# 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.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


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]