reval.rb

#!/usr/bin/env ruby
#
# Name: reval.rb
# License: MIT
# Author: postmodern (postmodern.mod3 at gmail.com)
# Description:
#
# Re-evaluates a specified Ruby file whenever the file changes.
# Reval was inspired by Giles Bowkett's kickass talk on Archaeopteryx at
# RubyFringe 2008, where Giles used some mad Ruby to re-evaluate his
# Achaeopteryx script as he edited it.
#
# Reval might come in handy, when you give that awesome breakthrough talk
# at some conference.
#

require 'digest/md5'

#
# Re-evaluate the contents of the specified _file_, whenever the file
# changes, using the given _options_.
#
# _options_ may contain the following keys:
# <tt>:pause</tt>:: Number of seconds to sleep between checking the _file_
#                   for changes. Defaults to +0.4+ if not given.
#
#   reval 'file.rb'
#
#   reval 'file.rb', :pause => 0.4
#
def reval(file,options={})
  pause = (options[:pause] || 0.4)

  last_time = Time.now
  this_time = Time.now

  last_fingerprint = nil
  fingerprint = nil

  if File.file?(file)
    last_fingerprint = Digest::MD5.hexdigest(File.read(file))

    load(file)
  end

  loop do
    begin
      this_time = File.mtime(file)

      if (this_time > last_time)
        fingerprint = Digest::MD5.hexdigest(File.read(file))

        if last_fingerprint != fingerprint
          load(file)

          last_fingerprint = fingerprint
        end

        last_time = this_time
      end
    rescue Errno::ENOENT
    end

    sleep(pause)
  end
end

reval(ARGV[0]) if ($0 =~ /reval/ && ARGV[0])