unlinked.rb

#!/usr/bin/env ruby
require 'set'

# Searches through the given markdown files to find reference-style links that
# are not defined.
abort "Usage: unlinked.rb [files...]" if ARGV.empty?

ARGV.each do |filename|
  defs = Set.new
  reqs = Set.new
  open(filename) do |file|
    contents = file.read
    contents.scan(/^\[(\w+)\]:/) { |l| defs << l }
    contents.scan(/\[[^\]]+\]\[([^\]]+)\]/) { |l| reqs << l }
  end

  diff = reqs - defs
  abort "#{filename} does not define: #{diff.to_a.join(', ')}" unless diff.empty?
end