From: Daniel Berger <djberg96@...>
Date: 2008-05-29T03:33:04+09:00
Subject: Re: Ensuring only one instance of a script is running



On May 25, 7:46 am, "Sean O'Halpin" <sean.ohal...@gmail.com> wrote:
> On Wed, May 21, 2008 at 5:25 AM, DanielBerger<djber...@gmail.com> wrote:
> > Hi all,
>
> > I'm probably late to the game on this, but I stumbled across an
> > interesting use for DATA. You can use it to ensure only one instance
> > of a given script is running by using flock:
>
> > class Foo
> >   def self.mainloop
> >      while true
> >         puts "Looping..."
> >         sleep 3
> >      end
> >   end
> > end
>
> > DATA.flock(File::LOCK_EX)
>
> > if $0 == __FILE__
> >   Foo.mainloop
> > end
>
> > __END_
>
> > The first run will work, but trying to start the program up again will
> > fail instantly because of the lock on DATA. I should probably do some
> > cleanup there, too, but I thought I'd toss this out there and see if
> > this is of interest to anyone.
>
> > Or was I was recovering from a hangover in college when they mentioned
> > this trick in class? Anyway, there you go.
>
> > Regards,
>
> > Dan
>
> Nice one! However, I don't get a failure (on Linux) - instead the
> second instance blocks waiting for the first instance to terminate at
> which point it executes. Also, needing to specify __END__ is a little
> awkward IMHO.
>
> I wonder, does the following work on Windows?
>
> if $0 == __FILE__
>   if File.open($0).flock(File::LOCK_EX|File::LOCK_NB)
>     Foo.mainloop
>   end
> end
>
> Wrapped up in a method:
>
> $ cat single_instance.rb
> def single_instance(&block)
>   if File.open($0).flock(File::LOCK_EX|File::LOCK_NB)
>     block.call
>   else
>     warn "Script #{ $0 } is already running"
>   end
> end
>
> $ cat self_locking.rb
> require 'single_instance'
> if __FILE__ == $0
>   single_instance do
>     Foo.mainloop
>   end
> end

Yep, that's definitely cleaner, thanks.

> I think I'll use this :)

Excellent. Good to know someone found it useful. :)

Regards,

Dan