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Bourne/bash shell script: while loop syntax

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:

Code:

while [ condition ] 
do 
    code block; 
done

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.

Code:

verify="n" 
while [ "$verify" != y ] 
do 
    echo "Enter option: " 
    read option 
    echo "You entered $option.  Is this correct? (y/n)" 
    read verify 
done

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):

Code:

while [ 1 ] 
do 
    ps -ef | grep [s]endmail | wc -l 
    sleep 5 
done

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 ~~endmail are a regular expression trick that prevents the 'grep ~~endmail' process from being counted.~~~~

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THE TECHNO CLUB [ TECHNOWORLDINC.COM ] => Shell Script => Topic started by: Mark David on March 16, 2007, 01:22:47 PM