Unix Shell - The until Loop

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The while loop is perfect for a situation where you need to execute a set of commands while some condition is true. Sometimes you need to execute a set of commands until a condition is true.

Syntax:

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

Here Shell command is evaluated. If the resulting value is false, given statement(s) are executed. If command is true then no statement would be not executed and program would jump to the next line after done statement.

Example:

Here is a simple example that uses the until loop to display the numbers zero to nine:

#!/bin/sh

a=0

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

This will produce following result:

0
1
2
3
4
5
6
7
8
9


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