Checking for processes and pid files

If you have a periodic task that might still be running when you try to run it and don't want both copies of the program running then you need to create a pid file and check it when your process starts.

# Returns true if it was able to create the pid file

def set_pid(filename)
  if File.exist?(filename)
    File.open(filename, "r").each do |l|
      old_pid = l.chomp.to_i

      begin
        Process.kill(0, old_pid)
        # The process is running
        return false
      rescue Errno::EPERM
        # The process is running and you don't have permission
        return false
      rescue
        # the process is not running or something
      end
    end

    File.delete(filename)
  end

  # Create the new process
  f = File.new(filename, "w")
  f.puts $$
  f.close

  return true
end

At the start of you program call it as follows

unless set_pid("/var/run/app.pid")
  puts "This process is already running!"
  exit
end

This is good for OS X and Linux, not sure if it will work for Windows. Probably will