Prevent SIGINT from interrupting current task while still passing information about SIGINT (and preserve the exit code)

Question

I have a quite long shell script and I'm trying to add signal handling to it.

The main task of the script is to run various programs and then clean up their temporary files.

I want to trap SIGINT. When the signal is caught, the script should wait for the current program to finish execution, then do the cleanup and exit.

Here is an MCVE:

#!/bin/sh

stop_this=0
trap 'stop_this=1' 2

while true ; do
    result="$(sleep 2 ; echo success)" # run some program
    echo "result: '$result'"
    echo "Cleaning up..." # clean up temporary files
    if [ $stop_this -ne 0 ] ; then
        echo 'OK, time to stop this.'
        break
    fi
done

exit 0

The expected result:

Cleaning up...
result: 'success'
Cleaning up...
^Cresult: 'success'
Cleaning up...
OK, time to stop this.

The actual result:

Cleaning up...
result: 'success'
Cleaning up...
^Cresult: ''
Cleaning up...
OK, time to stop this.

The problem is that the currently running instruction (result="$(sleep 2 ; echo success)" in this case) is interrupted. What can I do so it would behave more like I was set trap '' 2?

I'm looking for either a POSIX solution or one that is supported by most of shell interpreters (BusyBox, dash, Cygwin...)

I already saw answers for Prevent SIGINT from closing child process in bash script but this isn't really working for me. All of these solutions require to modify each line which shouldn't be interrupted. My real script is quite long and much more complicated than the example. I would have to modify hundreds of lines.

Answer 1

Both answers from @PSkocik and @WilliamPursell have helped me to get on the right track.

I have a fully working solution. It ain't pretty because it needs to use an external file to indicate that the signal didn't occurred but beside that it should work reliably.

#!/bin/sh

touch ./continue
trap 'rm -f ./continue' 2

( # the whole main body of the script is in a separate background process
trap '' 2 # ignore SIGINT
while true ; do
    result="$(sleep 2 ; echo success)" # run some program
    echo "result: '$result'"
    echo "Cleaning up..." # clean up temporary files
    if [ ! -e ./continue ] ; then # exit the loop if file "./continue" is deleted
        echo 'OK, time to stop this.'
        break
    fi
done
) & # end of the main body of the script
while ! wait ; do : ; done # wait for the background process to end (ignore signals)
wait $! # wait again to get the exit code
result=$? # exit code of the background process

rm -f ./continue # clean up if the background process ended without a signal

exit $result

---

EDIT: There are some problems with this code in Cygwin.

The main functionality regarding signals work. However, it seems like the finished background process doesn't stay in the system as a zombie. This makes the wait $! to not work. The exit code of the script has incorrect value of 127.

Solution to that would be removing lines wait $!, result=$? and result=$? so the script always returns 0. It should be also possible to keep the proper error code by using another layer of subshell and temporarily store the exit code in a file.