Am I executing the correct executable?
Q: When I run a command (let's say rails), which executable is it executing?
> which rails /Users/username/.rvm/gems/ruby-2.1.6/bin/rails
Q: Ah, I see which one it's running. And it's not the right one! Where are all the potential executables, given the current PATH?
> where rails /Users/username/.rvm/gems/ruby-2.1.6/bin/rails /usr/bin/rails
Now I know whether it's a PATH ordering issue, or whether it's not included PATH at all.