Paste this code snippet into your Gemfile. It let's you easily define gems for certain OSes.

gemfile_helper.rb

=begin
Paste this code snippet into your Gemfile. It let's you easily define gems for certain OSes using gem_for_darwin, gem_for_linux or gem_for_windows as gem-statement, e. g.

# notifications for file changes
gem_for_osx 'rb-fsevent', :platforms => :ruby
gem_for_linux 'rb-inotify', :platforms => :ruby
gem_for_windows 'rb-fchange', :platforms => [:mswin, :mingw]

# console color for windows
gem_for_windows 'win32console', :platforms => [:mswin, :mingw]

# To use growl ui-notifications
gem_for_osx 'ruby_gntp', :platforms => :ruby
=end

# ----- Helper Methods -----

def darwin_only(require_as)
  RbConfig::CONFIG['host_os'] =~ /darwin/ && require_as
end
alias :osx_only :darwin_only

def linux_only(require_as)
  RbConfig::CONFIG['host_os'] =~ /linux/ && require_as
end

def windows_only(require_as)
  RbConfig::CONFIG['host_os'] =~ /mingw|mswin/i && require_as
end
alias :win_only :windows_only

# OS must be one of: :darwin | :osx, :linux, :windows | :win
# Last elemt in args should be the options-hash:
# :require_as => Optional parameter, giving the name for the require statement
def gem_for_os(os, *args)
  if args.last.is_a?(Hash)
    opts = args.last
  else
    opts = {}
    args << opts
  end
  opts[:require] = send("#{os}_only", opts.delete(:require_as) || args.first)
  # ---
  gem(*args)
end

def gem_for_darwin(*args)
  gem_for_os(:darwin, *args)
end
alias :gem_for_osx :gem_for_darwin

def gem_for_linux(*args)
  gem_for_os(:linux, *args)
end

def gem_for_windows(*args)
  gem_for_os(:windows, *args)
end
alias :gem_for_win :gem_for_windows

# ----- End -----