From the command line, I can easily find the path to rubygems.rb$ gem which rubygems/usr/lib/ruby/1.9.1/rubygems.rband from a Ruby script I can also do thisrequire 'rubygems/commands/which_command'wc = Gem::Commands::WhichCommand.newputs wc.find_paths 'rubygems', $LOAD_PATHHowever is a simpler way available to do this, for example without usingrequire 'rubygems/commands/which_command', and without a system() call?Update from posted answerputs $.grep(/rubygems.rb/).firstUpdate from posted commentputs Gem.method(:dir).source_location.first
Return path to rubygems.rb
ruby;file system;modules
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  endThat'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 falseLoads 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\./