cache_bust.rb

# Simple plugin for hashing assets in Jekyll and adding a fragment of the
# hash to the file's path as a query string.
#
# Example:
#
#   <link rel="stylesheet" href="/{% bust css/main.css %}">
#
# Outputs:
#
#   <link rel="stylesheet" href="/css/main.css?5bbb9e1">
#
# You also have to turn this plugin on in your config.yml using the
# 'cache-bust' option:
#
#    cache-bust: true
#
# I use multiple _config.yml files to manage my different output formats
# and some cache bust and others don't.
#
# Lastly, this plugin does cache the MD5 hashes so that the file doesn't
# get rehashed over and over again when you build.
#
require 'digest/md5'

module Jekyll
  class CacheBustTag < Liquid::Tag
    MD5_CACHE = {}
    def initialize(tag_name, file_name, tokens)
      super
      @file_name = file_name.strip
    end

    def md5
      h = MD5_CACHE[@file_name]
      unless h
        h = MD5_CACHE[@file_name] = Digest::MD5.file(@file_name).hexdigest[0, 7]
      end
      h
    end

    # Gets Md5 contents of target file (assumed to be using the full path)
    # and appends a hash end of to asset file reference. Ensures deployed
    # asset files are "cachebust-ed" every time the file changes
    def render(context)
      if context.registers[:site].config['cache-bust']
        "#{@file_name}?#{md5}"
      else
        @file_name
      end
    end
  end
end

Liquid::Template.register_tag('bust', Jekyll::CacheBustTag)