Running Periodic Tasks Without cron or at

Problem

You want to write a self-contained Ruby program that performs a task in the background at a certain time, or runs repeatedly at a certain interval.

Solution

Fork off a new process that sleeps until its time to run the Ruby code.

Heres a program that waits in the background until a certain time, then prints a message:

	#!/usr/bin/ruby
	# lunchtime.rb

	def background_run_at(time)
	 fork do
	 sleep(1) until Time.now >= time
	 yield
	 end
	end

	today = Time.now
	noon = Time.local(today.year, today.month, today.day, 12, 0, 0)
	raise Exception, "Its already past lunchtime!" if noon < Time.now

	background_run_at(noon) { puts "Lunchtime!" }

The fork command only works on Unix systems. The win32-process third-party add on gives Windows a fork implementation, but its more idiomatic to run this code as a Windows service with win32-service.

Discussion

With this technique, you can write self-contained Ruby programs that act as though they were spawned by the at command. If you want to run a backgrounded code block at a certain interval, the way a cronjob would, then combine fork with the technique described in Recipe 3.12.

	#!/usr/bin/ruby
	# reminder.rb
	def background_every_n_seconds(n)
	 fork do
	 loop do
	 before = Time.now
	 yield
	 interval = n-(Time.now-before)
	 sleep(interval) if interval > 0
	 end
	 end
	end

	background_every_n_seconds(15*60) { puts Get back to work! }

Forking is the best technique if you want to run a background process and a foreground process. If you want a script that immediately returns you to the command prompt when it runs, you might want to use the Daemonize module instead; see Recipe 20.1.

See Also

  • Both the win32-process and the win32-service libraries are available at http://rubyforge.org/projects/win32utils/
  • Recipe 3.12, "Running a Code Block Periodically"
  • Recipe 20.1, "Running a Daemon Process on Unix"


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