Bash Infinite Loop Examples

How do I write an infinite loop in Bash script under Linux or UNIX like operating systems?

An infinite loop is nothing but a sequence of instructions which loops endlessly, either due to the loop having no terminating condition, having one that can never be met, or one that causes the loop to start over. The syntax is as follows using the while loop:

#!/bin/bash
while :
do
	echo "Press [CTRL+C] to stop.."
	sleep 1
done

This is a loop that will forever print “Press [CTRL+C] to stop..”. Please note that : is the null command. The null command does nothing and its exit status is always set to true. You can modify the above as follows to improve the readability:

#!/bin/bash
while true
do
	echo "Press [CTRL+C] to stop.."
	sleep 1
done

A single-line bash infinite while loop syntax is as follows:

 while :; do echo 'Hit CTRL+C'; sleep 1; done

 while true; do echo 'Hit CTRL+C'; sleep 1; done

Bash for infinite loop example

#!/bin/bash
 
for (( ; ; ))
do
   echo "Pres CTRL+C to stop..."
   sleep 1
done

How Do I Escape the Loop?

A for or while loop may be escaped with a break statement when certain condition is satisfied:

 ### for loop example ###
for (( ; ; ))
do
   echo "Pres CTRL+C to stop..."
   sleep 1
   if (disaster-condition)
   then
	break       	   #Abandon the loop.
   fi
done

 ### while loop example ###
while :
do
   echo "Pres CTRL+C to stop..."
   sleep 1
   if (disaster-condition)
   then
	break       	   #Abandon the loop.
   fi
done

You can also use the case statement to esacpe with a break statement:

while :
do 
     ### add some input and output here ###
     case $var in
       yes) do something ;;
       no) do something ;;
       quit) break ;;        ##Abandon the loop.
     ease
done

Example

A sample shell script to demonstrate the actual usage of an infinite loop and the break statement:

#!/bin/bash
# Purpose: Display various options to operator using menus
# Author: Vivek Gite < vivek @ nixcraft . com > under GPL v2.0+
# ---------------------------------------------------------------------------
# capture CTRL+C, CTRL+Z and quit singles using the trap
trap '' SIGINT
trap ''  SIGQUIT
trap '' SIGTSTP
 
# display message and pause 
pause(){
	local m="$@"
	echo "$m"
	read -p "Press [Enter] key to continue..." key
}
 
# set an 
while :
do
	# show menu
	clear
	echo "---------------------------------"
	echo "	     M A I N - M E N U"
	echo "---------------------------------"
	echo "1. Show current date/time"
	echo "2. Show what users are doing"
	echo "3. Show top memory & cpu eating process"
	echo "4. Show network stats"
	echo "5. Exit"
	echo "---------------------------------"
	read -r -p "Enter your choice [1-5] : " c
	# take action
	case $c in
		1) pause "$(date)";;
		2) w| less;;
		3) echo '*** Top 10 Memory eating process:'; ps -auxf | sort -nr -k 4 | head -10; 
		   echo; echo '*** Top 10 CPU eating process:';ps -auxf | sort -nr -k 3 | head -10; 
		   echo;  pause;;
		4) netstat -s | less;;
		5) break;;
		*) Pause "Select between 1 to 5 only"
	esac
done

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

14 comment

somevar=1 && while [ $somevar -lt 2 ]; do echo “Something”;done

Great article mate! Used this one yesterday :-)

An example in a book that I am reading is as follows:

#!/bin/sh

yes_or_no()
{
  echo "Is your name $* ?"
  while true
  do
    echo -n "Enter yes or no: "
    read x
    case "$x" in
      y | yes ) return 0;;
      n | no  ) return 1;;
      *       ) echo "Answers can only be yes or no";;
      esac
  done
}

echo "Original parameters are $*"

if yes_or_no "$1"
then
  echo "Hi $1, nice name."
else
  echo "Never mind."
fi

exit 0

I don’t understand what makes the infinate while loop get terminated in the yes_or_no() function.

Thanks

Carl says:
May 1, 2013 at 12:54 pm
In ilkata89’s code, the return statements cause the yes_or_no() function to end, thus terminating the loop.

The while loop is in a function, note the ().

The if else statement calls the function and if your name is the same as $0 then the condition is true and if not it returns 0 and prints out Never mind and exits.

Great example but do you have similar one to make program use up available memory?
Need to do some testing but need a memory hog program.

while true; do cat big.random.block; | dd of=/dev/st0 bs=1024

this is a bit of a script for overwriting random data via a file created that’s 10meg in size to tapes, But, it doesn’t stop when the tape is full,….
So, how should this “true” become untrue so it exits please?

Just what I needed! Thanks so much.

I’m using the following line:

$ for (( ;; )) do ping -c 1 google.com; sleep 15; done

This is for checking whether my internet connection has “recovered” after suspend/hibernate. It will always take quite awhile here, and I was tired of repeatedly hacking in “ping…” until there was some response…

Alexey Serikov says:
November 12, 2014 at 10:59 am
[code]
ping -i 15 google.co
[/code]
will have the same effect

Nice… use it for my server

thanks :) me ajudou muito

I always use this:

while [ true ] ; do something() ; done

The case “*)” in the “Example” above is not working:
*) Pause “Select between 1 to 5 only”

The message is not displayed if a number >5 is entered.

I’ve changed it to

*) echo “Select between 1 to 5 only”; pause

Then it’s working. Only for the sake of form :)

Cpt. Varel says:
May 16, 2017 at 8:50 am
Forget my 1st comment.
Just saw that “Pause” was written up.
If it is small-written, it works naturally ;)

nixCraft