Easy, Concurrent Temp Dirs with ExUnit ✅

Sometimes we need a temp directory in tests.

Our first inclination might be to reach for System.tmp_dir!/0. And that works, but ExUnit has a really nice helper just for us.

If we tag our test with :tmp_dir, ExUnit will pass a temp directory in the text metadata under tmp_dir:

@tag :tmp_dir
test "we need a temp directory!", %{tmp_dir: tmp_dir} do
  # we can now read and write from `tmp_dir`
end

The cool thing is that the tmp_dir generated by ExUnit includes the module and test names – making it unique. Thus, we can continue to run our tests asynchronously without worrying that we’ll accidentally clobber the directory with data across tests.

For more, check out ExUnit’s docs on Tmp Dir.