From: Sean O'Halpin <sean.ohalpin@...>
Date: 2008-05-25T22:46:11+09:00
Subject: Re: Ensuring only one instance of a script is running

On Wed, May 21, 2008 at 5:25 AM, Daniel Berger <djberg96@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

I think I'll use this :)

Thanks,
Sean