13. 07.

Bash While Loop Example – Bash While 循环实例

Q. How do I use bash while loop to repeat certain task under Linux / UNIX operating system?

A. Bash while loop is a control flow statement that allows code or

commands to be executed repeatedly based on a given condition. For

example, run echo command 5 times or read text file line by line or

evaluate the options passed on the command line for a script.

while loop syntax

while [ condition ]

do

command1

command2

command3

done

command1 to command3 will be executed repeatedly till condition is

true. The argument for a while loop can be any boolean expression.

Infinite loops occur when the conditional never evaluates to false. For

example following while loop will print welcome 5 times on screen:

#!/bin/bash

x=1

while [ $x -le 5 ]

do

echo "Welcome $x times"

x=$(( $x + 1 ))

done

Here is a sample shell code to calculate factorial using while loop:

#!/bin/bash

counter=$1

factorial=1

while [ $counter -gt 0 ]

do

factorial=$(( $factorial * $counter ))

counter=$(( $counter - 1 ))

done

echo $factorial

To run just type:

$ chmod +x script.sh

$ ./script.sh 5

Output:

120

While loops are frequently used for reading data line by line from file:

#!/bin/bash

FILE=$1

# read $FILE using the file descriptors

exec 3exec 0while read line

do

# use $line variable to process line

echo $line

done

exec 0

You can easily evaluate the options passed on the command line for a script. using while loop:

......

..

while getopts ae:f:hd:s:qx: option

do

case "${option}"

in

a) ALARM="TRUE";;

e) ADMIN=${OPTARG};;

d) DOMAIN=${OPTARG};;

f) SERVERFILE=$OPTARG;;

s) WHOIS_SERVER=$OPTARG;;

q) QUIET="TRUE";;

x) WARNDAYS=$OPTARG;;

\?) usage

exit 1;;

esac

done

.......

..

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