# Killing sons (Linux)

**URL:** <https://www.ruby-forum.com/t/killing-sons-linux/105287>\
**Category:** Ruby\
**Created:** [May 28, 2007, 1:31pm UTC](https://www.ruby-forum.com/t/killing-sons-linux/105287 "2007-05-28T13:31:20Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![ohad](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/o/f04885/32.png) [@ohad](https://www.ruby-forum.com/u/ohad)\
**Post date:** [May 28, 2007, 1:31pm UTC](https://www.ruby-forum.com/t/killing-sons-linux/105287/1 "2007-05-28T13:31:20Z")

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Maybe this isn’t strictly a Ruby question, but I hope someone here can  
help:

I have a job-management application, with a central daemon which  
receives job requests. Upon receiving this request, it forks and then  
runs “system” to run bash, which in turn runs the Matlab job. I use bash  
for this in order to redirect the input and output from Matlab. pstree  
output looks like this:

init-±apache2—8\*[apache2]  
|-atd  
…  
|-ruby-±4\*[ruby—bash—MATLAB-±matlab\_helper]  
| | `-15*[{MATLAB}]] |`-{ruby}  
…

Legend:  
daemon ^ ^ daemon fork

Now, my system also allows a ‘kill’ command, intended to stop the job in  
progress. This has been causing me a lot of trouble, and I suddenly  
(after quite a while the system has been in production, how embarassing)  
realized why - the PID I’m keeping is of the daemon fork. Killing it  
doesn’t kill all of its sons - it causes bash to get reparented to init!

Any idea of a clean, quick way to fix this?

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**Author:** ![ohad](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/o/f04885/32.png) [@ohad](https://www.ruby-forum.com/u/ohad)\
**Post date:** [May 28, 2007, 2:29pm UTC](https://www.ruby-forum.com/t/killing-sons-linux/105287/2 "2007-05-28T14:29:15Z")

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On 5/28/07, Ohad L. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Now, my system also allows a ‘kill’ command, intended to stop the job in  
> progress. This has been causing me a lot of trouble, and I suddenly  
> (after quite a while the system has been in production, how embarassing)  
> realized why - the PID I’m keeping is of the daemon fork. Killing it  
> doesn’t kill all of its sons - it causes bash to get reparented to init!
> 
> Any idea of a clean, quick way to fix this?

Make your parent process the leader of its own process group with  
setpgid(0,0). When you fork, add each child to the parent’s process  
group  
with setpgid(0, getppid()). If you fork subchildren, make sure they get  
added to the same process group. Now, to send a signal to the whole  
group,  
send it to (0 - pid), where pid is that of the parent. If you want them  
all  
to die without killing the leader, use a signal whose default behavior  
is  
terminate-process and ignore it in the parent.

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**Author:** ![ohad](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/o/f04885/32.png) [@ohad](https://www.ruby-forum.com/u/ohad)\
**Post date:** [May 28, 2007, 5:19pm UTC](https://www.ruby-forum.com/t/killing-sons-linux/105287/3 "2007-05-28T17:19:42Z")

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Francis C. wrote:

> On 5/28/07, Ohad L. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:
> 
> > Now, my system also allows a ‘kill’ command, intended to stop the job in  
> > progress. This has been causing me a lot of trouble, and I suddenly  
> > (after quite a while the system has been in production, how embarassing)  
> > realized why - the PID I’m keeping is of the daemon fork. Killing it  
> > doesn’t kill all of its sons - it causes bash to get reparented to init!
> > 
> > Any idea of a clean, quick way to fix this?
> 
> Make your parent process the leader of its own process group with  
> setpgid(0,0). When you fork, add each child to the parent’s process  
> group  
> with setpgid(0, getppid()). If you fork subchildren, make sure they get  
> added to the same process group. Now, to send a signal to the whole  
> group,  
> send it to (0 - pid), where pid is that of the parent. If you want them  
> all  
> to die without killing the leader, use a signal whose default behavior  
> is  
> terminate-process and ignore it in the parent.

Just to be sure - if I run the following Ruby code on a Linux system:

child = fork do  
Process::setpgid 0,0  
system ‘bash -c “sleep 300”’  
end  
Process::kill 9, -child

Then I am guaranteed that no child bash, sleep or ruby process will  
remain? It works, I just want to be sure I can count on that behaviour.  
For contrast, in my original code, bash gets reparented to init:

child = fork do  
system ‘bash -c “sleep 300”’  
end  
Process::kill 9, child

And this code doesn’t even work (ESRCH: No such process)

child = fork do  
system ‘bash -c “sleep 300”’  
end  
Process::kill 9, -child

Thank you for your help!

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<div class="post-metadata">

**Author:** ![ohad](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/o/f04885/32.png) [@ohad](https://www.ruby-forum.com/u/ohad)\
**Post date:** [May 28, 2007, 6:19pm UTC](https://www.ruby-forum.com/t/killing-sons-linux/105287/4 "2007-05-28T18:19:43Z")

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On 5/28/07, Ohad L. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> remain? It works, I just want to be sure I can count on that behaviour.  
> With signals, nothing is ever “guaranteed.”

Why are you using signal 9 instead of something more system-friendly? 9  
should be your last resort when all else fails, because it doesn’t give  
your  
processes a chance to exit cleanly.

I have the uncomfortable feeling that your code is working by accident,  
because the subprocesses which the “system” call creates aren’t  
explicitly  
added to the process group of the fork child. If you run this program in  
a  
shell, it will probably work, because of the process-group semantics  
defined  
by most shells. However, if you run it as a headless daemon or from a  
cron  
job, it may not work. Try it and see.

When I have to do what you’re trying to do, I usually avoid calling  
system.  
Instead I call fork, and in the child I call setpgid(0, getppid()), and  
then  
exec.

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<div class="post-metadata">

**Author:** ![ohad](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/o/f04885/32.png) [@ohad](https://www.ruby-forum.com/u/ohad)\
**Post date:** [May 30, 2007, 11:46am UTC](https://www.ruby-forum.com/t/killing-sons-linux/105287/5 "2007-05-30T11:46:19Z")

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I’ve learned how to do redirection from within ruby  
($stdwhatever.reopen), and switched to using exec instead of system - so  
now I avoid two levels of depth. I also use setpgid and kill the whole  
group for good measure.

Much thanks for your help!