BASH SHELL

Basic Shell Programming

First we will create a text file which contains the text as below,

[root@localhost root]# cat > f1

who

date

cal

Now we have to give the executive permission to the file “f1”,

[root@localhost root]# chmod u+x f1

[root@localhost root]# ls -l f1

-rwx-w-r-- root root 24 22 May 2004 f1

Now, we can execute this file by giving command as below,

[root@localhost root]# ./f1

ð It will display all users along with terminals which are running, current date and time and calendar of the current month. That means the contents of the file works as command.

"echo" is a function of bash shell which prints a given string on the monitor screen.

Example,

[root@localhost root]# echo “Linux is a OS”

Linux is a OS

[root@localhost root]# echo *

ð It will display all files’ name of current directory. If we want to display only a character of star "*", then we execute the command as below,

[root@localhost root]# echo \*

[root@localhost root]# echo '*'

The shell commands also can be performed through the echo command.

Example,

[root@localhost root]# echo `date`

Tue Jun 7 13:44:10 BDT 2005

ð It will show the current date.

[root@localhost root]# echo `cal`

June 2005

Su Mo Tu We Th Fr Sa

1 2 3 4

5 6 7 8 9 10 11

12 13 14 15 16 17 18

19 20 21 22 23 24 25

26 27 28 29 30

ð It shows the calendar of current month.

To show the text of “date” or “cal” or “ls” etc on the monitor screen by echo command, then we can execute the command as below,

[root@localhost root]# echo date

date

[root@localhost root]# echo cal

cal

[root@localhost root]# echo ls

We can use a variable in shell programming.

Example,

[root@localhost root]# new_var=5987

[root@localhost root]# echo $new_var

5987

ð Here new_var is an integer variable and its value is 5987 which is displayed on the monitor by echo command.

Some examples of shell programming are given below,

[root@localhost root]# cat > Math.sh

a=3

b=6

a=`expr $a '*' $b`

echo $a

[root@localhost root]# ./Math.sh

[root@localhost root]# cat > add.sh

a=5

b=5

a=`expr $a + $b`

echo The addition of A+B = $a

[root@localhost root]# ./Math.sh

The addition of A+B = 10

The bash shell and configuration

The shell is configured through a variety of mechanisms.

1. Local variables

2. Aliases and functions.

3. The set and shopt commands.

Variables

A variable is a label that has a value. Display variables and value using:

· set command to display all variables.

· env command to display environment variables.

· echo command to display value of a given variable.

Example,

The set, env and echo commands can be used to display all variables, environment variables and a single variable value, respectively.

[root@localhost root]# set | less

[root@localhost root]# env |less

[root@localhost root]# echo $HOME

Configuring the Shell (local variables)

Data is shell script and environment settings stored in variables.

Setting variable value: VARIABLE = VALUE

Examples,

[root@localhost root]# FAV_COLOR=blue

To retrieve variable value, use $before variable name,

[root@localhost root]# echo $FAV_COLOR

blue

[root@localhost root]# WELCOME=”Welcome to Red Hat”

[root@localhost root]# echo $WELCOME

Welcome to Red Hat

There are some common Local Variables which are used by shell:

HISTFILESIZE	Determines how many commands to be saved in the history file on logout.
COLUMNS	Sets the width of the terminal.
LINES	Sets the height of the terminal.
HISTFILE	Specifies the file in which history commands are stored on logout.
HISTSIZE	Specifies the number of history commands to keep while operating interactively.

Other Shell Configuration

Less common, but powerful commands to configure elements of the shell are,

1. set

2. shopt

Example: The set command can configure many different aspects of the shell. Such as:

Command	Performance
set -b	Respect terminal of background commands immediately rather than waiting for next prompt.
set –u	Unset variables generate an error.
set –o noclobber	Do not clobber files with > and > & operators.
set –o vi	Use vi syntax on bash command line instead of default emacs system.

Configuring commands (Environment Variables)

Shell variables exist only in current shell instance but Environment variables passed to sub-shells. Shell variables can be exported into environment.

Example,

$EDITOR=/user/bin/vim; export EDITOR

Common environment variables:
HOME	Path to user’s home directory.
LANG	Identification of default language programs should use; example: en_us, UTF-8 for U.S. English.
PWD	User’s current working directory.
EDITOR	Default editor programs should invoke for text editing.
LESS	Operations to pass to the less command.
The term environment variable:
TERM	Environment variable setting the terminal type.
reset	Command (not variable) used to reset a terminal should the screen be come corrupted.

PS1

You can change the shell’s style by the PS1 command,

[root@localhost root]# PS1="Enter you command # "

Enter you command # ls

ð It shows the command prompt in different style.

Some important option of PS1 (or the PS1 local variable) is given below:

\u - Display user name.

\h - Display Hostname.

\W - Display Current Directory.

\t - Display Current Time.

\d - Display Current Date.

\! -

\$ - Display for Normal User.

\# - Display for Super User.

Example,

Enter you command # PS1="[\u@\h \W]#"

[root@localhost root]#

MOTD

You can send message of the day to a user when h/she will login the system. The message are stored in the file is called /etc/motd.

[root@localhost root]# cat > /etc/motd

Welcome In Linux.

ð This message will display when user login the system.

.bash_logout file

You can execute some Linux command, when any user is going to logout the system. The command is written in the file called .bash_logout. This file is hidden and stored in every user's home directory.

[root@localhost root]# cat > .bash_logout

cal

date

ð The cal and date command will be executed at logout time.

Aliases

You can give one or more alias names to a command. Example, d is a new alias name of ls -l and it will execute as like ls –l command.

[root@localhost root]# alias d='ls -l'

[root@localhost root]# d

ð It will show all files and directories from /root directory.

All alias names of commands are usually stored in the file called bashrc which is stored in every user's home directory and it is a hidden file.

[root@localhost root]# vi /root/.bashrc

# .bashrc

# User specific aliases and functions

alias rm='rm -i'

alias cp='cp -i'

alias mv='mv -i'

alias mcdrom='mount /mnt/cdrom'

alias ucdrom='umount /mnt/cdrom'

# Source global definitions

if [ -f /etc/bashrc ]; then

. /etc/bashrc

Again there is another bashrc file in /etc directory. If you write any alias name of command in this file, every user can use that alias name.

[root@localhost root]# vi /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

# are we an interactive shell?

if [ "$PS1" ]; then

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"'

;;

screen)

if [ -e /etc/sysconfig/bash-prompt-screen ]; then

PROMPT_COMMAND=/etc/sysconfig/bash-prompt-screen

else

PROMPT_COMMAND='echo -ne "\033_${USER}@${HOSTNAME%%.*}:${PWD/#$HOME/~}\033\\"'

;;

[ -e /etc/sysconfig/bash-prompt-default ] && PROMPT_COMMAND=/etc/sysconfig/bash-prompt-default

;;

esac

# Turn on checkwinsize

shopt -s checkwinsize

[ "$PS1" = "\\s-\\v\\\$ " ] && PS1="[\u@\h \W]\\$ "

if [ "x$SHLVL" != "x1" ]; then # We're not a login shell

for i in /etc/profile.d/*.sh; do

if [ -r "$i" ]; then

. $i

done

unset i

# vim:ts=4:sw=4

ð This file contains some important data, such as, the value of umask, PS1 etc.

[root@localhost root]# alias

alias cdrom='cd /mnt/cdrom/Fedora/RPMS'

alias cp='cp -i'

alias floppy='mount /mnt/floppy'

alias l.='ls -d .* --color=tty'

alias ll='ls -l --color=tty'

alias ls='ls --color=tty'

alias mc='. /usr/share/mc/bin/mc-wrapper.sh'

alias mcdrom='mount /mnt/cdrom'

alias mv='mv -i'

alias rm='rm -i'

alias ucdrom='umount /mnt/cdrom'

alias ufloppy='umount /mnt/floppy'

alias vi='vim'

alias which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'

ð It shows all alias names which can be executed as command.

[root@localhost root]# unalias

Example,

[root@localhost root]# unalias d

ð To delete the alias name.

Anchors

Anchors match the beginning or end of a line or word,

Anchors Symbol	Performance
^	Line begins with.
$	Line ends with.
\<	Word begins with.
\>	Word ends with.

Example,

\ Any word beginning with “cat”.

cat \>

History

[root@localhost root]# history

1 isapnp isapnp.conf

2 cat /proc/interrupts

3 cat /proc/ioports

4 cat /proc/dma

5 cat /proc/pci

6 service sound start

7 rc.d/init.d/sound start

8 dmesg |grep *cry*

9 dmesg |grep *cr*

10 dmesg |grep *c*

11 dmesg |grep *Cr*

12 dmesg |grep *isa*

ð It shows all commands which are used before in command prompt.

[root@localhost root]# echo $HISTORYSIZE

1000

ð It shows the history size. That means how many commands can be stored in /etc/history file. We can change the history size by editing /etc/profile file.

[root@localhost root]# vi /etc/profile

# /etc/profile

# System wide environment and startup programs, for login setup

# Functions and aliases go in /etc/bashrc

pathmunge () {

if ! echo $PATH | /bin/egrep -q "(^|:)$1($|:)" ; then

if [ "$2" = "after" ] ; then

PATH=$PATH:$1

else

PATH=$1:$PATH

}

# Path manipulation

if [ `id -u` = 0 ]; then

pathmunge /sbin

pathmunge /usr/sbin

pathmunge /usr/local/sbin

pathmunge /usr/X11R6/bin after

unset pathmunge

# No core files by default

ulimit -S -c 0 > /dev/null 2>&1

USER="`id -un`"

LOGNAME=$USER

MAIL="/var/spool/mail/$USER"

HOSTNAME=`/bin/hostname`

HISTSIZE=1000

if [ -z "$INPUTRC" -a ! -f "$HOME/.inputrc" ]; then

INPUTRC=/etc/inputrc

export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE INPUTRC

for i in /etc/profile.d/*.sh ; do

if [ -r "$i" ]; then

. $i

done

unset i

ð This file contains some important data, such as, the value of history size etc.

DIR_COLORS

[root@localhost root]# vi /etc/DIR_COLORS

# Configuration file for the color ls utility

# This file goes in the /etc directory, and must be world readable.

# You can copy this file to .dir_colors in your $HOME directory to #override

# the system defaults.

# COLOR needs one of these arguments: 'tty' colorizes output to ttys, #but not

# pipes. 'all' adds color characters to all output. 'none' shuts #colorization

# off.

COLOR tty

# Extra command line options for ls go here.

# Basically these ones are:

# -F = show '/' for dirs, '*' for executables, etc.

# -T 0 = don't trust tab spacing when formatting ls output.

OPTIONS -F -T 0

# Below, there should be one TERM entry for each termtype that is #colorizable

TERM linux

TERM console

TERM con132x25

TERM con132x30

TERM con132x43

TERM con132x60

TERM con80x25

TERM con80x28

TERM con80x30

TERM con80x43

TERM con80x50

TERM con80x60

TERM cons25

TERM xterm

TERM rxvt

TERM xterm-color

TERM color-xterm

TERM vt100

TERM dtterm

TERM color_xterm

TERM ansi

TERM screen

TERM kon

TERM kterm

# EIGHTBIT, followed by '1' for on, '0' for off. (8-bit output)

EIGHTBIT 1

# Below are the color init strings for the basic file types. A color #init

# string consists of one or more of the following numeric codes:

# Attribute codes:

# 00=none 01=bold 04=underscore 05=blink 07=reverse 08=concealed

# Text color codes:

# 30=black 31=red 32=green 33=yellow 34=blue 35=magenta 36=cyan #37=white

# Background color codes:

# 40=black 41=red 42=green 43=yellow 44=blue 45=magenta 46=cyan #47=white

NORMAL 00 # global default, although everything should be something.

FILE 00 # normal file

DIR 01;34 # directory

LINK 01;36 # symbolic link

FIFO 40;33 # pipe

SOCK 01;35 # socket

BLK 40;33;01 # block device driver

CHR 40;33;01 # character device driver

ORPHAN 01;05;37;41 # orphaned syminks

MISSING 01;05;37;41 # ... and the files they point to

# This is for files with execute permission:

EXEC 01;32

# List any file extensions like '.gz' or '.tar' that you would like ls

# to colorize below. Put the extension, a space, and the color init string.

# (and any comments you want to add after a '#')

.cmd 01;32 # executables (bright green)

.exe 01;32

.com 01;32

.btm 01;32

.bat 01;32

.sh 01;32

.csh 01;32

.tar 01;31 # archives or compressed (bright red)

.tgz 01;31

.arj 01;31

.taz 01;31

.lzh 01;31

.zip 01;31

.z 01;31

.Z 01;31

.gz 01;31

.bz2 01;31

.bz 01;31

.tz 01;31

.rpm 01;31

.cpio 01;31

.jpg 01;35 # image formats

.gif 01;35

.bmp 01;35

.xbm 01;35

.xpm 01;35

.png 01;35

.tif 01;35

ð This file contains the setting of color of director file, ordinary file, tar file, zip file etc.

Relational condition of shell programming

Command Used in Shell Programming	Meaning
-gt	>
-ge	>=
-eq	=
-le	<=
-lt	<
-ne	!=

Conditional Statement

IF statement:

Syntax:

Example

if [ condition ]

then

statement

a=5

f [ $a -eq 5 ]

then

echo A is equal to 5

IF-ELSE statement:

Syntax:

Example

if [ condition ]

then

statement

else

statement

a=3

b=5

f [ $a -gt $b ]

then

echo A is greater then B

else

echo B is greater then A

LOOPING CONDITION

WHILE LOOPING

Syntax:

Example

while [ condition ]

statement

increment

done

a=1

b=10

while [ $a -le $b ]

echo $a

a=`expr $a + 1`

done

UNTIL LOOPING

Syntax:

Example

until [ condition is false ]

statement

increment

done

a=1

b=10

while [ $a -eq $b ]

echo $a

a=`expr $a + 1`

done

FOR LOOPING

Syntax:

Example

for variable in arg1 arg2 arg3 .. .. .. ..

statement

done

for var in 1 2 3 4

c=`expr $var '*' $var`

echo $var \* var = $c

done

CASE statement

Syntax:

Example

case $var in

value 1)

statement

;;

value 1)

statement

;;

value 1)

statement

;;

... ... ... ... ... ... ... ...

... ... ... ... .. .... ... ...

esac

echo "Enter two values : "

read a b

echo a b

echo 1. ADD

echo 2. SUB

echo 3. MUL

echo 4. DIV

read var

case $var in

1) c=`expr $a + $b`

;;

2) c=`expr $a - $b`

;;

3) c=`expr $a \* $b`

;;

5) c=`expr $a / $b`

;;

esac

echo Result = $c

Another example of CASE statement:

[root@localhost root]# cat . case1.sh

echo "Enter a key: "

read a

case $a in

[a-z])

echo "You have pressed a character :"

;;

[0-9])

echo "You have pressed a number :"

;;

echo "You have pressed a symbol :"

;;

echo "Incorrect value."

esac

ð In the program, the value "?" means all keyboard symbol and "*" means default value.

BREAK statement

break is a keyword, it breaks looping of the line.

Example,

i=1

while [ $i -le 10 ]

echo $i

i=`expr $i + 1`

if [ $i -eq 5 ]

then

break

done

echo "The Looping is breaked."

EXIT statement

exit is a keyword. It can exit the whole program.

CONTINUE statement

This keyword always sends the compiler again to the looping statement.

POSITION PARAMETER

It can take the value of variable from the arguments which are given on the program executive time. The variable can declare as $1 which takes the value from first argument, $2 which takes the value from second argument, so on.

Example,

[root@localhost root]# cat > parameter.sh

echo $1 $2

c=`expr $1 + $2`

echo $c

[root@localhost root]# sh parameter.sh 4 5

4 5

ð The output is coming by adding of the argument 4 and 5.