Gracefully exiting from console programs in Ruby

Imagine you write a CLI program or a Rake task which loops through some data performing some work on it. You run it and then you remembered something. You’d love to kill the process with ctrl-c, but that will raise an exception somewhere in the loop. What you want is for the iteration to complete and then you want the program to quit.

You could handle the Interrupt exception or add some conditions. But how about a cleaner and reusable way?

No problem - you can trap signals, which means we can trap the ctrl-c (which is an INT signal). Servers, like unicorn, use signals for graceful restarts.

How it works - you can use the trap method which accepts the name of the signal and a block to which replaces the system’s default handler.

trap "INT" {
  # You can do your own stuff here
  #
  # Remember, the signal dies here - if you don't
  # raise an exception or exit the process, nothing will happen
}

Here’s a simple example where I wrapped this pattern in a singleton class:

require "singleton"

class GracefulQuit
  include Singleton

  attr_accessor :breaker

  def initialize
    self.breaker = false
  end

  def self.enable
    trap('INT') {
      yield if block_given?
      self.instance.breaker = true
    }
  end

  def self.check(message = "Quitting")
    if self.instance.breaker
      yield if block_given?
      puts message
      exit
    end
  end

end

So now how to use it? First, enable it before the loop using GracefulQuit.enable and then somewhere within the loop call GracefulQuit.check. Like in this example:

GracefulQuit.enable
some_collection.each do |item|
  heavy_duty_work_on item
  GracefulQuit.check
end

And that’s it - when you press ctrl-c, an exception will not be raised but the program will quit when you want it to. You can even add your own handler for this event.

GracefulQuit.enable
some_collection.each do |item|
  heavy_duty_work_on item
  GracefulQuit.check do
    cleanup
  end
end

Of course, this doesn’t solve other problems, like picking up when you left off, but allows to have better control over console programs or Rake tasks.

Source:http://futuresimple.github.com/posts/2012-02-24-graceful-exiting-from-console-programs-in-ruby/