daemon.rb

#!/usr/bin/env ruby

class PidFile
  attr_reader :path

  def initialize(path)
    @path = path
  end

  def exists?
    File.exists? path
  end

  # Returns true if the process is running
  def running?
    return false unless exists?
    return false unless pid

    # Check if process is in existence
    # The simplest way to do this is to send signal '0'
    # (which is a single system call) that doesn't actually
    # send a signal
    begin
      Process.kill 0, pid
      return true
    rescue Errno::ESRCH
      return false
    rescue ::Exception   # for example on EPERM (process exists but does not belong to us)
      return true
    end
  end

  # Return the pid contained in the pidfile, or nil
  def pid
    return nil unless exists?

    content = File.read(path)
    content.empty? ? nil : content.to_i
  end

  def write
    File.open path, 'w' do |f|
      f.puts Process.pid
    end
  end
end

case ARGV.first
when 'start'
  pid_file = PidFile.new 'app.pid'
  raise "#{pid_file.pid} already running" if pid_file.running?

  Process.daemon true, true

  pid_file.write

  $stdout.reopen "app.log", "a"
  $stderr.reopen "app.log", "a"

  $stdout.sync = true
  $stderr.sync = true

  require_relative 'consumer'
when 'stop'
  pid_file = PidFile.new 'app.pid'

  if !pid_file.running?
    $stderr.puts "Process not running"
    exit 1
  end

  Process.kill 'TERM', pid_file.pid

  $stdout.puts "Sent TERM to #{pid_file.pid}"
end