Bash While Loop Examples

How do I use bash while loop to repeat specific task under Linux / UNIX operating system? How do I set infinite loops using while statement? Can you provide me the while loop examples?

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

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. Here is the while loop one-liner syntax:

while [ condition ]; do commands; done
while control-command; do COMMANDS; done

while [ condition ]; do commands; done
while control-command; do COMMANDS; done

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

#!/bin/bash
x=1
while [ $x -le 5 ]
do
echo "Welcome $x times"
x=$(( $x + 1 ))
done

And here is above code as a bash while one liner:
x=1; while [ $x -le 5 ]; do echo "Welcome $x times" $(( x++ )); 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

#!/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

The while loop in action on my Ubuntu Linux desktop

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

#!/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
.......
..

……
..
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

#!/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

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

#!/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

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

Recommended readings:

  • See all sample shell script in our bash shell directory
  • Bash loops from our Linux shell scripting tutorial guide
  • man bash
  • help while
  • help {
  • help break
  • help continue

Posted by: SXI ADMIN

The author is the creator of SXI LLC and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.