Bash Infinite Loop Examples

by nixCraft on January 20, 2011 · 9 comments· LAST UPDATED January 20, 2011

in BASH Shell

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