Working with the do while loop

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):

while condition do commands done

until condition do commands done

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...