bash script

 

http://web.theurbanpenguin.com/training/scripting/mastering-bash-shell-scripting/

http://web.theurbanpenguin.com/category/bash-scripting/

http://web.theurbanpenguin.com/category/bash-scripting/

if statements

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syntax

if [ expression ];  

then  

statements  

fi  

//////////////////////////////

example 1

read -p "Enter number:" number

if [ $number -gt 123 ];

then

echo "value is greater than 123"

fi

o/p:value is greater than 123

////////////////////////////////////

example 2

if [ "myfile" == "myfile" ];

then

        echo "its true"

fi

o/p:its true

//////////////////////////////////

For using multiple conditions with AND operator:

''''''''''''''''''''''''''''''''''''''''''''''''''''''

if [ expression_1 ] && [ expression_2 ];  

then  

statements  

fi

example 1

if [ 1 -lt 10 ] && [ 100 -gt 10 ];

then

echo "we won"

fi

o/p: we won

////////////////////////////

For using multiple conditions with OR operator:

if [ expression_1 ] || [ expression_2 ];  

then  

statements  

fi  

case statement example

echo "Enter the arithmetic operation you want to perform?"

echo "A for ADD, S for SUBTRACTION, M FOR MULTIPLY, D FOR DIVISION"

echo "Enter the  operands "

read x

read y

read -p "A|S|M|D" val

case $val in

        A|a)

 sum=$((y+x))

echo $sum

;;

  S|s)

          ((sub=$x-$y))

echo " Subtraction of $x - $y =$sub"

;;

      M|m)

              ((mul=$x*$y))

echo " Multiplication of $x * $y =$mul"

;;

      D|d)

        ((div=$x/$y))

echo "Division of $x/$y =$div"

;;

esac

forloop 

**********************************

syntax

for (( expression1; expression2; expression3 ))  

do  

commands  

done

///////////////////////////////////////

example 1

echo "FINDING SUM OF N NUMBERS"

read x

sum=0

for (( i=0; i<=x; i++ ))

do

        ((sum=sum+i))

done

echo "$sum"

o/p x=5

15

//////////////////////////////////////

syntax

for variable in list  

do  

commands  

done  

example

for num in {1..10}

do

echo $num

done

echo"series of numbers from 1 to 10"

o/p:

1

2

3

4

5

6

7

8

9

10

////////////////////////////////////////////////

For Loop to Read Array Variables

syntax

array=(  "element1" "element 2" .  .  "elementN" )  

  

for i in "${arr[@]}"  

do  

echo $i  

done  

example

arr=( "sun" "moon" "earth" "mars" "venus" "mercury" )

for i in "${arr[@]}"

do

echo $i

done

o/p

sun

moon

earth

mars

venus

mercury

/////////////////////////////////////////////////

For Loop with a Continue Statement

use the 'continue' statement inside the 'for' loop to skip any specific statement on a particular condition. It tells Bash to stop executing that particular iteration of the loop and process the next iteration.

example program to print Numbers from 1 to 20, ignoring from 6 to 15 using continue statement"

for ((i=1; i<=20; i++));

do

if [[ $i -gt 5 && $i -lt 16 ]];

then

continue

fi

echo $i

done

///////////////////////////////////////////////////////////////////

For Loop with a Break Statement

A 'break' statement can be used inside 'for' loop to terminate from the loop.

example

#!/bin/bash

#Table of 2

for table in {2..100..2}

do

        echo $table

        if [ $table == 20 ]; then

                break

        fi

done

o/p:

2

4

6

8

10

12

14

16

18

20

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

Infinite Bash For Loop

When there is no 'start, condition, and increment' in the bash three expressions for loop, it becomes an infinite loop. To terminate the infinite loop in Bash, we can press Ctrl+C.

example

i=1;

for (( ; ; ))

do

        sleep 1s

        echo "Current Number: $((i++))"

done

Bash While Loop

****************************

while [ condition ];

 do 

 commands; 

 done  

 

 example

 #!/bin/bash

#Script to get specified numbers

read -p "Enter starting number: " snum

read -p "Enter ending number: " enum

while [[ $snum -le $enum ]];

do

        echo $snum

        ((snum++))

done

echo "This is the sequence that you wanted."

o/p

Enter starting number: 2

Enter ending number: 9

2

3

4

5

6

7

8

9

This is the sequence that you wanted.

////////////////////////////////////////////////////////////

Infinite While Loop

An infinite loop is a loop that has no ending or termination. If the condition always evaluates to true, it creates an infinite loop. 

example

#!/bin/bash

#An infinite while loop

while :;

do

        echo "YOU r in a loop.";

done