After giving it some thought, this one seems like the most elegant solution.

Thanks everyone who took the time to share your knowledge, much appreciated, I didn’t expect so much feedback, you are a lovely community!

Yes, in Ruby it’s super easy to stub stuff, but it’s not necessarily good. In this case a suggestion to use Application.put_env/4 is of course good, but you are losing the ability to run tests async. Also, I would argue this is not very Elixir-ish way to do things.

I know it’s slightly off-topic, but perhaps you don’t need to keep the file name in an env variable? Environmental variables are to keep things about the environment. I know they are widely used in Ruby as a hack to pass data to commands, but copying tricks from other languages is rarely the best way to do things. You could, for example, just pass the file name as an argument to a mix tasks and the testing this would be trivial.

^ This pattern should be avoided, as the FILE_PATH env var will be read when the code is compiled for production, not in the running production environment itself—for example, using releases, or docker to build your app. runtime.exs should be preferred.