http://blog.nilsjonsson.com/post/546358154/wait-a-minute-strike-that-reverse-it-thank-you

directories_and_files.rb

module DirectoriesAndFiles
  
  module ClassMethods
    
    def directories(path)
      new(path).directories
    end
    
    def files(path)
      new(path).files
    end
    
  end
  
  def self.included(other_module)
    other_module.extend ClassMethods
  end
  
  def directories
    Dir.glob "#{path}/*/", File::FNM_DOTMATCH
  end
  
  def files
    filter_entries do |path|
      File.file? path
    end
  end
  
private
  
  def filter_entries
    entries.select do |e|
      yield "#{path}/#{e}"
    end
  end
  
end

Dir.class_eval do
  include DirectoriesAndFiles
end

directories_and_files_spec.rb

require 'spec'
require 'directories_and_files'

describe Dir do
  SANDBOX = 'tmp'
  
  def create_sandbox(options={})
    FileUtils.mkdir_p SANDBOX
    Array(options[:files]).each do |filename|
      FileUtils.touch "#{SANDBOX}/#{filename}"
    end
    Array(options[:directories]).each do |directory|
      FileUtils.mkdir_p "#{SANDBOX}/#{directory}"
    end
    Dir.new SANDBOX
  end
  
  def destroy_sandbox
    FileUtils.rm_rf SANDBOX
  end
  
  after :each do
    destroy_sandbox
  end
  
  describe 'class' do
    it 'should show DirectoriesAndFiles as an ancestor' do
      Dir.ancestors.should include(DirectoriesAndFiles)
    end
  end
  
  describe '-- when a path' do
    describe 'does not exist,' do
      describe 'and when sent class method' do
        describe "'directories' with that path --" do
          it 'should raise the expected error' do
            lambda {
              Dir.directories 'NONEXISTENT_PATH'
            }.should raise_error(Errno::ENOENT,
                                 'No such file or directory - NONEXISTENT_PATH')
          end
        end
        
        describe "'files' with that path --" do
          it 'should raise the expected error' do
            lambda {
              Dir.files 'NONEXISTENT_PATH'
            }.should raise_error(Errno::ENOENT,
                                 'No such file or directory - NONEXISTENT_PATH')
          end
        end
      end
    end
    
    describe 'contains' do
      describe 'no entries,' do
        before :each do
          create_sandbox
        end
        
        describe 'and when sent class method' do
          describe "'directories' with that path --" do
            it 'should return the expected names' do
              Dir.directories(SANDBOX).should == ["#{SANDBOX}/./",
                                                  "#{SANDBOX}/../"]
            end
          end
          
          describe "'files' with that path --" do
            it 'should return an empty array' do
              Dir.files(SANDBOX).should == []
            end
          end
        end
      end
      
      describe 'directories but no files,' do
        before :each do
          create_sandbox :directories => ['.foo', 'b a r']
        end
        
        describe 'and when sent class method' do
          describe "'directories' with that path --" do
            it 'should return the expected names' do
              Dir.directories(SANDBOX).should == ["#{SANDBOX}/./",
                                                  "#{SANDBOX}/../",
                                                  "#{SANDBOX}/.foo/",
                                                  "#{SANDBOX}/b a r/"]
            end
          end
          
          describe "'files' with that path --" do
            it 'should return an empty array' do
              Dir.files(SANDBOX).should == []
            end
          end
        end
      end
      
      describe 'files but no directories,' do
        before :each do
          create_sandbox :files => ['.foo', 'b a r']
        end
        
        describe 'and when sent class method' do
          describe "'directories' with that path --" do
            it 'should return the expected names' do
              Dir.directories(SANDBOX).should == ["#{SANDBOX}/./",
                                                  "#{SANDBOX}/../"]
            end
          end
          
          describe "'files' with that path --" do
            it 'should return the expected names' do
              Dir.files(SANDBOX).should == ['.foo', 'b a r']
            end
          end
        end
      end
      
      describe 'both directories and files,' do
        before :each do
          create_sandbox :directories => ['.foo', 'b a r'],
                         :files => ['.baz', 'b a t']
        end
        
        describe 'and when sent class method' do
          describe "'directories' with that path --" do
            it 'should return the expected names' do
              Dir.directories(SANDBOX).should == ["#{SANDBOX}/./",
                                                  "#{SANDBOX}/../",
                                                  "#{SANDBOX}/.foo/",
                                                  "#{SANDBOX}/b a r/"]
            end
          end
          
          describe "'files' with that path --" do
            it 'should return the expected names' do
              Dir.files(SANDBOX).should == ['.baz', 'b a t']
            end
          end
        end
      end
    end
  end
  
  describe 'with a path that contains' do
    describe 'no entries,' do
      before :each do
        @dir = create_sandbox
      end
      
      describe 'and when sent class method' do
        describe "'directories' with that path --" do
          it 'should return the expected names' do
            @dir.directories.should == ["#{SANDBOX}/./", "#{SANDBOX}/../"]
          end
        end
        
        describe "'files' with that path --" do
          it 'should return an empty array' do
            @dir.files.should == []
          end
        end
      end
    end
    
    describe 'directories but no files,' do
      before :each do
        @dir = create_sandbox(:directories => ['.foo', 'b a r'])
      end
      
      describe 'and when sent class method' do
        describe "'directories' with that path --" do
          it 'should return the expected names' do
            @dir.directories.should == ["#{SANDBOX}/./",
                                        "#{SANDBOX}/../",
                                        "#{SANDBOX}/.foo/",
                                        "#{SANDBOX}/b a r/"]
          end
        end
        
        describe "'files' with that path --" do
          it 'should return an empty array' do
            @dir.files.should == []
          end
        end
      end
    end
    
    describe 'files but no directories,' do
      before :each do
        @dir = create_sandbox(:files => ['.foo', 'b a r'])
      end
      
      describe 'and when sent class method' do
        describe "'directories' with that path --" do
          it 'should return the expected names' do
            @dir.directories.should == ["#{SANDBOX}/./", "#{SANDBOX}/../"]
          end
        end
        
        describe "'files' with that path --" do
          it 'should return the expected names' do
            @dir.files.should == ['.foo', 'b a r']
          end
        end
      end
    end
    
    describe 'both directories and files,' do
      before :each do
        @dir = create_sandbox(:directories => ['.foo', 'b a r'],
                              :files => ['.baz', 'b a t'])
      end
      
      describe 'and when sent class method' do
        describe "'directories' with that path --" do
          it 'should return the expected names' do
            @dir.directories.should == ["#{SANDBOX}/./",
                                        "#{SANDBOX}/../",
                                        "#{SANDBOX}/.foo/",
                                        "#{SANDBOX}/b a r/"]
          end
        end
        
        describe "'files' with that path --" do
          it 'should return the expected names' do
            @dir.files.should == ['.baz', 'b a t']
          end
        end
      end
    end
  end
end