run shellcheck in the test script

CONTRIBUTING.md

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+# Writing the Bible
+
+<!-- vim-markdown-toc GFM -->
+
+* [Adding Code to the Bible.](#adding-code-to-the-bible)
+* [Special meanings for code blocks.](#special-meanings-for-code-blocks)
+* [Writing tests](#writing-tests)
+* [Running the tests](#running-the-tests)
+
+<!-- vim-markdown-toc -->
+
+## Adding Code to the Bible.
+
+- The code must use only `bash` built-ins.
+    - A fallback to an external program is allowed if the code doesn't
+      always work.
+    - Example Fallback: `${HOSTNAME:-$(hostname)}`
+- If possible, wrap the code in a function.
+    - This allows tests to be written.
+    - It also allows `shellcheck` to properly lint it.
+    - An added bonus is showing a working use-case.
+- Write some examples.
+    - Show some input and the modified output.
+
+
+## Special meanings for code blocks.
+
+Use `sh` for functions that should be linted and unit tested.
+
+    ```sh
+    # Shellcheck will lint this and the test script will source this.
+    func() {
+        # Usage: func "arg"
+        :
+    }
+    ```
+
+Use `shell` for code that should be ignored.
+
+    ```shell
+    # Shorter file creation syntax.
+    :>file
+    ```
+
+## Writing tests
+
+The test file is viewable here: https://github.com/dylanaraps/pure-bash-bible/blob/master/test.sh
+
+Example test:
+
+```sh
+test_upper() {
+    result="$(upper "HeLlO")"
+    assert_equals "$result" "HELLO"
+}
+```
+
+Steps:
+
+1. Write the test.
+    - Naming is `test_func_name`
+    - Store the function output in a variable.
+    - Use `assert_equals` to test equality between the variable and the
+      expected output.
+2. ???
+3. The test script will automatically execute it. :+1:
+
+
+## Running the tests
+
+Running `test.sh` also runs `shellcheck` on the code.
+
+```sh
+cd pure-bash-bible
+./test.sh
+```