Daemonss.

daemonss.rb

class Daemon
  def self.run!(options = {}, &block)
    new(options, &block).run!
  end

  def initialize(options = {}, &block)
    check_pid
    daemonize
    write_pid
    trap_signals
    redirect_output

    @logfile = options[:logfile]
    @pidfile = options[:pidfile]
    @block = block
  end

  def logfile
    @logfile || 'log/{$PROGRAM_NAME}.log'
  end

  def pidfile
    @pidfile || 'log/pid/{$PROGRAM_NAME}.pid'
  end

  def run!
    @block.call
  end

  def daemonize
    exit if fork
    Process.setsid
    exit if fork
  end

  def redirect_output
    FileUtils.mkdir_p(File.dirname(logfile), mode: 0755)
    FileUtils.touch logfile
    File.chmod(0644, logfile)
    $stderr.reopen(logfile, 'a')
    $stdout.reopen($stderr)
    $stdout.sync = $stderr.sync = true
  end

  def suppress_output
    $stderr.reopen('/dev/null', 'a')
    $stdout.reopen($stderr)
  end

  def write_pid
    FileUtils.mkdir_p(File.dirname(pidfile), mode: 0755)

    File.open(pidfile, ::File::CREAT | ::File::EXCL | ::File::WRONLY) do |f|
      f.write(Process.pid.to_s)
    end

    at_exit do
      File.delete(pidfile) if File.exist?(pidfile)
    end
  rescue Errno::EEXIST
    check_pid
    retry
  end

  def check_pid
    case pid_status(pidfile)
    when :running, :not_owned
      puts "A server is already running. Check #{pidfile}"
      exit(1)
    when :dead
      File.delete(pidfile)
    end
  end

  def pid_status(pidfile)
    return :exited unless File.exist?(pidfile)
    pid = ::File.read(pidfile).to_i
    return :dead if pid == 0
    Process.kill(0, pid)
    :running
  rescue Errno::ESRCH
    :dead
  rescue Errno::EPERM
    :not_owned
  end

  def trap_signals
    trap(:QUIT) do
      exit
    end
  end
end

example.rb

require "sinatra/base"
require "daemonss"

class App < Sinatra::Base
  get "/" do
    "I'm a daemon!"
  end
end

Daemon.run! do 
  App.run!
end