How can I tell if I'm running from JRuby vs. Ruby?

I believe you can check the RUBY_PLATFORM constant.

As already have been stated here, there are multiple ways to detect JRuby. Here's the complete list, sorted by preference:

1. Simple and straightforward:

   RUBY_PLATFORM == "java"
   

2. This can be used for other impls as well. The downside is that MRI 1.8.6 and 1.8.7 do not support RUBY_ENGINE yet, so we need the defined? check:

   if defined?(RUBY_ENGINE) && RUBY_ENGINE == 'jruby'
   

3. If you wish to know a bit more than just whether it's JRuby or not:

   if defined? JRUBY_VERSION
   

   JRUBY_VERSION is only defined in JRuby and in provides the short version info, like "1.5.0.dev"

4. Least favorite way: require 'java' or require 'jruby' and rescue. Should not be used.

I don't quite understand your question. You seem to be mixing two completely different abstraction levels: Ruby is a programming language, JRuby is a compiler for the Ruby programming language. The question whether your program is running in Ruby or in JRuby just plain doesn't make sense: when it's running in JRuby, it is running in Ruby, because JRuby is an implementation of Ruby.

It's the same as asking "how can I tell if I'm driving in a Ford vs. a car?"

If you want to know in what Ruby implementation you're running, then you can check the global RUBY_ENGINE constant. It is supposed to universally and uniquely identify the engine you are running on, although it unfortunately fails for three reasons:

1. on some engines, it doesn't tell you what engine it is running on, for example on YARV I would expect RUBY_ENGINE to be 'yarv', but it is actually 'ruby'. So, it fails at the "identify" part.
2. even worse: on MRI, it is also 'ruby', which means that not only does it not tell you what engine you are running on, but there also totally different engines that return the same value. IOW, it also fails at the "unique" part.
3. and last but not least, RUBY_ENGINE is fairly new, so it is not yet supported by all engines, which means it fails at the "universal" part.

Still, for your purposes something like

if defined? RUBY_ENGINE && RUBY_ENGINE == 'jruby'

should work fine.

If you're debugging shell, irb, RVM or some type of plugin like Ruby text editor plugins I prefer the more verbose RUBY_DESCRIPTION.

If you're trying to run some code based on if you're running JRUBY or not, other constants commented are good: RUBY_ENGINE, JRUBY_VERSION or RUBY_PLATFORM.