The bash while loop is a control flow statement that allows code or commands to be executed repeatedly based on a given condition.

Most languages have the concept of loops: If we want to repeat a task twenty times, we don’t want to have to type in the code twenty times, with maybe a slight change each time.
As a result, we have for and while loops in the Bourne shell.

Types of Loops in Shell Scripting

Syntax of Shell Scripting

Control Flow of While Loop

Example of While Loop in Shell Script

Boolean Operators When Working With Shell Scripts

Apart from the boolean operators that you’ve seen in other programming languages, including ==, <, >, <=, >=, etc., these are shell script specific operators. The major usage difference is that these operators will only work when numeric values are provided. With string value, the operation will simply fail.

Creating a While Loop in Shell Script

A while loop doesn’t have a repetition limit by default. We have to explicitly provide it a condition that at some point during the code execution, will turn to false. Else you’ll have a loop that doesn’t end unless an interrupt signal is sent.

Save the script above by any name with the .sh extension. To run the file, you can either run it with the bash command like bash <filename>.sh or make the file executable using the chmod command. To make the script executable, run chmod +x <filename>.sh and then ./<filename>.sh will allow you to run the script.

Let’s understand the script line by line.

• i=0 – Here we set the variable $i to 0. Learn more about variables in our previous tutorial
• while [ $i -le 10 ] – Run the while loop only until the variable $i is lesser than or equal to 10. So this loop will run 11 times including the ZERO-th run.
• do – Marks the beginning of the while loop in shell scripts
• echo True – A simple command that will print the word “True” on our terminal
• ((i++)) – C style variable increments to add 1 to the variable i with every loop. There are various other ways to perform an increment include $i=((i+1)), ((i=i+1)), etc. Feel free to use either one.
• done – Marks the end of the while loop
• echo False – This again simply prints out the word False on the screen to indicate that condition has now turned false.

Example of nested While Loop

	# ############################
	# Program - 1
	# ############################
	#!/bin/sh
	a=0
	while [ $a -lt 10 ]
	do
	echo $a
	a=`expr $a + 1`
	done
	# ############################
	# Program - 2
	# ############################
	#!/bin/bash
	x=1
	while [ $x -le 5 ]
	do
	echo "Welcome $x times"
	x=$(( $x + 1 ))
	done
	# ############################
	# Program - 3
	# ############################
	# And here is above code as a bash while one liner:
	x=1; while [ $x -le 5 ]; do echo "Welcome $x times" $(( x++ )); done
	# ############################
	# Program - 4
	# ############################
	# 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"
	# ############################
	# Program - 5
	# ############################
	# 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 $line
	done
	exec 0<&3
	# ############################
	# Program - 6
	# ############################
	# Here is a bash while loop that read those IP address separated by Internal Field Separator ($IFS) to an octothorpe (#):
	#!/bin/bash
	# Script name: block_ips.sh
	# Set the Internal Field Separator to an octothorpe '#'
	IFS='#'
	# Set input file name here
	INPUT="bad-guys.ips.txt"
	# Read file line-by-line to get an IP and comment to block it using the iptables
	while read -r ip comment
	do
	/sbin/iptables -A INPUT -s "$ip" -m comment --comment "$comment" -j DROP
	done < "$INPUT"
	# ############################
	# Program - 7
	# ############################
	# Infinite for while can be created with empty expressions, such as:
	#!/bin/bash
	while :
	do
	echo "infinite loops [ hit CTRL+C to stop]"
	done
	# ############################
	# Program - 8
	# ############################
	# Conditional while loop exit with break statement
	#!/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
	# ############################
	# Program -9
	# ############################
	# Early continuation with the continue statement
	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
	# ############################
	# Program -10
	# ############################
	#!/bin/ksh
	# Script name: while.ksh
	num=1
	while (( num < 6 ))
	do
	print "The value of num is: $num"
	(( num = num + 1 )) # let num=num+1
	done
	print "Done."
	# ############################
	# Program -11
	# ############################
	#!/bin/ksh
	# Script name: readinput.ksh
	print -n "Enter a string: "
	while read var
	do
	print "Keyboard input is: $var"
	print -n "\nEnter a string: "
	done
	print "End of input."
	# ############################
	# Program -12
	# ############################
	#!/bin/ksh
	# Script name: internalredir.ksh
	# set the Internal Field Separator to a colon
	IFS=:
	while read name number
	do
	print "The phone number for $name is $number"
	done < phonelist
	# ############################
	# Program -13
	# ############################
	a=1
	until [ $a -ge 3 ]
	do
	echo “value of a=” $a
	a=`expr $a + 1`
	done
	# ############################
	# Program -14
	# ############################
	num=1
	while [ $num -le 5 ]
	do
	read var
	if [ $var -lt 0 ]
	then
	break
	fi
	num=`expr $num + 1`
	done
	echo “The loop breaks for negative numbers”
	# ############################
	# Program -15
	# ############################
	#!/bin/bash
	# While Loop Example with 2 table, print any given number table.
	# See Full Explanation of this above shell script [while loop](https://www.youtube.com/Techarkit?sub_confirmation=1)
	#START
	echo -e "Please provide one value: \c"
	read -r c
	i=1
	while [ $i -le 10 ]
	do
	b=`expr $c \* $i`
	echo "$c * $i = $b"
	i=`expr $i + 1`
	done
	#END

view raw while.sh hosted with by GitHub

Rajesh Kumar

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