Add Run The Test At A Specific Line Number as an elixir til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_788 TILs and counting..._
+_789 TILs and counting..._
 
 ---
 
@@ -158,6 +158,7 @@ _788 TILs and counting..._
 - [Root Directory Of A Project](elixir/root-directory-of-a-project.md)
 - [Round Floats To Integers](elixir/round-floats-to-integers.md)
 - [Run ExUnit Tests In A Deterministic Order](elixir/run-exunit-tests-in-a-deterministic-order.md)
+- [Run The Test At A Specific Line Number](elixir/run-the-test-at-a-specific-line-number.md)
 - [Same Functions Should Be Grouped Together](elixir/same-functions-should-be-grouped-together.md)
 - [String Interpolation With Just About Anything](elixir/string-interpolation-with-just-about-anything.md)
 - [Unique Indexes With Ecto](elixir/unique-indexes-with-ecto.md)

elixir/run-the-test-at-a-specific-line-number.md

@@ -0,0 +1,25 @@
+# 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:
+
+```bash
+$ mix test test/module/file_test.exs
+```
+
+You can even point it to a specific line number in that file like so:
+
+```bash
+$ 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:
+
+```bash
+$ 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`.