Run The Test At A Specific Line Number

You can tell mix test to only run tests that appear in a specific file by naming the file:

$ mix test test/module/file_test.exs

You can even point it to a specific line number in that file like so:

$ mix test test/module/file_test.exs:45

Behind the scenes, mix test is using tags to build a list of exclusions and inclusions that result in only the test at line 45 running. Here is an equivalent of the above command:

$ mix test test/module/file_test.exs --exclude test --include line:45

This will exclude every test. It will then re-include the test that corresponds to having a tag line:45.