Bah while

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 $FILE using the file descriptors exec 3<&0 exec 0<$FILE while read line do # use $line variable to process line echo $line done exec 0<&3

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 ....... ..