Check if PID exists in Bash

Question

I want to stall the execution of my BASH script until a process is closed (I have the PID stored in a variable). I'm thinking

while [PID IS RUNNING]; do sleep 500 done 

Most of the examples I have seen use /dev/null which seems to require root. Is there a way to do this without requiring root?

Thank you very much in advance!

Answer 1

kill -s 0 $pid will return success if $pid is running, failure otherwise, without actually sending a signal to the process, so you can use that in your if statement directly.

wait $pid will wait on that process, replacing your whole loop.

Answer 2

It seems like you want

wait $pid 

which will return when $pid finishes.

Otherwise you can use

ps -p $pid 

to check if the process is still alive (this is more effective than kill -0 $pid because it will work even if you don't own the pid).