How to check if a process id (PID) exists

Question

In a bash script, I want to do the following (in pseudo-code):

if [ a process exists with $PID ]; then

    kill $PID 

fi

What's the appropriate expression for the conditional statement?

Answer 1

To check for the existence of a process, use

kill -0 $pid

But just as @unwind said, if you want it to terminate in any case, then just

kill $pid

Otherwise you will have a race condition, where the process might have disappeared after the first kill -0.

If you want to ignore the text output of kill and do something based on the exit code, you can

if ! kill $pid > /dev/null 2>&1; then
    echo "Could not send SIGTERM to process $pid" >&2
fi