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Linux Shell Programming

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Overview

Command line arguments Arithmetic in shell scripts Read and echo Commands in shell

scriptsTaking decision:

If then-fi

If then else-fi The test command(file test,string tests) Nested if-elsesThe Case control structure

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Overview

The loop control structure

The while ,until and for loop structure

The break and continue statementsDirectory stacks manipulation

Job control, history and processes

Built in functions

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Command line arguments

cat sample.sh

echo Program:$0

The number of arguments is $#The arguments are $*

grep$1 $2

echo \n Job Over$sample.sh hello test.txt

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Command line arguments

Special variables are assigned toarguments in shell procedure

First argument $1Second argument $2

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Arithmetic in shell scripts

Use to perform arithmetic operations. Syntax:

expr op1 math-operator op2

Examples:$ expr 1 + 3$ expr 2 - 1$ expr 10 / 2$ expr 20 % 3

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Arithmetic in shell scripts

$ expr 10 \* 3

$ echo `expr 6 + 3`

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Read and echoCommands

cat search.sh#search for pattern in a file

echo \n Enter the string to be searched:\c

read pnameecho \n Enter the file name:\c

read fname

echo \n Searching for $pname from file $fname \n

grep $pname $fnameecho \n Search completed\n

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Taking decision

If then-fiif condition is true

then

execute commandsfi

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If then else-fi

if condition is true

then

execute commands

elseexecute commands

fi

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If then else-fi

if grep test sample.txt

then

echo String foundelse

echo String not found

fi

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test command ( numeric)

Operator

Meaning

-eq Equal to-ne Not equal to

-gt Greater than

-ge Greater than or equal to

-lt Less than

-le Less than or equal to

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test command ( numeric)

test : uses operator to evaluate the condition, returns true orfalse

exit : used by if for taking decisions

$x=5; y=7; z=7.2

$test $x eq $y ; echo $?

1

$test $x lt $y ; echo $?

0

$test $z gt $y ; echo $?

1

$test $z eq $y ; echo $?

0

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test command

if test $# -ne 3

then

echo 3 arguments are required

exit 3

elseif grep $1 $2 >$3

then

echo String found

elseecho String not found

fi

fi

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test command (file test)

Test Exit status

-e file True if file exists

-f file True if file exists and is a regular file-r file True if file exists and is readable

-w file True if file exists and is writeable

-x file True if file exists and is executable

-d file True if file exists and is directory

-s file True if file exists and has size > 0

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test command (file test)

ls l sample.txt

-rw-rw-rw- 1 student SENG 870 Jun 8 15:52sample.txt

$[ -f sample.txt]; echo $?0$ [-x sample.txt]; echo $?

1

$ [! w sample.txt ] || echo File is writableFile is writable

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test command (file test)

$cat testfile.sh

if [! e $1] ; then

echo File does not exists

elif [! r $1] ; then

echo File is not readable

elif [! w $1] ; then

echo File is not writable

else

echo File is both readable and writable

fi

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The Case control structure

case expression in

pattern1) execute command;;

pattern2) execute command;;pattern3) execute command;;

esac

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The Case control structure

$ cat menu.shecho MENU\n

1.List of files\n2.Processes\n 3.Todays Date\n4.Users ofsystem\n5.Quit \n Enter your choice :\c

read choicecase $choice in

1) ls l ;;

2) ps ;;

3) date ;;4) who ;;

5) exit

esac

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The loop control structure

while

until

for loopBreak

continue

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The while ,until and for loopstructure

while condition is true

do

execute commandsdone

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Job control processes

ps list the processes running on the system

kill send a signal to one or more processes (usually to "kill"

a process)

jobs listing processes

bg put a process in the background

fg put a process in the foreground

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history

To display the command list

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Built in functions

hash Command hash command maintains a hash table When a command is executed

it searches for a command in variable$PATH.

$ hash

hits command

1 /usr/bin/cat2 /usr/bin/ps

4 /usr/bin/ls

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Built in functions

$ hash -d cat

$ hash

hits command2 /usr/bin/ps

4 /usr/bin/ls

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Built in functions

set is a shell built-in command:

- used to set and modify the internal

variables

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Built in functions

$ cat set.sh

var="Welcome to CDAC"

set -- $var

echo "\$1=" $1echo "\$2=" $2

echo "\$3=" $3

$ ./set.sh

$1=Welcome$2=to

$3=CDAC

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unset

- set the shell variable to null

$ cat unset.sh

var="welcome to CDAC"echo $var

unset var

echo $var$ ./unset.sh

welcome to CDAC

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let

Performs arithmetic operations on shell variables

$ cat arith.shlet arg1=12

let arg2=11let add=$arg1+$arg2

let sub=$arg1-$arg2

let mul=$arg1*$arg2

let div=$arg1/$arg2

echo $add $sub $mul $div

$ ./arith.sh

23 1 132 1

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Reference

1.http://www.thegeekstuff.com/2010/08/bash-shell-builtin-commands/

2.UNIX Concepts And Application

Sumitabha Das3.http://linuxcommand.org/lts0080.php