Similar to the for command, while is also the command for loop operations. The command next to while is evaluated. If it is successful or 0 then the commands inside do and done are executed.

The purpose of a loop is to test a certain condition or expression and execute a given command while the condition is true (while loop) or until the condition becomes true (until loop):

The following is the while_01.sh script, in which we read a file and display its it's content:

#!/bin/bash
file=/etc/resolv.conf
while IFS= read -r line    # IFS : inter field separator
do
   # echo line is stored in $line
  echo $line
done < "$file"

Let's test the program:

$ chmod +x while_01.sh
$ ./while_01.sh

The following will be the output after executing the preceding commands:

nameserver 192.168.168.2
search localdomain

In the following script while_02.sh, we are printing number 1–10 on the screen using the while loop:

#!/bin/bash
declare -i x
x=0
while [ $x -le 10 ]
do
  echo $x
  x=$((x+1))
done

Let's test the program:

$ chmod +x while_02.sh
$ ./while_02.sh

The following will be the output after executing the preceding commands:

0
1
2
3
4
5
6
7
8
9
10

In the following script while_03.sh, we ask the user to input the test. If the input of the text is quit, then we terminate the loop; otherwise, we print the text on the screen:

#!/bin/bash
INPUT=""
while [ "$INPUT" != quit ]
do
   echo ""
   echo 'Enter a word (quit to exit) : '
   read INPUT
   echo "You typed : $INPUT"
done

Let's test the program:

$ chmod +x while_03.sh
$ ./while_03.sh

The following will be the output after executing the preceding commands:

Enter a word (quit to exit) :
GANESH
You typed : GANESH
Enter a word (quit to exit) :
Naik
You typed : Naik

Enter a word (quit to exit) :
quit
You typed : quit

In the following while_04.sh script, we print the content of variable num on screen. We are starting with value 1. In the loop, we increment the value of the num variable by 1. When the value of the variable num reaches 6, then the while loop is terminated:

#!/bin/bash
num=1
while (( num < 6 ))
do
  echo "The value of num is: $num"
  (( num = num + 1 ))             # let num=num+1
done
echo "Done."

Let's test the program:

$ chmod +x while_04.sh
$ ./while_04.sh

The following will be the output after executing the preceding commands:

The value of num is: 1
The value of num is: 2
The value of num is: 3
The value of num is: 4
The value of num is: 5
Done.

The while_05.sh script prints a series of odd numbers on the screen. We are passing a total number of odd numbers required as command-line parameter:

#!/bin/bash
count=1
num=1
while [ $count -le $1 ]
do
  echo $num
  num=`expr $num + 2`
  count=`expr $count + 1`
done

Let's test the program:

$ chmod +x while_05.sh
$ ./while_05.sh 5

The following will be the output after executing the preceding commands:

1
3
5
7
9