a.rb

#! /usr/bin/env ruby


status = DATA.flock(File::LOCK_EX | File::LOCK_NB)

if status == 0
  puts "we have the lock..."
  sleep

else
  puts "another process has the lock..."
  exit
end


__END__

### this data section MUST be here for DATA.flock to function.

### the user locking the script MUST have write privledges to the script

this is a simple technique that portably prevents two copies of a script from running at the same time. if you have ruby scripts in your crontab you will burn in hell if you aren't using this or a similar technique to prevent script stacking taking your box down. same applies for background jobs.

Great idea! Just today I wrote a backup script and made use of this technique.