insert magic comment to **/*.rb

magic.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

Dir['**/*.rb'].each do |f|
  File.open(f, 'r+') do |io|
    buf = io.readlines
    unless /coding:/ =~ buf[0]
      io.rewind
      buf.unshift "# -*- coding: utf-8 -*-\n\n"
      io.write buf.join
    end
  end
end