A Liquid tag for Jekyll sites that allows embedding Gists and showing code for non-JavaScript enabled browsers and readers.

gist_tag.rb

# A Liquid tag for Jekyll sites that allows embedding Gists and showing code for non-JavaScript enabled browsers and readers.
# by: Brandon Tilly
# Source URL: https://gist.github.com/1027674
# Post http://brandontilley.com/2011/01/31/gist-tag-for-jekyll.html
#
# Example usage: {% gist 1027674 gist_tag.rb embed %} //embeds a gist for this plugin

require 'cgi'
require 'digest/md5'
require 'net/https'
require 'uri'

module Jekyll
  class GistTag < Liquid::Tag

    def initialize(tag_name, text, token)
      super
      @text           = text
      @cache_disabled = false
      @embed_code     = false
      @config         = Jekyll.configuration({ 'quiet' => true })
      @cache_folder   = @config['gist_tag']['cache'] || File.expand_path( "../.gist-cache", File.dirname(__FILE__) )
      @encoding       = @config['encoding'] || 'UTF-8'
      FileUtils.mkdir_p @cache_folder
    end

    def render(context)
      if @text.match(/^[a-zA-Z\d]*\s.*?$/)
        string = @text.gsub(/\s+/, ' ').strip
        gist, file, @embed_code = string.split(' ')
      else
        gist, file = @text.strip, ""
      end
      if gist.empty?
        ""
      else
        script_url = script_url_for(gist, file)
        if @embed_code
          code = get_cached_gist(gist, file) || get_gist_from_web(gist, file, script_url)
          html_embed code
        else
          gist_url = get_gist_url_for(gist, file)
          code = get_cached_gist(gist, file) || get_gist_from_web(gist, file, gist_url)
          javascript_embed script_url, code
        end
      end
    end

    def html_embed(code)
      "<div>#{code}</div>" 
    end

    def javascript_embed(script_url, code)
      code = CGI.escapeHTML code
      <<-HTML
<div><script src="#{script_url}"></script>
<noscript><pre><code>#{code}</code></pre></noscript></div>
      HTML
    end

    def script_url_for(gist_id, filename)
      url = "https://gist.github.com/#{gist_id}.js"
      url = "#{url}?file=#{filename}" unless filename.nil? or filename.empty?
      url
    end

    def get_gist_url_for(gist, file)
      "https://gist.githubusercontent.com/raw/#{gist}/#{file}"
    end

    def cache(gist, file, data)
      cache_file = get_cache_file_for gist, file
      File.open(cache_file, "w") do |io|
        io.write data
      end
    end

    def get_cached_gist(gist, file)
      return nil if @cache_disabled
      cache_file = get_cache_file_for gist, file
      File.read cache_file if File.exist? cache_file
    end

    def get_cache_file_for(gist, file)
      bad_chars = /[^a-zA-Z0-9\-_.]/
      gist      = gist.gsub bad_chars, ''
      file      = file.gsub bad_chars, ''
      md5       = Digest::MD5.hexdigest "#{gist}-#{file}"
      File.join @cache_folder, "#{gist}-#{file}-#{md5}.cache"
    end

    def get_gist_from_web(gist, file, gist_url)
      data = get_web_content(gist_url)

      locations = Array.new
      while (data.code.to_i == 301 || data.code.to_i == 302)
        data = handle_gist_redirecting(data)
        break if locations.include? data.header['Location']
        locations << data.header['Location']
      end

      if data.code.to_i != 200
        raise RuntimeError, "Gist replied with #{data.code} for #{gist_url}"
      end

      # Cleanup embed version
      code = data.body.force_encoding("UTF-8").encode(@encoding)
      code = code.gsub( /document\.write\('/, '' ).gsub( /'\)/, '' ) # JS
      code = code.gsub( /\\"/, '"' ).gsub( /\\\//, '/' ) # escaped stuff
      code = code.gsub( /\\n/, "\n" ) # returns
      code = code.encode(@encoding) # encode

      cache(gist, file, code) unless @cache_disabled
      code
    end

    def handle_gist_redirecting(data)
      redirected_url = data.header['Location']
      if redirected_url.nil? || redirected_url.empty?
        raise ArgumentError, "GitHub replied with a 302 but didn't provide a location in the response headers."
      end

      get_web_content(redirected_url)
    end

    def get_web_content(url)
      raw_uri           = URI.parse url
      proxy             = ENV['http_proxy']
      if proxy
        proxy_uri       = URI.parse(proxy)
        https           = Net::HTTP::Proxy(proxy_uri.host, proxy_uri.port).new raw_uri.host, raw_uri.port
      else
        https           = Net::HTTP.new raw_uri.host, raw_uri.port
      end
      https.use_ssl     = true
      https.verify_mode = OpenSSL::SSL::VERIFY_NONE
      request           = Net::HTTP::Get.new raw_uri.request_uri
      data              = https.request request
    end
  end

  class GistTagNoCache < GistTag
    def initialize(tag_name, text, token)
      super
      @cache_disabled = true
    end
  end
end

Liquid::Template.register_tag('gist', Jekyll::GistTag)
Liquid::Template.register_tag('gistnocache', Jekyll::GistTagNoCache)