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

The best way is:

if ps -p $PID > /dev/null
then
   echo "$PID is running"
   # Do something knowing the pid exists, i.e. the process with $PID is running
fi

The problem with:

kill -0 $PID

is the exit code will be non-zero even if the pid is running and you dont have permission to kill it. For example:

kill -0 1

and

kill -0 $non-running-pid

have an indistinguishable (non-zero) exit code for a normal user, but the init process (PID 1) is certainly running.

DISCUSSION

The answers discussing kill and race conditions are exactly right if the body of the test is a "kill". I came looking for the general "how do you test for a PID existence in bash".

The /proc method is interesting, but in some sense breaks the spirit of the "ps" command abstraction, i.e. you dont need to go looking in /proc because what if Linus decides to call the "exe" file something else?