Unix / Linux Shell - The while Loop

The while loop enables you to execute a set of commands repeatedly until some condition occurs. It is usually used when you need to manipulate the value of a variable repeatedly.

Syntax

while command
do
   Statement(s) to be executed if command is true
done

Here the Shell command is evaluated. If the resulting value is true, given statement(s) are executed. If command is false then no statement will be executed and the program will jump to the next line after the done statement.

Example

Here is a simple example that uses the while loop to display the numbers zero to nine −

#!/bin/sh

a=0

while [ $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done

Upon execution, you will receive the following result −

0
1
2
3
4
5
6
7
8
9

Each time this loop executes, the variable a is checked to see whether it has a value that is less than 10. If the value of a is less than 10, this test condition has an exit status of 0. In this case, the current value of a is displayed and later a is incremented by 1.