Jekyll Rakefile to generate tag pages without plugins for use in gh-pages sites.

Rakefile

# This is a ruby port of Long Qian's:
#
# https://github.com/qian256/qian256.github.io/blob/master/tag_generator.py
#
# which is a python script that accompanies this useful webpage on creating 
# tags for Jekyll without plugins:
#
# http://longqian.me/2017/02/09/github-jekyll-tag/
#
# This Rakefile uses Jekyll's internals to generate the list, as opposed to
# crawling through "_posts/", like the python original. Hence, it works for all 
# collections. ("posts" are a default collection)
#
# Simply put, it scans all of the site's *collections* (so posts and anything 
# else that's defined as a collection in _config.yml) for the "tags" key and
# builds an array out of them. Then, it autogenerates a page for each tag.
# It assumes you've followed Long Qian's instructions for structuring tags, etc.
#
# Unfortunately, every time a new tag is added, this rake task needs to be run
# again.
#
# To install:
#   1. Put this file in the root directory of your Jekyll install (the same
#      folder as _config.yml)
#   2. Add "gem 'rake'" to your Gemfile
#   3. Install rake with "bundle install"
#
# To use:
#   1. "bundle exec rake build_tags"

require "jekyll"

task :build_tags do
  puts "Building up the tag files…"
  tags_dir = "tags" # set the tags folder
  if File.directory?(tags_dir) # create the tags folder if it doesn't exist
    puts "Tags directory (./#{tags_dir}) exists."
  else
    puts "Tags directory (./#{tags_dir}) does not exist. Creating it…"
    Dir.mkdir(tags_dir)
  end
  site = Jekyll::Site.new(Jekyll.configuration) # init Jekyll site
  reader = Jekyll::CollectionReader.new(site) # init Jekyll collections reader
  tags = reader.read.keys.map do |collection| # iterate over the collections
    reader.read[collection].docs.map do |document| # and over every coll's docs
      document.data["tags"] # add its tags array to the array
    end
  end
  tags.flatten!.uniq!.each do |tag| # flatten array of arrays, use only uniques, and iterate
    File.open("./#{tags_dir}/#{tag}.md", "w") do |f| # create a blank file for the tag
      f.puts "---"
      f.puts "title: \"Tag: #{tag}\""
      f.puts "layout: tagpage"
      f.puts "tag: #{tag}"
      f.puts "robots: noindex"
      # add any other metadata you want to include—permalink, etc.—here.
      f.puts "---"
    end
  end
  puts "#{tags.count} tags generated!"
end