how do I run ruby in Xcode?

Check out this link:

http://stackoverflow.com/questions/6015182/coding-running-ruby-in-xcode

Keep in mind Shawn that the stackoverflow link is from 2011, and MacRuby has not had an update in a LONG while, and by their own post, are on indefinite hiatus. There is no inherent Ruby build solution in Xcode 6.2 or later. Apple has squashed the Ruby Cocoa scripting bridge that allowed one to mix Ruby and Cocoa/Objective-C code. Ruby/Cocoa which reinstates this scripting bridge functionality has not been updated since Fall 2014 — but does work.

One can write an Objective-C program that uses NSTask to run Ruby scripts. One can Goog "NSTask Ruby Script" for examples of using NSTask.

I write Ruby code in Sublime Text 3 — not in Xcode. I use the Rubocop gem to validate my syntax.