A category list tag for jekyll

category_list.rb

# place this file in your plugins directory and add the tag to your sidebar
#$ cat source/_includes/custom/asides/categories.html 
#<section>
#  <h1>Categories</h1>
#  <ul id="categories">
#    {% category_list %}
#  </ul>
#</section>

module Jekyll
  class CategoryListTag < Liquid::Tag
    def render(context)
      html = ""
      categories = context.registers[:site].categories.keys
      categories.sort.each do |category|
        posts_in_category = context.registers[:site].categories[category].size
        html << "<li class='category'><a href='/categories/#{category}/'>#{category} (#{posts_in_category})</a></li>\n"
      end
      html
    end
  end
end

Liquid::Template.register_tag('category_list', Jekyll::CategoryListTag)

I had trouble with ampersands in category names, eg 'Tech & Prod'. The above generated a url that was missing the '-and-' that Jekyll creates. I found this worked

category_url = File.join(category_dir, category.to_url)