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# The test command

When writing a book, you sometimes need to automate some tests. For example, The Rust Programming Book uses a lot of code examples that could get outdated.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 Rust tests are supported.

#### 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.

    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`). The path should point to the Cargo build cache `deps` directory that contains the build output of your project. For example, if your Rust project’s book is in a directory named `my-book`, the following command would include the crate’s dependencies when running `test`:

    mdbook test my-book -L target/debug/deps/

See the `rustdoc` command-line documentation for more information.

#### `--dest-dir`

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

#### `--chapter`

The `--chapter` (`-c`) option allows you to test a specific chapter of the book using the chapter name or the relative path to the chapter.