How do I use bash while loop to repeat certain task under Linux / UNIX operating system? How do I set infinite loops using while statement? The 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.
bash while loop syntax
The syntax is as follows:
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
.......
..
How do I use while as infinite loops?
Infinite for while can be created with empty expressions, such as:
#!/bin/bash
while :
do
echo "infinite loops [ hit CTRL+C to stop]"
done
Conditional while loop exit with break statement
You can do early exit with the break statement inside the whil loop. You can exit from within a WHILE using break. General break statement inside the while loop is as follows:
while [ condition ]
do
statements1 #Executed as long as condition is true and/or, up to a disaster-condition if any.
statements2
if (disaster-condition)
then
break #Abandon the while lopp.
fi
statements3 #While good and, no disaster-condition.
done
In this example, the break statement will skip the while loop when user enters -1, otherwise it will keep adding two numbers:
#!/bin/bash
while :
do
read -p "Enter two numnbers ( - 1 to quit ) : " a b
if [ $a -eq -1 ]
then
break
fi
ans=$(( a + b ))
echo $ans
done
Early continuation with the continue statement
To resume the next iteration of the enclosing WHILE loop use the continue statement as follows:
while [ condition ]
do
statements1 #Executed as long as condition is true and/or, up to a disaster-condition if any.
statements2
if (condition)
then
continue #Go to next iteration of I in the loop and skip statements3
fi
statements3
done
