# Controlling a forked process

**URL:** <https://www.ruby-forum.com/t/controlling-a-forked-process/160415>\
**Category:** Ruby\
**Created:** [February 16, 2009, 5:32am UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415 "2009-02-16T05:32:24Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 16, 2009, 5:32am UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/1 "2009-02-16T05:32:24Z")

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If I start a process, is there some way that I can maintain control of  
it. More specifically, can I start a process and then at some point  
based on a user action kill that process. I see hints of that in the  
documentation, but I don’t see anything that specifically addresses this  
case.

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<div class="post-metadata">

**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 16, 2009, 9:22am UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/2 "2009-02-16T09:22:05Z")

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Michael S. wrote:

> If I start a process, is there some way that I can maintain control of  
> it. More specifically, can I start a process and then at some point  
> based on a user action kill that process. I see hints of that in the  
> documentation, but I don’t see anything that specifically addresses this  
> case.

Yes. The parent process should have the process ID of the child. With  
that you can do things like:

if (pid = fork)  
sleep 1  
Process.kill(‘TERM’, pid)  
else  
loop do  
puts ‘KILL ME’  
end  
end

Or you could kill with ‘KILL’, if you really need it dead RIGHT NOW –  
but you might want to read ‘man kill’ first.

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<div class="post-metadata">

**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 16, 2009, 7:45pm UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/3 "2009-02-16T19:45:09Z")

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David M. wrote:

> Yes. The parent process should have the process ID of the child. With  
> that you can do things like:
> 
> if (pid = fork)  
> sleep 1  
> Process.kill(‘TERM’, pid)  
> else  
> loop do  
> puts ‘KILL ME’  
> end  
> end

on the line  
if (pid = fork)

Is fork returning the pid of the spawned process or of the current (e.g.  
parent) process? More to the point, assume that I spawn multiple  
processes. How can I address each of them?

Thanks  
—Michael

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<div class="post-metadata">

**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 16, 2009, 9:08pm UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/4 "2009-02-16T21:08:46Z")

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Hi Michael,

you can:

fork1\_pid = fork { # fork 1 body }

fork2\_pid = fork { # fork 2 body }

Process.kill(‘HUP’, fork1\_pid)  
Process.kill(‘HUP’, fork2\_pid)

does this answer your question?

2009/2/16 Michael S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)

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<div class="post-metadata">

**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 16, 2009, 9:29pm UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/5 "2009-02-16T21:29:16Z")

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Louis-Philippe wrote:

> fork1\_pid = fork { # fork 1 body }
> 
> fork2\_pid = fork { # fork 2 body }
> 
> Process.kill(‘HUP’, fork1\_pid)  
> Process.kill(‘HUP’, fork2\_pid)
> 
> does this answer your question?

Close … and it may just be that I’m a bit dense today.

What is causing the first call to fork to be tied to the first process  
and the second call to fork to be tied to the second process? Is it  
simply that it’s returning the pid of the most recently spawned process?

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<div class="post-metadata">

**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 16, 2009, 9:31pm UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/6 "2009-02-16T21:31:08Z")

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Michael S. wrote:

> > loop do  
> > puts ‘KILL ME’  
> > end  
> > end
> 
> on the line  
> if (pid = fork)
> 
> Is fork returning the pid of the spawned process or of the current (e.g.  
> parent) process?

It’s returning the pid of the spawned process in the parent process, and  
nil in the spawned process. So in the code above, the ‘if’ clause is in  
the parent process, and the ‘else’ clause is in the spawned process.

> More to the point, assume that I spawn multiple  
> processes. How can I address each of them?

With multiple pids. Following my pattern above, you’d do:

if pid1 = fork  
if pid2 = fork  
# parent process  
else  
# inside pid2  
end  
else

# inside pid1

end

You could also create arbitrary process trees:

if child = fork

# parent process

elsif grandchild = fork

# child process

else

# grandchild process

end

Now, Louise-Philippe has some more elegant syntax, where you can pass a  
block that will be executed in the spawned process. But the idea is the  
same. Also worth noting, my examples above are the same relatively  
low-level concept used everywhere else on Unix.

So, translating my example above:

child = fork do  
grandchild = fork do  
# grandchild process  
end

# child process

end

# parent process

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<div class="post-metadata">

**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 17, 2009, 12:47am UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/7 "2009-02-17T00:47:59Z")

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Michael S. wrote:

> What is causing the first call to fork to be tied to the first process  
> and the second call to fork to be tied to the second process?

Because the first call to fork is both spawning and returning the  
process id that was spawned.

> Is it  
> simply that it’s returning the pid of the most recently spawned process?

Close. It is returning the pid of the process that this particular call  
to fork spawned.

This is effectively the same, but you’re overthinking it. Let me make it  
as simple as I possibly can:

this\_pid = fork { this child body }

No matter where you do that in your code, or how often you do it, that  
code will always do what you expect. The variable on the left will  
always receive the pid corresponding to the process which is now running  
the code on the right.

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<div class="post-metadata">

**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 16, 2009, 10:19pm UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/8 "2009-02-16T22:19:21Z")

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"What is causing the first call to fork to be tied to the first process  
and the second call to fork to be tied to the second process? Is it  
simply that it’s returning the pid of the most recently spawned process?

essentially with:

fork { # code body }

or also

fork do

# code body

end

syntax,

you’re just executing a block of code as a child process, completely  
separated from its parent from the point it forks, there is no  
complicated  
tie over there,  
the fork returns its pid when its initiated, you can then use  
Process.kill  
to kill it but may also want to kill it from the inside with Kernel.exit

2009/2/16 David M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)

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<div class="post-metadata">

**Author:** ![wickie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/ba9def/32.png) [@wickie](https://www.ruby-forum.com/u/wickie)\
**Post date:** [February 17, 2009, 1:39am UTC](https://www.ruby-forum.com/t/controlling-a-forked-process/160415/9 "2009-02-17T01:39:45Z")

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David M. wrote:

> This is effectively the same, but you’re overthinking it.

On that, you were right. I was separating things that are not separable.  
Thank you.