Bourne/bash shell script: while loop syntax

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From the computer of: Rex (39 recipes)
Created: Jan 09, 2004     Updated: Aug 30, 2004

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A while loop allows execution of a code block an arbitrary number of times until a condition is met. This recipe describes the while loop syntax for the various Bourne shells (sh, ksh, bash, zsh, etc.) and provides examples.

General syntax:

Any valid conditional expression will work in the while loop. The following code asks the user for input and verifies the input asking for a y or n and will repeat the process until a y answer is received.

Another useful while loop is a simple one, and infinite loop. If the conditional is "1" the loop will run until something else breaks it (such as the break keyword):

This while loop will display the number of sendmail processes running every five seconds until the loop or executing shell is killed. The brackets in [s]endmail are a regular expression trick that prevents the 'grep [s]endmail' process from being counted.