From: Csaba Henk <csaba@..._for_avoiding_spam.org>
Date: 2005-04-02T23:34:55+09:00
Subject: Re: Getting process id of started process

On 2005-03-30, 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?

To ensure that the called program really runs as the child of the main
process (and not a further descendent), you have to code it in a
relatively low-level way; in the above case it would look like

     def start_process
         @process_id = fork do
             File.reopen $stdin, File.open("/dev/null","w")
             exec %w(cat /dev/zero)
         end
     end

(In this case, 'exec "cat /dev/zero"' would do as well. Ruby's exec is a
bastardized one, not just an interface to the posix exec* calls: it
tries to find out by itself whether to exec the program directly or to
do it via shell. In general, passing an array to exec protects from
shell expansion.)

I'm working on a lib which makes it easier, eg. you could do with it:

     def start_process
         po = Pope.new
         po.do(%w(cat /dev/zero), 1 => "/dev/null").end
         @process_id = po.actors[0].child 
     end

Csaba