Bash While Loop Example
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
.......
..
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Tags: bash while do loop syntax, control flow, echo command, flow statement, Linux, loop syntax, shell code, UNIX, unix operating system, while command
March 16th, 2008 at 1:02 am
Nice to see the Bash shell,
How do you break out of a loop
is there a break statement, or do you have to use a goto?
March 18th, 2008 at 10:22 am
Yes there is a break and continue statement. man bash has more information.