If you have a process ID but aren’t sure whether it’s valid, you can use the most unlikely of candidates to test it: the kill command. If you don’t see any reference to this on the kill(1) man page, check the info pages. The man/info page states that signal 0 is special and that the exit code from kill tells whether a signal could be sent to the specified process (or processes).
So kill -0 will not terminate the process, and the return status can be used to determine whether a process is running. For example:
$ echo $$ # show our process id
12833
$ /bin/bash # create new process
$ echo $$ # show new process id
12902
$ kill -0 12902
$ echo $? # exists, exit code is 0
0
$ exit # return to previous shell
$ kill -0 12902
bash: kill: (12902) - No such process
$ echo $? # doesn’t exist, exit code is 1
1
Many UNIX dæmons store their process IDs in a file in /var/run when they are started. Using kill -0 to test the pid is a lot easier than parsing ps output. For example, to test whether cron is running, do the following:
# kill -0 $(cat /var/run/cron.pid)
# echo $?
0
