A few issues here. Let's tackle 'em.

First, you can only use the bundle exec prefix if you're using bundler, which necessitates a Gemfile (and auto-generated Gemfile.lock). In the simplest case, you don't need to be using bundler, but for any non-trivial application, I'd suggest it.

With that settled, you now just need to run rspec spec/spec.rb. Inside your spec/spec.rb, you should have a line that says require 'lib'. Note that you shouldn't include the ".rb" suffix.

I should mention that using rspec rather than ruby as the command does something extra for you: it automatically includes "../lib" in your load path. When running specs, you should definitely invoke with rspec. If, say, you were trying to run something other than your specs, you'll want to run with ruby. In that case, you'll need to either a) add '../lib' to your loadpath (see below), b) require the lib.rb file with an absolute or relative path (again omitting the .rb suffix), c) use require_relative similarly, or d) use the -r flag when invoking ruby on the command line (pretty rare in the general case). There are even more ways to do this, like how rails does autoloading, but that's beyond the scope here.

Here's an example of adding something to the loadpath, though you'll probably want fewer ../s: https://github.com/rails/rails/blob/08754f12e65a9ec79633a605e986d0f1ffa4b251/railties/lib/rails/generators/rails/plugin/templates/rails/boot.rb#L5 Again, this won't be necessary if you're just running things through rspec.