How Jekyll works

Last saturday, I decided to build a Jekyll plugin to get the quotes from my Tumblr account (through a quotes.json file conveniently saved using a Rake task, inspired by [1]) and generate a “quotes index”.

So… I needed a generator. (d'oh)

There’s an example in the Github wiki:

module Jekyll

  class CategoryPage < Page
    def initialize(site, base, dir, category)
      @site = site
      @base = base
      @dir = dir
      @name = 'index.html'

      self.process(@name)
      self.read_yaml(File.join(base, '_layouts'), 'category_index.html')
      self.data['category'] = category

      category_title_prefix = site.config['category_title_prefix'] || 'Category: '
      self.data['title'] = "#{category_title_prefix}#{category}"
    end
  end

  class CategoryPageGenerator < Generator
    safe true

    def generate(site)
      if site.layouts.key? 'category_index'
        dir = site.config['category_dir'] || 'categories'
        site.categories.keys.each do |category|
          site.pages << CategoryPage.new(site, site.source, File.join(dir, category), category)
        end
      end
    end
  end

end

After some hacking, I started to think it wasn’t as easy as it appeared. Ok, coding while being sleep-deprived isn’t very smart.

I started again after some hours of sleep and sufficient caffeine, but this time desiring to understand how Jekyll works. I imagined some kind of data structure holding all the generators, some other holding the converters and they were called during the “compilation” of the site. But something was missing: when are the files read from the filesystem? How to generate a file without an initial template, effectively creating the file during “compile-time”?

I started with the Jekyll executable. It’s pretty simple: the command-line parameters, the defaults and the _config.yml (through Jekyll::configuration method) are used to create an options hash and then a new site is instantiated:

# Create the Site
site = Jekyll::Site.new(options)

After that, it starts to watch the necessary directories if the --auto option was used.

if options['auto']
  require 'directory_watcher'
  puts "Auto-regenerating enabled: #{source} -> #{destination}"
    # ...
else
  puts "Building site: #{source} -> #{destination}"
    # ...
end

The site is built through a call to site.process, the main method in the Jekyll::Site class. Finally, it runs the local server if --server was specified.

I was happy, but didn’t yet understand how my blog was created. Well, the site.process call was the obvious answer, so I opened lib/jekyll/site.rb on TextMate:

def process
  self.reset
  self.read
  self.generate
  self.render
  self.cleanup
  self.write
end

And it turned out to be very similar to my initial guess. For sake of completeness: site.reset and site.setup are called during initialization of the site to initialize its data structures and to load libraries, plugins, generators and converters, respectively. Ok, let’s see what each of these methods do:

So if I wanted to create a generator, I just needed a generate method. Pretty easy. But there were some doubts still: How to specify a layout for my page through code, without creating a previous file on the filesystem?

The Jekyll::Site.read method stores the contents from the filesystem in internal data structures. This is the stage where the files in _layouts, _posts and possibly other directories are read and their respective objects (Layout, Post, Page and StaticFile) are stored inside arrays.

So, my page should be created dynamically by my generator. Nice. I did it, but after some tests, the quotes/ directory wasn’t being created.

The catch is the Jekyll::Site.cleanup method: it’ll remove from the destination everything that isn’t inside the internal data structures, including empty directories. So the solution was easy:

self.pages << quotes_page

Just add the page to the site.pages array before returning. To understand what I’m saying, take a look at the plugin (after all this code archeology, it’s amazing how simple it turned out to be). The current code for the generator can be found in this gist.

I wrote a How Jekyll works? page for the wiki in Jekyll’s repository to help others interested in this.

Sources

[1] Generating Jekyll pages from data [2] Jekyll git repository

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