Wait for a program (non-child) to finish and execute a command

Michael O. :

I have a program running on a remote computer which shouldn't be stopped. I need to track when this program is stopped and immediately execute a command. PID is known. How can I do that?

Thomas :

You cannot wait for non-child processes.

Probably the most efficient way in a shell would be to poll using the exit code of kill -0 <pid> to check if the process still exists:

while kill -0 $PID 2>/dev/null; do sleep 1; done

This is both simpler and more efficient than any approaches involving ps and grep. However, it only works if your user has permission to send signals to that process.