Monitoring Processes with Kill
September 23rd, 2008 by Rich Lundeen in
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
__________________________
Special Magazine Offer -- Free Gift with Subscription
Receive a free digital copy of Linux Journal's System Administration Special Edition as well as instant online access to current and past issues. CLICK HERE for offer
Linux Journal: delivering readers the advice and inspiration they need to get the most out of their Linux systems since 1994.
Subscribe now!
The Latest
Newsletter
Tech Tip Videos
- Jul-01-09
- Jun-29-09
Recently Popular
From the Magazine
July 2009, #183
News Flash: Linux Kernel 3.0 to include an on-the-go Expresso machine interface! Ok, maybe not, but Linux is definitely going mobile, from phones to e-readers. Find out more inside about Android, the Kindle 2, the Western Digital MyBook II, The Bug, and Indamixx (a portable recording studio). And if you've gone mobile and you been wanting more Emacs in your life then check out Conkeror.
To compliment the mobile we've got the stationary: parsing command line options with getopt, checking your Ruby code with metric_fu, and building a secure Squid proxy. How is this stationary you ask? What can we say? It's not. We just wanted to see if anybody actually read this part of the page :) .
All this and more, and all you have to do is get your hot sweaty hands on the latest copy of Linux Journal.
Delicious
Digg
StumbleUpon
Reddit
Facebook








ps "parsing"
On September 24th, 2008 Anonymous says:
I think you should parse the ps output because in theory it is possible that your process died and (another) new process got its PID. It just depends on how often you check if the PID is still running and how often new processes get spawned on your system.
But by giving ps the right parameters it does all the work for you:
$ ps hp $(cat /var/run/crond.pid) o comm
cron
This way you can confirm that the given process name is indeed the program you are monitoring.
Post new comment