pid_file.rb

class PidFile
  attr_reader :path

  def initialize(path)
    @path = path
  end

  def write
    file = File.open(@path, "w")
    file << Process.pid
    file.close
  end

  def unlink
    File.unlink(@path)
  end

  def running?
    return false unless File.exist?(@path)
    return false unless pid = read_from_file
    
    begin
      Process.kill(0, pid)
      return true
    rescue Errno::ESRCH
      return false
    end
  end
  
  def if_not_running
    unless running?
      write
      yield
      unlink
    end
  end
  
  private  
  
  def read_from_file
    lines = File.readlines(@path)
    return if lines.empty?
    lines.first.chomp.to_i
  end
end