From: Sam Roberts <sroberts@...>
Date: 2005-03-31T22:55:06+09:00
Subject: Re: Getting process id of started process

Wrote Joe Van Dyk <joevandyk@gmail.com>, on Thu, Mar 31, 2005 at 07:58:11AM +0900:
> Say I want to start a long running shell process (and monitor that process).
> 
> 
>     def start_process
>         @process_id = fork do
>             Kernel.system "cat /dev/zero > /dev/null"
>         end
>     end
> 
> However, @process_id doesn't contain the pid of the cat process.  How
> can I capture the pid of that process?

You can't, not like this anyhow.  You aren't running cat directly, you
are calling the shell, /bin/sh, which is processing the redirect command
(>/dev/null). Only the shell knows the pid of cat. You actually have

  ruby --fork()--> ruby --system()--> /bin/sh --fork()/exec()--> /bin/cat

So there are 3 processes inherited from your main ruby executable.

Kernel.fork/exec/system are fairly thin wrappers around the C
equivalents, so a unix prog manual should cover whats happening, as well
as ways of doing whatever you are trying to do.

Sam

-- 
Sam Roberts <sroberts@certicom.com>