# The test command

When writing a book, you sometimes need to automate some tests. For example,
[The Rust Programming Book](https://doc.rust-lang.org/stable/book/) uses a lot
of code examples that could get outdated. Therefore it is very important for
them to be able to automatically test these code examples.

mdBook supports a `test` command that will run all available tests in a book. At
the moment, only rustdoc tests are supported, but this may be expanded upon in
the future.

#### Disable tests on a code block

rustdoc doesn't test code blocks which contain the `ignore` attribute:

    ```rust,ignore
    fn main() {}
    ```

rustdoc also doesn't test code blocks which specify a language other than Rust:

    ```markdown
    **Foo**: _bar_
    ```

rustdoc *does* test code blocks which have no language specified:

    ```
    This is going to cause an error!
    ```

#### Specify a directory

The `test` command can take a directory as an argument to use as the book's root
instead of the current working directory.

```bash
mdbook test path/to/book
```

#### --library-path

The `--library-path` (`-L`) option allows you to add directories to the library
search path used by `rustdoc` when it builds and tests the examples. Multiple
directories can be specified with multiple options (`-L foo -L bar`) or with a
comma-delimited list (`-L foo,bar`).

#### --dest-dir

The `--dest-dir` (`-d`) option allows you to change the output directory for the
book. If not specified it will default to the value of the `build.build-dir` key
in `book.toml`, or to `./book` relative to the book's root directory.