In short, it points the binary in /bin for ruby to other installed locations of ruby.

This does require ruby version > 1.9 and requires the psych ruby gem for yaml, which comes as standard since ruby 1.9.2.

I have two versions of Ruby. One from Mac OS X and the other from MacPorts.

./rptr.rb where /usr/bin/ruby
./rptr.rb add /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby system
./rptr.rb add /opt/local/bin/ruby1.9 port

To then use the MacPorts version

./rptr.rb use port

For help please use ./rptr.rb help