From: Ben Giddings <bg-rubytalk@...>
Date: 2005-03-31T09:13:36+09:00
Subject: Re: Getting process id of started process

Joe Van Dyk wrote:
> I've attatched code and test code at
> http://rafb.net/paste/results/2Rj4rb87.html , if someone wants to take
> a look.  Hopefully it's clear about what I want to do.
> 
> 
> On Wed, 30 Mar 2005 14:57:51 -0800, Joe Van Dyk <joevandyk@gmail.com> wrote:
> 
>>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?

I think what you need is "$?", which is "The status of the last executed
child process."

You might want to look at the docs here:

http://www.ruby-doc.org/core/classes/Kernel.html#M001773

It says that in block form, fork returns zero, but it might set $?, I'm
not sure.

In any case, you might need to use the non-block form of fork to do what
you want.  That way you'll either get the process ID or nil:

def start_process
  @process_id = fork
  if @process_id
    # We're in the parent
  else
    system("cat /dev/zero > /dev/null")
    Process.exit!(0)
  end

  @process_id
end

Anyhow, the Process documentation seems to be pretty good:

http://www.ruby-doc.org/core/classes/Process.html

Ben