make sure a ruby program can run only one copy of itself

a.rb

#! /usr/bin/env ruby  

# it's trivial ot make a ruby program run once.
#

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

  unless status == 0
    puts "another process has the lock"
    exit(42)
  else
    puts Process.pid
    puts "holding the lock until you hit ctrl-c..."
    sleep
  end


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