"Old School" Ruby Daemon Example Code

daemon.rb

#!/usr/bin/env ruby

# What does getrlimit tell us?
# What are we doing here and why?
# If we are on linux, is there a better alternative?
(3).upto(Process.getrlimit(Process::RLIMIT_NOFILE)[0]) do |n|
  begin
    if(fd = File.for_fd(n))
      puts "Closing FD #{n}"
      fd.close
    end
  rescue Errno::EBADF, ArgumentError
  end
end


# What does the system call fork() do?
# What does fork() return in Ruby or C?
pid1 = Process.fork
if pid1.nil?
  puts "[#{Process.pid}] I am the first child."

  # What does setsid do here?
  # What would happen if we didn't do this?
  Process.setsid

  pid2 = Process.fork
  if pid2.nil?
    puts "[#{Process.pid}] I am the second child."

    # Where would output go if we didn't do this?
    $stdin.reopen("/dev/null")
    $stdout.reopen("/dev/null", "a")
    $stderr.reopen($stdout)

    # What is a umask?
    # What is the consequence of doing this?
    File.umask 0

    # What could happen if we didn't do this?
    Dir.chdir "/"

    while sleep 5 do
      $0 = "my-daemon: still running #{Time.now}"
    end
  else
    exit 0
  end
else
  exit 0
end

# But wait, there's more!
#
# What didn't we handle that we may have if we were paranoid or a more
# complicated program?  Well, why and how would you want to deal with:
#
# - Signal handlers
# - Signal mask
# - Preventing another copy from running if necessary
# - Dropping priviledges
#
# Also if our program was more complicated, we've exited 0 in the top
# level process without knowing if our daemon is completely
# initialized.
#