setsid-fork.rb

Process.setproctitle("foo-parent")

ppid = Process.pid
fork do
  exit if fork            # You can only setsid if you're in a child process so first we fork
  Process.setsid          # and exit the parent then setsid in the child.  Now that we have a
  exit if fork            # new session create a child process in it and exit the parent again.
  Dir.chdir("/")          # Finally, change directory to one that can't be deleted or moved.

  Process.setproctitle("foo-child")
  while true
    puts 'child'
    sleep 0.5
  end
end

while true
  puts 'parent'
  sleep 0.5
end