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Announcing ruby-processing-2.4.1, which is now available as a rubygem.
gem install ruby-processing
You do need to have regular processing installed (processing-2.0.3 Mac and processing-2.1.0 Linux and Windows), and configure the path to the PROCESSING_ROOT in .rp5rc to run ruby-processing. Ideally will have installed a version of JRuby (1.7+) on your system, failing that you can get by with some other version of ruby and our vendored JRuby-Complete, which is added by running install_jruby_complete on the command line, post install (requires wget) for full instructions see wiki.
Running sketches is as simple as rp5 run sketch.rb or if you are using the vendored jruby-complete rp5 --nojruby run sketch.rb.