From: Brian Candler <b.candler@...>
Date: 2009-03-09T04:41:03+09:00
Subject: Re: Script Won't Exit After Call to System

David Sokoloff wrote:
> I have a small script (see http://pastie.org/411010) that will check to
> see if my ssh tunnel has died, and if so, will create a new one. It
> works, except that if it does need to re-open a tunnel (through Kernel's
> 'system' call), the script never exits. Maybe it isn't a big deal (say
> so if you think so), but I'm not sure why the script doesn't exit. It
> *does* re-open the tunnel successfully. Furthermore, if you execute from
> the command line the same ssh command I'm executing through Ruby's
> 'system' call, the tunnel does successfully go to the background.
> 
> Thanks for any insight.

Rather than mess around with how the ssh command-line client may or may 
not background itself, I suggest two alternatives:

1. Use Net::SSH as the client and get it to do the forwarding for you. 
This is a pure Ruby solution and hence no issues with forked processes. 
From the docs:

    ssh.forward.local(17055, "mail.example.com", 143)

2. If you're going to use the openssh client, run it in the foreground 
and hold onto it from the Ruby client. Then you'll know immediately when 
it has terminated, zombie-free, and can restart it when necessary; 
scanning the output of 'ps' is no longer necessary.

You might do this using IO.popen or the pty library, but even if just 
using system() this can be done in a Ruby thread.

Aside: I don't know if you realise, but you can open multiple forwarding 
ports in the same ssh command line:

  ssh -Lxx:xx:xx -Lyy:yy:yy -Lzz:zz:zz host.example.com

HTH,

Brian.
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