files_to_lower.rb

#!/usr/bin/env ruby

def traverse_and_rename(dir)
  Dir.entries(dir).each do |entry|
    next if %w(. ..).include?(entry) || !File.directory?(entry)

    if entry[/[A-Z]/]
      File.rename(entry, entry.downcase)
    else
      Dir.chdir(entry) do
        traverse_and_rename('.')
      end
    end
  end
end

traverse_and_rename('.')