mdBook/guide/src/for_developers/preprocessors.md

Preprocessors

A preprocessor is simply a bit of code which gets run immediately after the book is loaded and before it gets rendered, allowing you to update and mutate the book. Possible use cases are:

• Creating custom helpers like \{{#include /path/to/file.md}}
• Updating links so [some chapter](some_chapter.md) is automatically changed to [some chapter](some_chapter.html) for the HTML renderer
• Substituting in latex-style expressions ($$ \frac{1}{3} $$) with their mathjax equivalents

Hooking Into MDBook

MDBook uses a fairly simple mechanism for discovering third party plugins. A new table is added to book.toml (e.g. preprocessor.foo for the foo preprocessor) and then mdbook will try to invoke the mdbook-foo program as part of the build process.

While preprocessors can be hard-coded to specify which backend it should be run for (e.g. it doesn't make sense for MathJax to be used for non-HTML renderers) with the preprocessor.foo.renderer key.

[book]
title = "My Book"
authors = ["Michael-F-Bryan"]

[preprocessor.foo]
# The command can also be specified manually
command = "python3 /path/to/foo.py"
# Only run the `foo` preprocessor for the HTML and EPUB renderer
renderer = ["html", "epub"]

Once the preprocessor has been defined and the build process has started, MdBook will execute the command defined in the preprocessor.foo.command key passing the arguments support and the renderer name, monitoring the status code of the executed command. If the status code retrieved is 0, the library will be sending through stdin both the context and the book representation serialized in JSON format, and it'll be capturing the response from stdout, which will be the modified book which has to also be serialized in json format.

The easiest way to get started is by creating your own implementation of the Preprocessor trait (e.g. in lib.rs) and then creating a shell binary which translates inputs to the correct Preprocessor method. For convenience, there is an example no-op preprocessor in the examples/ directory which can easily be adapted for other preprocessors.

Example no-op preprocessor

// nop-preprocessors.rs

{{#include ../../../examples/nop-preprocessor.rs}}

Hints For Implementing A Preprocessor

By pulling in mdbook as a library, preprocessors can have access to the existing infrastructure for dealing with books.

For example, a custom preprocessor could use the CmdPreprocessor::parse_input() function to deserialize the JSON written to stdin. Then each chapter of the Book can be mutated in-place via Book::for_each_mut(), and then written to stdout with the serde_json crate.

Chapters can be accessed either directly (by recursively iterating over chapters) or via the Book::for_each_mut() convenience method.

The chapter.content is just a string which happens to be markdown. While it's entirely possible to use regular expressions or do a manual find & replace, you'll probably want to process the input into something more computer-friendly. The pulldown-cmark crate implements a production-quality event-based Markdown parser, with the pulldown-cmark-to-cmark allowing you to translate events back into markdown text.

The following code block shows how to remove all emphasis from markdown, without accidentally breaking the document.

fn remove_emphasis(
    num_removed_items: &mut usize,
	    chapter: &mut Chapter,
) -> Result<String> {
    let mut buf = String::with_capacity(chapter.content.len());

    let events = Parser::new(&chapter.content).filter(|e| {
        let should_keep = match *e {
            Event::Start(Tag::Emphasis)
            | Event::Start(Tag::Strong)
            | Event::End(Tag::Emphasis)
            | Event::End(Tag::Strong) => false,
            _ => true,
        };
        if !should_keep {
            *num_removed_items += 1;
        }
        should_keep
    });

    cmark(events, &mut buf, None).map(|_| buf).map_err(|err| {
        Error::from(format!("Markdown serialization failed: {}", err))
    })
}

For everything else, have a look at the complete example.

implementing a preprocessor with a different language

The fact that MdBook utilizes stdin and stdout to communicate with the preprocessors, makes it easy for developers to implement them in a language different than rust. The following code shows how to implement a simple preprocessor in python, which will modify the content of the first chapter. The example will follow the configuration shown above with preprocessor.foo.command actually pointing to a python script. The code of said script is below:

import json
import sys


if __name__ == '__main__':
    if len(sys.argv) > 1: # we check if we received any argument
        if sys.argv[1] == "supports": 
            # then we are good to return an exit status code of 0, since the other argument will just be the renderer's name
            sys.exit(0)

    # we will load both the context and the book representations from stdin
    stdin = sys.stdin
    context, book = json.load(stdin)
    # and now, we can just modify the content of the first chapter
    book['sections'][0]['Chapter']['content'] = '<h3>Hello</h3>'
    # we are done with the book's modification, we can just print it to stdout, 
    print(json.dumps(book))