Cache busting with Sass

bust_url.rb

# Use as: sass -r ./bust_url.rb

require 'digest/md5'

module Sass::Script::Functions
  def bust_url(url)
    url = url.value
    source = options[:filename]
    dir = File.dirname(source)
    file = File.join(dir, url)
    hex = Digest::MD5.hexdigest(File.read(file))
    Sass::Script::String.new("url(#{url}?#{hex[0, 10]})")
  end
  declare :bust_url, :args => [:string]
end