How to check if a process id (PID) exists

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?

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 that the exit code will be non-zero even if the process is running and you don't have permission to kill it. For example:

kill -0 $known_running_pid

and

kill -0 $non_running_pid

have a non-zero exit codes that are indistinguishable for a normal user, but one of them is by assumption running, while the other is not.

Partly related, additional info provided by AnrDaemon: The init process (PID 1) is certainly running on all Linux machines, but not all POSIX systems are Linux. PID 1 is not guaranteed to exist there:

kill -0 1 
-bash: kill: (1) - No such process … 

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 don't need to go looking in /proc because what if Linus decides to call the exe file something else?

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

If your system implements procfs interface, you can just check if there is a special file /proc/$PID/status:

if [ -e /proc/$PID/status ]; then
    echo "process exists"
fi

otherwise you can use ps program:

if [ -n "$(ps -p $PID -o pid=)" ]

In the latter form, -o pid= is an output format to display only the process ID column with no header. The quotes are necessary for non-empty string operator -n to give valid result.