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
Code:
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:
Code:
#!/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:
Code:
#!/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:
Code:
#!/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 $line
done
exec 0<&3
You can easily evaluate the options passed on the command line for a script using while loop:
......
..
while getopts ae:f:hd:s:qx: option
Code:
do
case "${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
.......
Reference:
http://www.cyberciti.biz/faq/bash-while-loop/
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