# Configuration

You can configure the parameters for your book in the book.toml file.

Here is an example of what a book.toml file might look like:

    [book]
    title = "Example book"
    author = "John Doe"
    description = "The example book covers examples."

    [rust]
    edition = "2018"

    [build]
    build-dir = "my-example-book"
    create-missing = false

    [preprocessor.index]

    [preprocessor.links]

    [output.html]
    additional-css = ["custom.css"]

    [output.html.search]
    limit-results = 15

## Supported configuration options

It is important to note that any relative path specified in the configuration will always be taken relative from the root of the book where the configuration file is located.

### General metadata

This is general information about your book.

- title: The title of the book
- authors: The author(s) of the book
- description: A description for the book, which is added as meta information in the html `<head>` of each page
- src: By default, the source directory is found in the directory named `src` directly under the root folder. But this is configurable with the `src` key in the configuration file.
- language: The main language of the book, which is used as a language attribute `<html lang="en">` for example.

book.toml

    [book]
    title = "Example book"
    authors = ["John Doe", "Jane Doe"]
    description = "The example book covers examples."
    src = "my-src"  # the source files will be found in `root/my-src` instead of `root/src`
    language = "en"

### Rust options

Options for the Rust language, relevant to running tests and playground integration.

- edition: Rust edition to use by default for the code snippets. Default is "2015". Individual code blocks can be controlled with the `edition2015` or `edition2018` annotations, such as:

      ```rust,edition2015
      // This only works in 2015.
      let try = true;
      ```

### Build options

This controls the build process of your book.

- build-dir: The directory to put the rendered book in. By default this is `book/` in the book's root directory.

- create-missing: By default, any missing files specified in `SUMMARY.md` will be created when the book is built (i.e. `create-missing = true`). If this is `false` then the build process will instead exit with an error if any files do not exist.

- use-default-preprocessors: Disable the default preprocessors of (`links` & `index`) by setting this option to `false`.

  If you have the same, and/or other preprocessors declared via their table of configuration, they will run instead.

  - For clarity, with no preprocessor configuration, the default `links` and `index` will run.
  - Setting `use-default-preprocessors = false` will disable these default preprocessors from running.
  - Adding `[preprocessor.links]`, for example, will ensure, regardless of `use-default-preprocessors` that `links` it will run.

## Configuring Preprocessors

The following preprocessors are available and included by default:

- `links`: Expand the `{{ #playground }}`, `{{ #include }}`, and `{{ #rustdoc_include }}` handlebars helpers in a chapter to include the contents of a file.
- `index`: Convert all chapter files named `README.md` into `index.md`. That is to say, all `README.md` would be rendered to an index file `index.html` in the rendered book.

book.toml

    [build]
    build-dir = "build"
    create-missing = false

    [preprocessor.links]

    [preprocessor.index]

### Custom Preprocessor Configuration

Like renderers, preprocessor will need to be given its own table (e.g. `[preprocessor.mathjax]`). In the section, you may then pass extra configuration to the preprocessor by adding key-value pairs to the table.

For example

    [preprocessor.links]
    # set the renderers this preprocessor will run for
    renderers = ["html"]
    some_extra_feature = true

#### Locking a Preprocessor dependency to a renderer

You can explicitly specify that a preprocessor should run for a renderer by binding the two together.

    [preprocessor.mathjax]
    renderers = ["html"]  # mathjax only makes sense with the HTML renderer

### Provide Your Own Command

By default when you add a `[preprocessor.foo]` table to your `book.toml` file, `mdbook` will try to invoke the `mdbook-foo` executable. If you want to use a different program name or pass in command-line arguments, this behaviour can be overridden by adding a `command` field.

    [preprocessor.random]
    command = "python random.py"

## Configuring Renderers

### HTML renderer options

The HTML renderer has a couple of options as well. All the options for the renderer need to be specified under the TOML table `[output.html]`.

The following configuration options are available:

- theme: mdBook comes with a default theme and all the resource files needed for it. But if this option is set, mdBook will selectively overwrite the theme files with the ones found in the specified folder.
- default-theme: The theme color scheme to select by default in the 'Change Theme' dropdown. Defaults to `light`.
- preferred-dark-theme: The default dark theme. This theme will be used if the browser requests the dark version of the site via the 'prefers-color-scheme' CSS media query. Defaults to `navy`.
- curly-quotes: Convert straight quotes to curly quotes, except for those that occur in code blocks and code spans. Defaults to `false`.
- mathjax-support: Adds support for MathJax. Defaults to `false`.
- copy-fonts: Copies fonts.css and respective font files to the output directory and use them in the default theme. Defaults to `true`.
- google-analytics: If you use Google Analytics, this option lets you enable it by simply specifying your ID in the configuration file.
- additional-css: If you need to slightly change the appearance of your book without overwriting the whole style, you can specify a set of stylesheets that will be loaded after the default ones where you can surgically change the style.
- additional-js: If you need to add some behaviour to your book without removing the current behaviour, you can specify a set of JavaScript files that will be loaded alongside the default one.
- no-section-label: mdBook by defaults adds section label in table of contents column. For example, "1.", "2.1". Set this option to true to disable those labels. Defaults to `false`.
- fold: A subtable for configuring sidebar section-folding behavior.
- playground: A subtable for configuring various playground settings.
- search: A subtable for configuring the in-browser search functionality. mdBook must be compiled with the `search` feature enabled (on by default).
- git-repository-url: A url to the git repository for the book. If provided an icon link will be output in the menu bar of the book.
- git-repository-icon: The FontAwesome icon class to use for the git repository link. Defaults to `fa-github`.
- redirect: A subtable used for generating redirects when a page is moved. The table contains key-value pairs where the key is where the redirect file needs to be created, as an absolute path from the build directory, (e.g. `/appendices/bibliography.html`). The value can be any valid URI the browser should navigate to (e.g. `https://rust-lang.org/`, `/overview.html`, or `../bibliography.html`).
- input-404: The name of the markdown file used for misssing files. The corresponding output file will be the same, with the extension replaced with `html`. Defaults to `404.md`.
- site-url: The url where the book will be hosted. This is required to ensure navigation links and script/css imports in the 404 file work correctly, even when accessing urls in subdirectories. Defaults to `/`.

Available configuration options for the `[output.html.fold]` table:

- enable: Enable section-folding. When off, all folds are open. Defaults to `false`.
- level: The higher the more folded regions are open. When level is 0, all folds are closed. Defaults to `0`.

Available configuration options for the `[output.html.playground]` table:

- editable: Allow editing the source code. Defaults to `false`.
- copyable: Display the copy button on code snippets. Defaults to `true`.
- copy-js: Copy JavaScript files for the editor to the output directory. Defaults to `true`.
- line-numbers Display line numbers on editable sections of code. Requires both `editable` and `copy-js` to be `true`. Defaults to `false`.

Available configuration options for the `[output.html.search]` table:

- enable: Enables the search feature. Defaults to `true`.
- limit-results: The maximum number of search results. Defaults to `30`.
- teaser-word-count: The number of words used for a search result teaser. Defaults to `30`.
- use-boolean-and: Define the logical link between multiple search words. If true, all search words must appear in each result. Defaults to `false`.
- boost-title: Boost factor for the search result score if a search word appears in the header. Defaults to `2`.
- boost-hierarchy: Boost factor for the search result score if a search word appears in the hierarchy. The hierarchy contains all titles of the parent documents and all parent headings. Defaults to `1`.
- boost-paragraph: Boost factor for the search result score if a search word appears in the text. Defaults to `1`.
- expand: True if search should match longer results e.g. search `micro` should match `microwave`. Defaults to `true`.
- heading-split-level: Search results will link to a section of the document which contains the result. Documents are split into sections by headings this level or less. Defaults to `3`. (`### This is a level 3 heading`)
- copy-js: Copy JavaScript files for the search implementation to the output directory. Defaults to `true`.

This shows all available HTML output options in the book.toml:

    [book]
    title = "Example book"
    authors = ["John Doe", "Jane Doe"]
    description = "The example book covers examples."

    [output.html]
    theme = "my-theme"
    default-theme = "light"
    preferred-dark-theme = "navy"
    curly-quotes = true
    mathjax-support = false
    copy-fonts = true
    google-analytics = "UA-123456-7"
    additional-css = ["custom.css", "custom2.css"]
    additional-js = ["custom.js"]
    no-section-label = false
    git-repository-url = "https://github.com/rust-lang/mdBook"
    git-repository-icon = "fa-github"
    site-url = "/example-book/"
    input-404 = "not-found.md"

    [output.html.fold]
    enable = false
    level = 0

    [output.html.playground]
    editable = false
    copy-js = true
    line-numbers = false

    [output.html.search]
    enable = true
    limit-results = 30
    teaser-word-count = 30
    use-boolean-and = true
    boost-title = 2
    boost-hierarchy = 1
    boost-paragraph = 1
    expand = true
    heading-split-level = 3
    copy-js = true

    [output.html.redirect]
    "/appendices/bibliography.html" = "https://rustc-dev-guide.rust-lang.org/appendix/bibliography.html"
    "/other-installation-methods.html" = "../infra/other-installation-methods.html"

### Markdown Renderer

The Markdown renderer will run preprocessors and then output the resulting Markdown. This is mostly useful for debugging preprocessors, especially in conjunction with `mdbook test` to see the Markdown that `mdbook` is passing to `rustdoc`.

The Markdown renderer is included with `mdbook` but disabled by default. Enable it by adding an empty table to your `book.toml` as follows:

    [output.markdown]

There are no configuration options for the Markdown renderer at this time; only whether it is enabled or disabled.

See the preprocessors documentation for how to specify which preprocessors should run before the Markdown renderer.

### Custom Renderers

A custom renderer can be enabled by adding a `[output.foo]` table to your `book.toml`. Similar to preprocessors this will instruct `mdbook` to pass a representation of the book to `mdbook-foo` for rendering. See the alternative backends chapter for more detail.

The custom renderer has access to all the fields within its table (i.e. anything under `[output.foo]`). mdBook checks for two common fields:

- command: The command to execute for this custom renderer. Defaults to the name of the renderer with the `mdbook-` prefix (such as `mdbook-foo`).
- optional: If `true`, then the command will be ignored if it is not installed, otherwise mdBook will fail with an error. Defaults to `false`.

## Environment Variables

All configuration values can be overridden from the command line by setting the corresponding environment variable. Because many operating systems restrict environment variables to be alphanumeric characters or `_`, the configuration key needs to be formatted slightly differently to the normal `foo.bar.baz` form.

Variables starting with `MDBOOK_` are used for configuration. The key is created by removing the `MDBOOK_` prefix and turning the resulting string into `kebab-case`. Double underscores (`__`) separate nested keys, while a single underscore (`_`) is replaced with a dash (`-`).

For example:

- `MDBOOK_foo` -> `foo`
- `MDBOOK_FOO` -> `foo`
- `MDBOOK_FOO__BAR` -> `foo.bar`
- `MDBOOK_FOO_BAR` -> `foo-bar`
- `MDBOOK_FOO_bar__baz` -> `foo-bar.baz`

So by setting the `MDBOOK_BOOK__TITLE` environment variable you can override the book's title without needing to touch your `book.toml`.

> Note: To facilitate setting more complex config items, the value of an environment variable is first parsed as JSON, falling back to a string if the parse fails.

> This means, if you so desired, you could override all book metadata when building the book with something like

>     $ export MDBOOK_BOOK="{'title': 'My Awesome Book', authors: ['Michael-F-Bryan']}"
>     $ mdbook build

The latter case may be useful in situations where `mdbook` is invoked from a script or CI, where it sometimes isn't possible to update the `book.toml` before building.