does not give all users the same priveleges. Users only have access to their own files, while certain group members have access to group files; the root or superuser has priveleged access to all files. Thus, the spread of viruses is inhibited.

Users should choose difficult to choose passwords, which are not dictionary words and including special characters.

Login is accomplished by entering a valid user name and password. Commands in Linux have command-line options (preceded by a -).

Basic User commands: Try these basic commands after logging in:

• $ who
• $ date
• $ passwd
• $ exit

Further information about a particular command can be obtained from the in-built system manuals :

• $ man who
• $ info who
• $ who -h
• $ who --help
• $ apropos who

II. File System

a. Editing Files

• pico (`pico') [ simple file editor with few commands ]
  • Commands
    ^X (`eXit' :save and exit)
    ^J (`justify' :format paragraphs)
    ^T (`thesaurus' :spell checker)
    ^O (`out' :write to specified output file)
  • Examples
    • $pico f.txt     (edit file f.txt)
• pilot (`pilot') [ file browser program used with pico ]
  • Examples
    • $pilot .     (view present dir)
• vi (`vi') [ advanced file editor ]
  • Modes: vi has two modes: command mode (entered via `ESC` and edit mode (entered into by typing `i`)
  • Command Mode
    :q! (`quit' :quit without saving)
    :w (`write' :write)
    :wq (`write and quit' :write and quit)
    :%s/world/Globe/g (`search' :global search and replace)
  • Examples
    • $pico f.txt     (edit file f.txt)

b. Listing and Viewing Files

• ls (`list') [ lists contents of a directory ]
  • Options
    -a (`all' :display all files, including dot files)
    -l (`long' :long listing)
    -R (`recursive' :listing of subdirectories)
    -t (`time' :sorted by modification time)
  • Examples
    • $ls -la     (long listing of all files)
    • $ls -lR | less     (long listing of subdirectories piped to less)
    • $ls --color     (list in color; defaults changed by
      LS_COLORS=*.tex=1:*.html=34:*.txt=35; export LS_COLORS which can also be set via dircolors)
• cat (`concatenate') [ append files and print to output ]
  • Options
    -n (`number' :number all output lines)
  • Examples
    • $cat f.txt     (display file)
    • $cat f1.txt f2.txt > f3.txt     (concatenate two files, redirecting output to a new file)
  • Editing : By redirecting the output from cat to a file, files can be created, whence Ctrl-D terminates the process, eg.

         $cat > f.txt
         Hello Everybody!
         Samar Abbas was here
         ^D 

    Using $cat >> f.txt would instead append the text to an existing file.
• tac (`reverse concatenate') [ concatenate and write files in reverse ]
  • Options
    -r (`regex' :treat separator as regular expression)
  • Examples
    • $tac f.txt     (display file in reverse)
• head (`head') [ output the first part of file, first 10 lines by default ]
  • Options
    -c N (`char' :print first N characters)
  • Examples
    • $head -c 5 f.txt     (display first 5 characters of file)
• tail (`tail') [ output the last part of file, last 10 lines by default ]
  • Options
    -n N (`number' :display last N lines)
  • Examples
    • $tail -5 f.txt   (or $tail -n 5 f.txt )     (display last 5 lines of file)
• wc (`word count') [ count words in a file ]
  • Options
    -w (`words')
    -l (`lines')
    -c (`characters')
  • Examples
    • $wc f.txt     (count lines, words and characters of file f.txt)
    • $wc -w f.txt   (or $wc --words f.txt )     (count words in a file)
    • $wc -c f.txt   (or $wc --chars f.txt )     (count characters in a file)
• more (`more') [ pagewise text-viewing program, deprecated in favour of less ]
  • Options
    -n (`number' :display n rows)
  • Commands
    SPACE (`forward') display next screenful
    b or ^B (`back') back one screenful
  • Examples
    • $more -25 f.txt     (display 25 rows of file at a time)
• less (`less is not less than more') [ pagewise text-viewing program ]
  • Options
    -V (`version')
  • Commands
    SPACE (`forward') display next screenful
    b or ^B (`back') back one screenful
    f or ^F (`forward') forward one screenful
  • Examples
    • $less f.txt     (standard usage)
• cp (`copy')
  • Examples
    • $cp f1.txt f2.txt     (copy file f1.txt onto f2.txt)
    • $cp ../dir/f.txt .     (copy file f.txt in a directory dir under the superdirectory to the present dir `.`)
• mv (`move')
  • Examples
    • $mv f1.txt f2.txt     (standard usage)
    • $mv dir1 dir2     (move directory dir1 into directory dir2)
    • $mv f1.txt ~/dir/subdir     (move file to a subdirectory subdir in a directory called dir in the home directory)
• rm (`remove')
  • Examples
    • $rm f.txt     (remove file f.txt)
    • $rm *     (remove all files)
    • $rm -i *.temp     (interactively remove all files ending in .temp, prompting user)
    • $rm *.d??     (erase all files having a three-lettered suffix followed by two characters)
    • $rm -r cache/     (recursively remove all files and subdirectories of a directory called cache)

c. Formatting Files

• nl (`number lines')
  • Options
    -b style(`body numbering' :numbering style, style types include `a' (number all lines), `pREGEXP' (number only lines that contain a match for REGEXP), etc.
    -i number (`increment')
    -n format (`number format' :format is ln, rn or rz)
  • Examples
    • $nl f.txt     (number lines)
    • $nl -i 5 f.txt     (number lines with increment 5)
    • $nl -n rn f.txt     (number lines right-aligned)
• fmt (`format')
  • Options
    -w n (`width')
    -u (`uniform spacing')
  • Examples
    • $fmt -w 20 f.txt   (or $fmt --width=20 f.txt )     (width 20)
    • $fmt -u f.txt     (uniform spacing)
    • $fmt is equivalent to resizing the Xwindow in pico and using ^L

d. Directories

• cd (`change directory')
  • Examples
    • $cd mydir     (go to directory dir)
    • $cd     (return to home directory)
    • $cd ..     (up one directory)
    • $cd /usr/local/bin     (using fully qualified path names)
• mkdir (`make directory')
  • Options
    -mode ijk (`mode')
  • Examples
    • $mkdir d     (make directory d)
    • $mkdir -m 700 d     (close new directory for all other users)
• rmdir (`remove directory') [ remove directory if empty ]
  • Examples
    • $rmdir d     (remove directory d)
• df (`disk filesystem usage') [ reports the amount of disk space used and available on filesystems in units of 1024 bytes]
  • Examples
    • $df     (show usage)
    • $df -m     (show usage in MB)
• du (`disk usage') [ reports the disk space for the current or specified directory in units of 1024 bytes]
  • Options
    -a (`all' :show counts for all files, not just directories)
    -k (`kilobytes' :display size in kilobytes)
    -m (`megabytes' :display size in megabytes)
  • Examples
    • $du     (show directory usage)
    • $du -ak     (show directory and all file sizes in kB)
• pwd (`present working directory')
  • Examples
    • $pwd     (show current directory)

e. Finding and Searching Files

• find (`find file')
  • Examples
    • $find . -name '*.txt'     (search current directory tree for files ending in txt)
    • $find . -size +30k     (search current directory tree for files larger than 30k)
    • $find . -type f -print |xargs grep train     (find files containing string train)
    • $info find     (more information on find)
• grep (`grep') [ search for lines matching a pattern ]
  • Options
    -i (`ignore case' :ignore case in the pattern)
    -r (`recursive searoh' :search all files in a directory and its subdirectories)
  • Examples
    • $grep myword *.txt     (search all files ending in txt for myword)
    • $grep -e '-some' *.???     (search all files with a three letters suffix for -some)
    • $ grep -ri Samar mydir|less     (search all files in mydir and subdirectories for string Samar, ignoring case, piping output to less)
• xargs (`execute arguments') [ execute command lines from standard input ]
  • Examples
    • $ find . -name '*.txt' | xargs grep myword     (find all files in full directory tree ending in text, piping the output to a match for myword)
• spell (`check spelling') [ verify spelling ]
  • Several different varieties of spell checkers exist, such as :
    • $ spell f.txt     (lists wrong words only without correcting the original file; this is linux aspell script mimicking the original Unix spell)
    • $ ispell f.txt     (interactive spell checker, user simply enters number of closest choice from matched words, `quit' to quit)
    • $ aspell -c f.txt or $ aspell check f.txt     (linux spell checker)
    • $ aspell --help | less     (linux spell checker help)

f. File Permissions

• chmod (`change mode') [ change permissions ]
  • Examples
    • $chmod 744 *.txt     (change permission for global read access)
• chgrp (`change group') [ change permissions ]
• umask (`user mask') [ set the digits to be subtracted from full permission set ]

g. Compressing Files

• gzip (`gzip') [ compress files using Lempel-Ziv coding (LZ77) ]
  • Options
    -d (`decompress' :same as $gunzip)
    -r (`recursive' :descend into subdirectories)
  • Examples
    • $gzip -9 *.txt     (ultra-compression in maximum time)
    • $gunzip *.gz     (uncompress all gzipped files)
    • $ cat file1 file2 | gzip > foo.gz     (concatenate files and then compress)
• zip (`zip') [ compress files in a format compatible with PKZIP (Phil Katz's ZIP for MSDOS systems) ]
  • Examples
    • $zip z.zip *.txt     (zip all txt files into a file called z.zip)
    • $unzip z.zip     (unzip the given file)
• tar (`tape archive')
  • Options
    -c (`create')
    -v (`verbose')
    -f (`file' :use archival file name)
  • Examples
    • $tar -cvf t.tar *.jpg     (create tar file containing all jpeg images)
    • $tar -tvf t.tar     (verbosely tabulate the contents of the tar file)
    • $tar -xvf t.tar     (verbosely extract the contents of the tar file)
• compress (`compress')

h. Root File System

III. Processes

a. Process Commands

• ps (`process status') [ display processes ]
  • Options
    -e (`everything')
    -u (`user' :display user's processes)
    -f (`full')
  • Examples
    • $ps     (display processes using default settings)
    • $ps -ef | less     (write all processes)
    • $ps -u samar     (show samar's processes)
• sleep (`sleep') [ delay for a specified amount of seconds ]
  • Examples
    • $sleep 10     (suspend for 10 seconds)
    • $sleep 100 &     (suspend and place in background)
    • $ps -u samar     (show samar's processes)
• kill (`kill') [ terminate a process ]
  • Examples
    • $kill -2 27645     (terminate process ID 27645, which can be obtained from $ps)
    • $kill -9 103     (definitely kill process ID 103)
• at (`at') [ execute process at specified time ]
  • Examples
    • $at 11:53 < mpeg.ftp     (execute commands given in a text file mpeg.ftp:)

      mpeg.ftp

      ftp mm-ftp.cs.berkeley.edu > log anonymous abbas@ lcd /user2/abbas/bin/ cd pub/mpeg/play/ bin get mpeg_play-linux-elf.tar.gz bye

    • $atq     (at queue:show list of at jobs after executing at)

    • $atrm 4     (remove job in at queue)

    • $at 12:30
      at> date
      at> who
      at> <^D>

          (execute using simple date format)
• crontab (`cron table') [ edit crontab file ]
• top (`show all processes') [ show all processes ]

IV. Programming Languages

a. Compilers

• perl (`practical extraction and reporting language') [ language ]
  • $perl p.pl     (perl compiler)
• java (`java') [ programming language ]
  • $javac f.java     (java compiler)
  • $java f     (java virtual machine)
• C (`C') [ programming language ]
  • $cc f.c     (C compiler)
  • $gcc f.c     (GNU C and C++ Compiler)
• Fortran (`Formula Translation') [ programming language ]
  • $f77 p.f     (Fortran 77 compiler)
  • $f90 p.f or $fort90 p.f     (Fortran 90 Compiler)

b. Mathematical Aids

• bc (`best calculator') [ arbitrary precision calculator using scale, the number of decimal digits ]
  • $bc    
    scale=4
    2/3
    0.6666
    quit
• dc (`desk calculator') [ arbitrary precision calculator ]

V. X Windows

a. Initialization

• ~/.xinitrc (`x initialization file') [ init file ]
  • Default: /etc/X11/xinit/xinitrc
  • Format : Most commands should be run in background, last client in the foreground, when it exits the window session exits.

    xclock &
    xsetroot -sold blue &
    xterm -name "Main Window" 
    

• ~/.Xresources (`x resources') [ init file ]
  • Default: /etc/X11/xinit/xinitrc

b. Commands

• startx (`start x windows') [ start command ]
  • $startx     (new x terminal)
• xterm (`x terminal') [ new terminal ]
  • $xterm &     (new x terminal)
  • $xterm -g 80x24+0+0 -name "Samar Abbas"&     (new x terminal at 0,0, 80 digits wide, 24 lines high named as given)
• xsetroot (`set background') [ change bg color ]
  • $xsetroot -solid gray     (sets background)
• xclock (`x clock') [ clock ]
  • $xclock -geometry 80x80 &     (displays clock)
• animate (`animate') [ display series of images ]
  • $animate f*.jpg     (display images)

Games which can be started from the command prompt in xwindows: xevil, xdino, battlestar, atc, trek, robots, worm, doom, etc. cf. /usr/x11/bin

Desktop can be changed using Gnome Footprint | Programs | System | Desktop Switching Tool. To make the Taskbar visible, please click on the K Menu icon on the K Panel. Next, click on Settings > Applications > Panel. Then in the K Panel Configuration pop-up box, please move the dot under Taskbar from Hidden to Bottom by clicking on Bottom. Please see Figure 4 (above) and Figure 5 (below).

VI. Gnome

x. AbiWord ( MS Word clone )

• Download : http://www.abisource.com/free.phtml
• Starting : Click on AbiWord logo in menu.
• Features
  • Handles MSWord, rtf

x. GIMP   (`gnome image manipulation program', a free replacement of Adobe Photoshop )

• Download : http://www.gimp.org/download.html ; not necessary with Red Hat Linux 7+ as already installed.
• Starting : Footprint | Programs | raphics | The GIMP
• Features
  • Full suite of painting tools including Brush, Pencil, Airbrush, Clone, etc.
  • Converts images to various formats

x. Gnome-DB   (`gnome database')

• Download : http://www.gnome-db.org/download.php
• Starting : Click on Gnome-DB logo in menu.
• Features
  • Supports multiple database backends, including Postgresql, Mysql, Oracle, ODBC, Sybase

x. Guppi   (`graphing and plotting program')

• Download : http://www.gnome.org/guppi/
• Starting : Click on Guppi logo in menu.
• Features
  • Extensible Design

x. Dia   (`Diagram')

• Download : http://www.gnome.org/gnome-office/dia.shtml
• Starting : Click on Dia logo in menu.
• Features
  • Support for flow charts, structure diagrams, etc...

x. OpenWriter   (OpenOffice wordprocessor.)

Sun is integrating their OpenOffice Suite with GNOME, which means all the applications of OpenOffice will become part of GNOME Office.

• Download : sw.openoffice.org
• Starting : Click on Open Office logo.
• Features
  • Full word processor
  • Handles MS Word 95, 97, 2000

x. StarOffice   (`Sun Office suite')

• Download : http://www.sun.com/products/staroffice/get.html
• Starting : Click on Star Office logo.
• Features
  • Full office suite.

VII. Games

Not all of these games may be present on the particular linux installation, all however can be downloaded for free from various websites.

a. Adventure and Text Games

• battlestar : /usr/games/battlestar   (space adventure game)
• paranoia : /usr/games/paranoia   (spy adventure game)

b. War Games

• doom (`full graphics war grame') [ arbitrary precision calculator using scale, the number of decimal digits ]
  • $doom    
  • $xdoom
• xevil

c. Board Games

• xcarrom (`carrom')
• gnuchess (graphical) : /usr/bin/xboard   (graphical chess game for x windows)
• gnuchess (text) : /usr/bin/gnuchess   (character-based chess game)

d. Video Games

• xbill : Start | Programs | Games   (destroy an evil hacker)
• xeyes : Start | Programs | Games   (big eyes)

e. Card Games

• battlestar (`text game')

VIII. Networking

a. E-Mail

• pine (`pine') [ simple e-mail program ]
  • Commands (virtually identical to pico)
    ^X (`exit' :send a message)
    ^J (`justify' :format paragraphs)
    ^T (`thesaurus' :spell check)
  • Examples
    • $pine    
    • ~/.pinerc    
• mail (`send mail') [ command-line email ]
  • Examples
    • $mail     (view mail, use v 12 to view message no. 12)
    • $mailx     (extended mail program available on some systems)
    • $mail dcswain@hotmail.com     (send mail to given address)
    • $mail -s "hello samar" abbas@samarstan < f.txt     (send mail with given subject, using f.txt as body; useful for shell scripting)
    • ~/.forward     (file used for forwarding emails)
    • ~/.mailrc     (mail configuration file)
• elm (`elm') [ elm ]
  • Examples
    • $elm     (elm opened)

b. Chatting

• write (`write letter') [ chat to a user logged in ]
  • Examples
    • $write saroj     whence saroj must reply $write samar
    • $write samar tty03     (write samar on terminal tty03)
    • $write santosh@venus.xly.com     (write across the internet)
  • Conditions
    • Login: User must be logged in.
    • Permission: User must have granted talk permission: $mesg y   .Entering $mesg n disables all input.
• wall (`write all') [ broadcast message to all users ]
  • Examples
    • $wall "Hello from Samar Abbas"
    • $write santosh@venus.xly.com     (write across the internet)
  • Conditions
    • Login: User must be logged in.
    • Permission: User must have granted talk permission: $mesg y  
      Entering $mesg n disables all input.
    • On some systems, only root can send a wall message usually stored in /etc/wall
• finger (`finger user') [ find out info about user ]
  • Examples
    • $finger samar
    • $finger abbas@unix.mit.edu     (finger across the internet)
    • ~/.plan : file executed by finger
• talk (`talk') [ chatting program ]
  • Examples
    • $talk samar@webtech.com     (talk to samar across internet)
  • Conditions
    • Login: User must be logged in.
    • Permission: User must have granted talk permission: $mesg y  
      Entering $mesg n disables all input.
    • talkd (talk daemon) must be running under the superserver daemon, , cf file /etc/inetd.conf.

c. Telnet

To telnet to another machine, use the hostname $telnet mars or the IP address, $telnet 198.100.11.15 and then supply required username and password.

Use netstat to check on network peroformance.

d. ftp

To use ftp services use the hostname $ftp venus.ensindia.com or use the IP address $ftp 198.100.11.15, supplying required username and password.

The following commands are used to retrieve files:

• > get f.txt
• > put f.txt
• > mput *

e. www

• netscape (`internet scape') [ graphical web browser ]
  • Running Netscape:
    • $startx     (start x mode, not necessary if window mode running)
    • $netscape     (or click on Netscape logo)
• lynx (`links') [ fast text browser ]
  • Examples
    • $lynx http://www.google.com     (works for direct internet connection)
    • $lynx .     (browse home dir)
    • $lynx -source http://www.ibm.com/index.html > new.html &     (download file in background)
    • $http_proxy ="http://some.server.dom:port/"
      $export http_proxy
          (proxy server settings, other variables are ftp_proxy, gopher_proxy, then use) $lynx http://www.google.com
  • Commands
    • q - quit

f. Other Commands: rlogin, ping, traceroute

• ping (`ping pong') [ send ICMP ECHO_REQUEST packets to network hosts ]
  • Examples
    • $ping 198.100.11.14     (test connection to given host)
    • $ping mars.venus.com     (ping using hostname)
• traceroute (`draw route') [ print the route packets take to network host ]
  • Examples
    • $ /usr/sbin/traceroute 198.100.11.14
    • $ /usr/sbin/traceroute hotmail.com     (find route across network)
• rlogin (`remote login') [ login to a remote host ]
  • Examples
    • $ rlogin 198.100.11.14     (remote login)
    • $ rlogin -l user mars     (remote login)

IX. bash (Bourne Again Shell)

a. Features of bash

• Command-Line Completion : bash can automatically complete the command-line via the TAB key, eg. $pi<TAB><TAB> displays alll possible commands beginning with 'pi', while entering $pico f<TAB> either completes the name of the file, or displays all files beginning with f.
• Command History : bash keeps track of a certain number of previous commands that have been entered into the shell. The number of commands is given by a shell variable called HISTSIZE in a default file ~/.bash_history set using the HISTFILE bash variable. The up- and down-arrow keys scroll through the commands that have been previously entered.
  • $history     - displays entire history file
  • $history 10     - displays last 10 commands
  • $cat ~/.bash_history     - displays entire history file
• Command Aliasing : bash supports the ability to give nicknames to longer commands.
  • $alias ll="ls -l"     - alias ll to long listing
  • $alias copy='cp'     - alias copy to cp
  • $unalias mycommand     - unalias mycommand
  • $alias     - display all aliases
• IO Redirection : bash permits input and output to be redirected from the standard output (ie.the monitor).
  • $ls -lR > l.txt     - rediret output to a file
  • $ls -lR | less     - pipe listing to less fileviewing program
• Job Control : bash provides job control via the $fg and $bg and ^Z commands.

b. Shell Initialization and De-Initialization

1. /etc/profile is the default initialization file.
2. ~/.bash_profile
   ~/.bash_login
   ~/.profile   (bash searches for the first of these files and executes them; ~/.bashrc is often made to point to any of these so as to permit non-interactive login initialization)
3. ~/.bash_logout   (executed on logout)
4. ~/.bashrc(subshell or non-login shell initialization file)

c. Shell Environment Variables

Using $export causes the value of a variable to be made visible to all subprocesses that belong to the current shell.

• PS1 (`prompt symbol') [ First user prompt ], bash has two levels of user prompt. The first level (default $) is what you see when bash is waiting for a command to be typed, The second level of prompt is displayed when bash is expecting more input from you in order to complete a command. The default for the second level prompt is >. eg.
  • PS1=" Are You Samar ? "     - changes prompt to given text string, ie. Are You Samar ?
  • PS1="Time: \t * "     - changes prompt to given time, ie. Time: 11:51:58 *
  • PS2="\u % "     - changes second-level prompt to user login with percent symbol, ie. abbas %
  • PS1="\u@\h # "     - changes prompt to user login with percent symbol, ie. abbas@venus #
  • PS1="`hostname`:`pwd`> "     - changes prompt to user login with percent symbol, ie. abbas@venus #
  • PS1="\h:\w $ "; export PS1     - changes prompt to hostname and working directory and export the value mars:~/dir $
  • PROMPT_COMMAND=/usr/games/fortune; export PROMPT_COMMAND     - sets prompt command and exports it to subshells

  Character	Meaning
  \!	Displays the history number of this command.
  \#	Displays the command number of the current command.
  \$	Displays a $ in the prompt unless the user is root. When the user is root, it displays a #.
  \\	Displays a backslash.
  \d	Displays the current date.
  \h	Displays the host name of the computer on which the shell is running.
  \n	Prints a newline character. This will cause the prompt to span more than one line.
  \nnn	Displays the character that corresponds to the octal value of the number nnn.
  \s	The name of the shell that is running.
  \t	Displays the current time.
  \u	Displays the username of the current user.
  \W	Displays the base name of the current working directory.
  \w	Displays the current working directory.

  Table: Prompt special character codes.
  Source: Red Hat Linux Unleashed, Ch.10
• TERM (`terminal')
  • TERM = xterm; export TERM     - sets the terminal to standard x terminal
  • TERM = vt220; export TERM     - sets the terminal to vt220 emulation
• TMOUT (`timeout') [ Idle time before automatic logout ]
  • unset TMOUT     - unset time out.

#! /bin/bash
who
echo '******************'
ls
echo "******************"
# This is a comment
ps
echo '******************'

echo '******************'
echo "hello $LOGNAME"
echo "******************"