spawn a cmd redirect stdout/stder/stdin (helpful in cron)

spawn-cmd.rb

#!/usr/bin/ruby

require 'optparse'


class SpawnerOptions
  def self.parse(args)
    options = {}
    options[:stdin] = '/dev/null'
    options[:stdout] = '/dev/null'
    options[:stderr] = '/dev/null'
    options[:wdir] = '/tmp'

    opts = OptionParser.new do |opts|
      opts.banner = "Usage: #{__FILE__} [options]"
      opts.separator ""

      opts.on("--stdout FILE", "Redirect STDOUT to FILE, "\
              "default #{options[:stdout]}") do |f|
        options[:stdout] = f
      end

      opts.on("--stderr FILE", "Redirect STDERR to FILE, "\
                "default #{options[:stderr]}") do |f|
        options[:stderr] = f
      end

      opts.on("--stdin FILE", "Redirect STDIN to FILE, "\
                "default #{options[:stdin]}") do |f|
        options[:stdin] = f
      end

      opts.on("--pid-file FILE", "Write and do PID check to/from FILE") do |f|
        options[:pidfile] = f
      end

      opts.on("--command CMD", "Exec CMD") do |c|
        options[:cmd] = c
      end

      opts.on("--wdir DIR", "Change to dir DIR, default #{options[:wdir]}") do |c|
        options[:wdir] = c
      end

      opts.on_tail("-h", "--help", "Show this message") do
        puts opts
        exit
      end
    end

    begin
      opts.parse!(args)
      raise OptionParser::MissingArgument, "--pid-file, no pid file given."\
        if not options[:pidfile]
      raise OptionParser::MissingArgument, "--command, no command given."\
        if not options[:cmd]
    rescue SystemExit
      exit
    rescue Exception => e
      puts "Error " +  e.to_s, "#{__FILE__} -h for options."
      exit
    end
    options
  end
end

opts = SpawnerOptions.parse(ARGV)

if File.exists? opts[:pidfile]
  pid = File.open(opts[:pidfile]).read.strip.to_i
  begin
    if Process.kill(0, pid)
      puts "#{Time.now.to_s} Already running pid #{pid}"
      exit(1)
    end
  rescue Errno::ESRCH
  end
end

pid = fork do
  Dir.chdir(opts[:wdir])
  STDIN.reopen(opts[:stdin], 'r')
  STDOUT.reopen(opts[:stdout], 'w')
  STDERR.reopen(opts[:stderr], 'w')
  exec opts[:cmd]
end

File.open opts[:pidfile], 'w' do |f|
  f.write pid
  puts "#{Time.now.to_s} wrote pid #{pid} to #{opts[:pidfile]}"
end