# Configuring Preprocessors

Preprocessors are extensions that can modify the raw Markdown source before it gets sent to the renderer.

The following preprocessors are built-in and included by default:

• links: Expands the {{ #playground }}, {{ #include }}, and {{ #rustdoc_include }} handlebars helpers in a chapter to include the contents of a file. See Including files for more.
• 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.

The built-in preprocessors can be disabled with the build.use-default-preprocessors config option.

The community has developed several preprocessors. See the Third Party Plugins wiki page for a list of available preprocessors.

For information on how to create a new preprocessor, see the Preprocessors for Developers chapter.

## Custom Preprocessor Configuration

Preprocessors can be added by including a preprocessor table in book.toml with the name of the preprocessor. For example, if you have a preprocessor called mdbook-example, then you can include it with:

[preprocessor.example]


With this table, mdBook will execute the mdbook-example preprocessor.

This table can include additional key-value pairs that are specific to the preprocessor. For example, if our example prepocessor needed some extra configuration options:

[preprocessor.example]
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.example]
renderers = ["html"]  # example preprocessor only runs with the HTML renderer


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"


## Require A Certain Order

The order in which preprocessors are run can be controlled with the before and after fields. For example, suppose you want your linenos preprocessor to process lines that may have been {{#include}}d; then you want it to run after the built-in links preprocessor, which you can require using either the before or after field:

[preprocessor.linenos]

[preprocessor.links]

Preprocessors having the same priority specified through before and after are sorted by name. Any infinite loops will be detected and produce an error.