Bash Infinite Loop Examples

by on January 20, 2011 · 7 comments· LAST UPDATED January 20, 2011

in

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

OR

 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
 

OR

 ### 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

See also:

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{ 7 comments… read them below or add one }

1 Alexander Lagerström March 10, 2011 at 5:37 pm

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

Reply

2 Eric Smith July 19, 2011 at 2:13 pm

Great article mate! Used this one yesterday :-)

Reply

3 ilkata89 October 7, 2012 at 7:40 pm

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

Reply

4 Carl 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.

Reply

5 rickard August 2, 2013 at 7:13 am

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.

Reply

6 Robert November 25, 2013 at 3:47 pm

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.

Reply

7 NT4Boy December 2, 2013 at 2:36 pm

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?

Reply

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