Return path to rubygems.rb

From the command line, I can easily find the path to rubygems.rb

$ gem which rubygems
/usr/lib/ruby/1.9.1/rubygems.rb

and from a Ruby script I can also do this

require 'rubygems/commands/which_command'
wc = Gem::Commands::WhichCommand.new
puts wc.find_paths 'rubygems', $LOAD_PATH

However is a simpler way available to do this, for example without using

require 'rubygems/commands/which_command', and without a system() call?

Update from posted answer

puts $".grep(/rubygems.rb/).first

Update from posted comment

puts Gem.method(:dir).source_location.first

Whether a simpler solution exists depends on what your motivation is.

Let's look inside the implementation of Gem::Commands::WhichCommand#find_paths.

def find_paths(package_name, dirs)
    result = []

    dirs.each do |dir|
      Gem.suffixes.each do |ext|
        full_path = File.join dir, "#{package_name}#{ext}"
        if File.exist? full_path and not File.directory? full_path then
          result << full_path
          return result unless options[:show_all]
        end
      end
    end

    result
  end

That's the code you need. If there were a simpler way, find_paths would have used it.

However, there is a way to cheat: Kernel#require needs to do a very similar thing when it actually tries to load a module:

require(name) → true or false


Loads the given name, returning true if successful and false if
the feature is already loaded.


If the filename does not resolve to an absolute path, it will be
searched for in the directories listed in $LOAD_PATH ($:).


If the filename has the extension “.rb”, it is loaded as a source
file; if the extension is “.so”, “.o”, or “.dll”, or the default
shared library extension on the current platform, Ruby loads the
shared library as a Ruby extension. Otherwise, Ruby tries adding
“.rb”, “.so”, and so on to the name until found. If the file named
cannot be found, a LoadError will be raised.


For Ruby extensions the filename given may use any shared library
extension. For example, on Linux the socket extension is “socket.so”
and require 'socket.dll' will load the socket extension.


The absolute path of the loaded file is added to $LOADED_FEATURES
($"). A file will not be loaded again if its path already appears in
$". For example, require 'a'; require './a' will not load a.rb
again.

Or, it might be more accurate to say that find_paths simulates what require would do.

Therefore, if you are willing to let Ruby actually load the file first, then you could ask Ruby after the fact:

require 'rubygems'
$LOADED_FEATURES.grep /\/rubygems\./

StackExchange Code Review Q#48200, answer score: 3