How to stop a infinite while loop running in background?

Question

Is there a option to let a endless while loop, when it's running in background as a function, stop and start running at any given moment with a local variable? I already did try a lot of options, only i was not able to find a neat solution. The only way i could get it working is reading a external text file from within the while loop. And at specified points in the program writing a 0 or 1 to that text file.

What i'm doing now is:

    #!/bin/bash

intr(){ while true                  # function endless while loop start 
        do 
        sleep 0.5                   # execute function every x time
        var1=`grep "1" 0or1.txt`    # read file 0or1.txt 
        if [ -n "$var1" ] ; then    # if text =1 execute function, 
        # do some magic..
        fi
        done
        }                           # end function
        intr &                      # execute function as bg process

#some code                          # located some where in the script
echo "1" > 0or1.txt                 # write 1 to start function 
#some code                          # this should be a local variable??


#some code                          # located some where in the script               
echo "0" > 0or1.txt                 # write 0 to stop function
#some code                          # this should be a local variable??

Answer 1

From the discussion in the comments bellow the question it became apparent that what OP wants is to essentially pause the background function. Quote:

@ serg, the main objective is to let this code functioning as a interrupt. At several points in the program i need to disable the "interrupt" and at several points i need to enable it. (it's not a one time event.)

It is possible to do so with -SIGSTOP and -SIGCONT. These two signals are essentially pause and play buttons of a process. Bellow is modified demo script from what I've originally posted. Essentially , I've spawned function endless as background process, and first send -SIGSTOP and then -SIGCONT to it. The big important point here is that the background process will continue to run even if the main process exits, so in my example bellow the interrupt wouldn't stop outputting to the console, unless I issued kill 1234 form another terminal, where 1234 is the function PID reported from the script. So watch out for that

$ ./someInfiniteLoop.sh                                        
We are in the main, about to spawn the interrupt function
Parent process (the script itself)  7119
Captured 7120
>>>> HEY I'M THE 1 SECOND INTERRUPT
>>>> HEY I'M THE 1 SECOND INTERRUPT
>>>> HEY I'M THE 1 SECOND INTERRUPT
Sending SIGSTOP to  7120
Funciton paused; waiting 3 seconds
You could do something else when function is paused
Function resumed
>>>> HEY I'M THE 1 SECOND INTERRUPT
>>>> HEY I'M THE 1 SECOND INTERRUPT
>>>> HEY I'M THE 1 SECOND INTERRUPT

Demo script source code:

#!/bin/bash
# use the line bellow if you want to silence error messages
# exec 2>/dev/null
endless()
{
  while true
  do

     echo ">>>> HEY I'M THE 1 SECOND INTERRUPT"
  sleep 1
  done
}

echo 'We are in the main, about to spawn the interrupt function'
echo "Parent process (the script itself)  $$"

endless &
endless_pid="$!"
echo "Captured $endless_pid"
sleep 3

echo 'Sending SIGSTOP to ' $endless_pid
kill -SIGSTOP $endless_pid
[ $? -eq 0  ] && echo 'Funciton paused; waiting 3 seconds' || echo 'Something is wrong'

echo 'You could do something else when function is paused'

sleep 3

kill -SIGCONT $endless_pid

[ $? -eq 0   ] && echo 'Function resumed' || echo 'Something is wrong'

sleep 3 && exit 0 

You may wanna read also :

• bash: silently kill background function process
• suppress stderr in bash script

---

Answer 2

Use the break builtin to stop the while loop.

From help break:

break: break [n]

Exit for, while, or until loops.

Exit a FOR, WHILE or UNTIL loop.  If N is specified, break N enclosing
loops.

Exit Status:
The exit status is 0 unless N is not greater than or equal to 1.

So in your snippet, you can do:

while :; do
    if [ -n "$var1" ] ; then
        break
    fi
done

Or even shorter:

while :; do
    [ -n "$var1" ] && break
done

---

To pass any input to the function, use positional parameters i.e. arguments. The first argument can be retrieved by $1, second $2 and so on.

For example, if you call function foobar by:

foobar spam

In the function you can use get spam by using $1:

$ foobar () { echo "This is $1" ;}

$ foobar spam
This is spam