SIMPLE SHELL SCRIPTING

05 May 2020

A shell script is a computer program designed to be run by the Unix/Linux shell which could be one of the following:

• The Bourne Shell
• The C Shell
• The Korn Shell
• The GNU Bourne-Again Shell

A shell is a command-line interpreter and typical operations performed by shell scripts include file manipulation, program execution, and printing text.

---

create a simple .sh file

root@kali:~#vi try.sh

#!/bin/bash/

echo "1st Shell scripting"

root@kali:~# chmod +x try.sh
root@kali:~# ./try.sh
hello world

---

TIPS: For executing a .sh file you cane use ./{file_name} or

root@kali:~# bash -f {file_name}

---

Using conditional statements

root@kali:~#vi ifelse.sh

#!/bin/bash\

var1=7
var2=12

if [ $var1 > $var2 ]
then
        echo "var1 is greater than var2"
elif [ $var1 == $var2 ]
then
        echo "var1 is equal to var2"
else
        echo "var2 is greater than var1"
fi

root@kali:~# chmod +x ifelse.sh
root@kali:~# ./ifelse.sh
var1 is greater than var2

U sing logical ‘and’ ‘or’ and dynamic variable

#!/bin/bash

if (( $1 > $2 & $1 > $3 ))
then
	echo "var1 is greatest"
elif (( $1 == $2 || $3 == $2 || $1 == $3 ))
then
	echo "any two or all three are same"
else
	echo "no condtion"
fi

root@kali:~# ./ifelse.sh 12 23 12
any two or all three are same
root@kali:~# ./ifelse.sh 35 2 19
var1 is greatest
root@kali:~# ./ifelse.sh 5 14 12
no condtion

Switch Case

root@kali:~#vi case.sh

#!/bin/bash/

case $1 in
        "if")
                echo "if condition"
        ;;
        "elif")
                echo "elif condition"
        ;;
        "else")
                echo "else condition"
        ;;
esac

root@kali:~# chmod +x case.sh
root@kali:~# ./case.sh if
if condition
root@kali:~# ./case.sh elif
elif condition
root@kali:~# ./case.sh else
else condition

LOOP STRUCTURES

FOR LOOP

root@kali:~#vi for.sh

#!/bin/bash/

for i in 1 2 3 4 5
do
        echo " $i times this statement executed "
done

for (( i=0 ; i<=5 ; i++ ))
do
        echo " $i "
done

root@kali:~# chmod +x for.sh**
root@kali:~# ./for.sh
 1 times this statement executed
 2 times this statement executed
 3 times this statement executed
 4 times this statement executed
 5 times this statement executed
 0
 1
 2
 3
 4
 5

WHILE LOOP

---

NOTE

• ‘-lt’ is ‘<’
• ‘-le’ is ‘<=’
• ‘-gt’ is ‘>’
• ‘-ge’ is ‘<=’
• ‘-ne’ is ‘!=’
• ‘-e’ is ‘==’

---

root@kali:~#vi while.sh

#!/bin/bash/

i=0

while [ $i -lt 5 ]
do
        echo " printing $i times "
        i=`expr $i + 1`
done

root@kali:~# chmod +x while.sh
root@kali:~# ./while.sh
 printing 0 times
 printing 1 times
 printing 2 times
 printing 3 times
 printing 4 times