trap.rb

child_processes = 3 
dead_processes = 0
# We fork 3 child processes. 
child_processes.times do
  fork do
    # They sleep for 3 seconds. 
    sleep 3
  end 
end

# Our parent process will be busy doing some intense mathematics.
# But still wants to know when one of its children exits.
# By trapping the :CHLD signal our process will be notified by the kernel # when one of its children exits.
trap(:CHLD) do
  # Since Process.wait queues up any data that it has for us we can ask for it
  # here, since we know that one of our child processes has exited.
  puts Process.wait
  dead_processes += 1
  # We exit explicitly once all the child processes are accounted for. 
  exit if dead_processes == child_processes
end

# Work it.
loop do
  (Math.sqrt(rand(44)) ** 8).floor
  sleep 1
end

begin
  while pid = Process.wait(-1, Process::WNOHANG)
    puts pid
    dead_processes += 1
    # We exit ourselves once all the child processes are accounted for. 
    exit if dead_processes == child_processes
  end
rescue Errno::ECHILD 
end

改成这样可以保证每次 children process death 都能 catch。