I’ve done some research and my conclusion is “not too much unless your company puts pure Ruby code into their cookbooks/manifests.”
From a Chef perspective many blog entries say that the Just Enough Ruby for Chef cheat sheet is sufficient and a post from the official Opscode blog seems to confirm it too. I’m not going to pretend to be a Chef expert but by looking at cookbooks on the Supermarket it seems about right: Variables, booleans, hashes, arrays, conditionals, case statements, a few methods, and that’s about it. From a Puppet perspective this question in their FAQ addresses the question.
Both Chef and Puppet use what are called DSLs or Domain Specific Languages. A DSL is a mini-language or a sub-set of a language that is specialized to a particular domain. In this case the domain is Chef, or Puppet, or configuration management. DSLs provide a level of abstraction so you don’t need to know a bunch of pure Ruby to interact with the software.
So why did I write this silly post? Because at several interviews I’ve had people insist that I need to know actual Ruby to work with Chef or Puppet. At the time I didn’t have the sources to back up my side argument but now I do!
Bottom line: Unless your company uses pure Ruby in it’s cookbooks or manifests you can learn the software itself and not dig deep into actual Ruby. Don’t be intimidated because it uses a Ruby-based DSL, start learning!