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<main>
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<h1 id="preprocessors"><a class="header" href="#preprocessors">Preprocessors</a></h1>
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||
<p>A <em>preprocessor</em> 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
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||
the book. Possible use cases are:</p>
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<ul>
|
||
<li>Creating custom helpers like <code>{{#include /path/to/file.md}}</code></li>
|
||
<li>Substituting in latex-style expressions (<code>$$ \frac{1}{3} $$</code>) with their
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||
mathjax equivalents</li>
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||
</ul>
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||
<p>See <a href="../format/configuration/preprocessors.html">Configuring Preprocessors</a> for more information about using preprocessors.</p>
|
||
<h2 id="hooking-into-mdbook"><a class="header" href="#hooking-into-mdbook">Hooking into MDBook</a></h2>
|
||
<p>MDBook uses a fairly simple mechanism for discovering third party plugins.
|
||
A new table is added to <code>book.toml</code> (e.g. <code>[preprocessor.foo]</code> for the <code>foo</code>
|
||
preprocessor) and then <code>mdbook</code> will try to invoke the <code>mdbook-foo</code> program as
|
||
part of the build process.</p>
|
||
<p>Once the preprocessor has been defined and the build process starts, mdBook executes the command defined in the <code>preprocessor.foo.command</code> key twice.
|
||
The first time it runs the preprocessor to determine if it supports the given renderer.
|
||
mdBook passes two arguments to the process: the first argument is the string <code>supports</code> and the second argument is the renderer name.
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||
The preprocessor should exit with a status code 0 if it supports the given renderer, or return a non-zero exit code if it does not.</p>
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||
<p>If the preprocessor supports the renderer, then mdbook runs it a second time, passing JSON data into stdin.
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||
The JSON consists of an array of <code>[context, book]</code> where <code>context</code> is the serialized object <a href="https://docs.rs/mdbook-preprocessor/latest/mdbook_preprocessor/struct.PreprocessorContext.html"><code>PreprocessorContext</code></a> and <code>book</code> is a <a href="https://docs.rs/mdbook-preprocessor/latest/mdbook_preprocessor/book/struct.Book.html"><code>Book</code></a> object containing the content of the book.</p>
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<p>The preprocessor should return the JSON format of the <a href="https://docs.rs/mdbook-preprocessor/latest/mdbook_preprocessor/book/struct.Book.html"><code>Book</code></a> object to stdout, with any modifications it wishes to perform.</p>
|
||
<p>The easiest way to get started is by creating your own implementation of the
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<code>Preprocessor</code> trait (e.g. in <code>lib.rs</code>) and then creating a shell binary which
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translates inputs to the correct <code>Preprocessor</code> method. For convenience, there
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||
is <a href="https://github.com/rust-lang/mdBook/blob/master/examples/nop-preprocessor.rs">an example no-op preprocessor</a> in the <code>examples/</code> directory which can easily
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||
be adapted for other preprocessors.</p>
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<details>
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<summary>Example no-op preprocessor</summary>
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<pre class="playground"><code class="language-rust edition2018">// nop-preprocessors.rs
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//! A basic example of a preprocessor that does nothing.
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use crate::nop_lib::Nop;
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use clap::{Arg, ArgMatches, Command};
|
||
use mdbook_preprocessor::book::Book;
|
||
use mdbook_preprocessor::errors::Result;
|
||
use mdbook_preprocessor::{Preprocessor, PreprocessorContext};
|
||
use semver::{Version, VersionReq};
|
||
use std::io;
|
||
use std::process;
|
||
|
||
fn make_app() -> Command {
|
||
Command::new("nop-preprocessor")
|
||
.about("A mdbook preprocessor which does precisely nothing")
|
||
.subcommand(
|
||
Command::new("supports")
|
||
.arg(Arg::new("renderer").required(true))
|
||
.about("Check whether a renderer is supported by this preprocessor"),
|
||
)
|
||
}
|
||
|
||
fn main() {
|
||
let matches = make_app().get_matches();
|
||
|
||
// Users will want to construct their own preprocessor here
|
||
let preprocessor = Nop::new();
|
||
|
||
if let Some(sub_args) = matches.subcommand_matches("supports") {
|
||
handle_supports(&preprocessor, sub_args);
|
||
} else if let Err(e) = handle_preprocessing(&preprocessor) {
|
||
eprintln!("{e:?}");
|
||
process::exit(1);
|
||
}
|
||
}
|
||
|
||
fn handle_preprocessing(pre: &dyn Preprocessor) -> Result<()> {
|
||
let (ctx, book) = mdbook_preprocessor::parse_input(io::stdin())?;
|
||
|
||
let book_version = Version::parse(&ctx.mdbook_version)?;
|
||
let version_req = VersionReq::parse(mdbook_preprocessor::MDBOOK_VERSION)?;
|
||
|
||
if !version_req.matches(&book_version) {
|
||
eprintln!(
|
||
"Warning: The {} plugin was built against version {} of mdbook, \
|
||
but we're being called from version {}",
|
||
pre.name(),
|
||
mdbook_preprocessor::MDBOOK_VERSION,
|
||
ctx.mdbook_version
|
||
);
|
||
}
|
||
|
||
let processed_book = pre.run(&ctx, book)?;
|
||
serde_json::to_writer(io::stdout(), &processed_book)?;
|
||
|
||
Ok(())
|
||
}
|
||
|
||
fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
|
||
let renderer = sub_args
|
||
.get_one::<String>("renderer")
|
||
.expect("Required argument");
|
||
let supported = pre.supports_renderer(renderer).unwrap();
|
||
|
||
// Signal whether the renderer is supported by exiting with 1 or 0.
|
||
if supported {
|
||
process::exit(0);
|
||
} else {
|
||
process::exit(1);
|
||
}
|
||
}
|
||
|
||
/// The actual implementation of the `Nop` preprocessor. This would usually go
|
||
/// in your main `lib.rs` file.
|
||
#[allow(unreachable_pub, reason = "wouldn't be a problem in a proper lib.rs")]
|
||
mod nop_lib {
|
||
use super::*;
|
||
|
||
/// A no-op preprocessor.
|
||
pub struct Nop;
|
||
|
||
impl Nop {
|
||
pub fn new() -> Nop {
|
||
Nop
|
||
}
|
||
}
|
||
|
||
impl Preprocessor for Nop {
|
||
fn name(&self) -> &str {
|
||
"nop-preprocessor"
|
||
}
|
||
|
||
fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book> {
|
||
// In testing we want to tell the preprocessor to blow up by setting a
|
||
// particular config value
|
||
match ctx
|
||
.config
|
||
.get::<bool>("preprocessor.nop-preprocessor.blow-up")
|
||
{
|
||
Ok(Some(true)) => anyhow::bail!("Boom!!1!"),
|
||
Ok(_) => {}
|
||
Err(e) => anyhow::bail!("expect bool for blow-up: {e}"),
|
||
}
|
||
|
||
// we *are* a no-op preprocessor after all
|
||
Ok(book)
|
||
}
|
||
|
||
fn supports_renderer(&self, renderer: &str) -> Result<bool> {
|
||
Ok(renderer != "not-supported")
|
||
}
|
||
}
|
||
|
||
#[cfg(test)]
|
||
mod test {
|
||
use super::*;
|
||
|
||
#[test]
|
||
fn nop_preprocessor_run() {
|
||
let input_json = r##"[
|
||
{
|
||
"root": "/path/to/book",
|
||
"config": {
|
||
"book": {
|
||
"authors": ["AUTHOR"],
|
||
"language": "en",
|
||
"src": "src",
|
||
"title": "TITLE"
|
||
},
|
||
"preprocessor": {
|
||
"nop": {}
|
||
}
|
||
},
|
||
"renderer": "html",
|
||
"mdbook_version": "0.4.21"
|
||
},
|
||
{
|
||
"items": [
|
||
{
|
||
"Chapter": {
|
||
"name": "Chapter 1",
|
||
"content": "# Chapter 1\n",
|
||
"number": [1],
|
||
"sub_items": [],
|
||
"path": "chapter_1.md",
|
||
"source_path": "chapter_1.md",
|
||
"parent_names": []
|
||
}
|
||
}
|
||
]
|
||
}
|
||
]"##;
|
||
let input_json = input_json.as_bytes();
|
||
|
||
let (ctx, book) = mdbook_preprocessor::parse_input(input_json).unwrap();
|
||
let expected_book = book.clone();
|
||
let result = Nop::new().run(&ctx, book);
|
||
assert!(result.is_ok());
|
||
|
||
// The nop-preprocessor should not have made any changes to the book content.
|
||
let actual_book = result.unwrap();
|
||
assert_eq!(actual_book, expected_book);
|
||
}
|
||
}
|
||
}</code></pre>
|
||
</details>
|
||
<h2 id="hints-for-implementing-a-preprocessor"><a class="header" href="#hints-for-implementing-a-preprocessor">Hints for implementing a preprocessor</a></h2>
|
||
<p>By pulling in <code>mdbook-preprocessor</code> as a library, preprocessors can have access to the
|
||
existing infrastructure for dealing with books.</p>
|
||
<p>For example, a custom preprocessor could use the
|
||
<a href="https://docs.rs/mdbook-preprocessor/latest/mdbook_preprocessor/fn.parse_input.html"><code>parse_input()</code></a> function to deserialize the JSON written to
|
||
<code>stdin</code>. Then each chapter of the <code>Book</code> can be mutated in-place via
|
||
<a href="https://docs.rs/mdbook-preprocessor/latest/mdbook_preprocessor/book/struct.Book.html#method.for_each_mut"><code>Book::for_each_mut()</code></a>, and then written to <code>stdout</code> with the <code>serde_json</code>
|
||
crate.</p>
|
||
<p>Chapters can be accessed either directly (by recursively iterating over
|
||
chapters) or via the <code>Book::for_each_mut()</code> convenience method.</p>
|
||
<p>The <code>chapter.content</code> 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 <a href="https://docs.rs/mdbook-markdown/latest/mdbook_markdown/"><code>mdbook-markdown</code></a> crate exposes the <a href="https://crates.io/crates/pulldown-cmark"><code>pulldown-cmark</code></a> crate used by mdBook to parse Markdown. The <a href="https://crates.io/crates/pulldown-cmark-to-cmark"><code>pulldown-cmark-to-cmark</code></a> crate can be used to translate events back into markdown text.</p>
|
||
<p>The following code block shows how to remove all emphasis from markdown,
|
||
without accidentally breaking the document.</p>
|
||
<pre class="playground"><code class="language-rust edition2018"><span class="boring">//! This is a demonstration of an mdBook preprocessor which parses markdown
|
||
</span><span class="boring">//! and removes any instances of emphasis.
|
||
</span><span class="boring">
|
||
</span><span class="boring">use mdbook_preprocessor::book::{Book, Chapter};
|
||
</span><span class="boring">use mdbook_preprocessor::errors::Result;
|
||
</span><span class="boring">use mdbook_preprocessor::{Preprocessor, PreprocessorContext};
|
||
</span><span class="boring">use pulldown_cmark::{Event, Parser, Tag, TagEnd};
|
||
</span><span class="boring">use std::io;
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> let mut args = std::env::args().skip(1);
|
||
</span><span class="boring"> match args.next().as_deref() {
|
||
</span><span class="boring"> Some("supports") => {
|
||
</span><span class="boring"> // Supports all renderers.
|
||
</span><span class="boring"> return;
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> Some(arg) => {
|
||
</span><span class="boring"> eprintln!("unknown argument: {arg}");
|
||
</span><span class="boring"> std::process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> None => {}
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> if let Err(e) = handle_preprocessing() {
|
||
</span><span class="boring"> eprintln!("{e}");
|
||
</span><span class="boring"> std::process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">struct RemoveEmphasis;
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl Preprocessor for RemoveEmphasis {
|
||
</span><span class="boring"> fn name(&self) -> &str {
|
||
</span><span class="boring"> "remove-emphasis"
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> fn run(&self, _ctx: &PreprocessorContext, mut book: Book) -> Result<Book> {
|
||
</span><span class="boring"> let mut total = 0;
|
||
</span><span class="boring"> book.for_each_chapter_mut(|ch| match remove_emphasis(&mut total, ch) {
|
||
</span><span class="boring"> Ok(s) => ch.content = s,
|
||
</span><span class="boring"> Err(e) => eprintln!("failed to process chapter: {e:?}"),
|
||
</span><span class="boring"> });
|
||
</span><span class="boring"> eprintln!("removed {total} emphasis");
|
||
</span><span class="boring"> Ok(book)
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>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| match e {
|
||
Event::Start(Tag::Emphasis) | Event::Start(Tag::Strong) => {
|
||
*num_removed_items += 1;
|
||
false
|
||
}
|
||
Event::End(TagEnd::Emphasis) | Event::End(TagEnd::Strong) => false,
|
||
_ => true,
|
||
});
|
||
|
||
Ok(pulldown_cmark_to_cmark::cmark(events, &mut buf).map(|_| buf)?)
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">pub fn handle_preprocessing() -> Result<()> {
|
||
</span><span class="boring"> let pre = RemoveEmphasis;
|
||
</span><span class="boring"> let (ctx, book) = mdbook_preprocessor::parse_input(io::stdin())?;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let processed_book = pre.run(&ctx, book)?;
|
||
</span><span class="boring"> serde_json::to_writer(io::stdout(), &processed_book)?;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(())
|
||
</span><span class="boring">}</span></code></pre>
|
||
<p>Take a look at the <a href="https://github.com/rust-lang/mdBook/tree/master/examples/remove-emphasis/">full example source</a> for more details.</p>
|
||
<h2 id="implementing-a-preprocessor-with-a-different-language"><a class="header" href="#implementing-a-preprocessor-with-a-different-language">Implementing a preprocessor with a different language</a></h2>
|
||
<p>The fact that mdBook utilizes stdin and stdout to communicate with the preprocessors makes it easy to implement them in a language other 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 below follows the configuration shown above with <code>preprocessor.foo.command</code> actually pointing to a Python script.</p>
|
||
<pre><code class="language-python">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)
|
||
|
||
# load both the context and the book representations from stdin
|
||
context, book = json.load(sys.stdin)
|
||
# and now, we can just modify the content of the first chapter
|
||
book['items'][0]['Chapter']['content'] = '# Hello'
|
||
# we are done with the book's modification, we can just print it to stdout,
|
||
print(json.dumps(book))
|
||
</code></pre>
|
||
|
||
</main>
|
||
|
||
<nav class="nav-wrapper" aria-label="Page navigation">
|
||
<!-- Mobile navigation buttons -->
|
||
<a rel="prev" href="../for_developers/index.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
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<div style="clear: both"></div>
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</nav>
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</div>
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</div>
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||
<nav class="nav-wide-wrapper" aria-label="Page navigation">
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</a>
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</nav>
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</div>
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<template id=fa-eye><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M288 32c-80.8 0-145.5 36.8-192.6 80.6C48.6 156 17.3 208 2.5 243.7c-3.3 7.9-3.3 16.7 0 24.6C17.3 304 48.6 356 95.4 399.4C142.5 443.2 207.2 480 288 480s145.5-36.8 192.6-80.6c46.8-43.5 78.1-95.4 93-131.1c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C433.5 68.8 368.8 32 288 32zM432 256c0 79.5-64.5 144-144 144s-144-64.5-144-144s64.5-144 144-144s144 64.5 144 144zM288 192c0 35.3-28.7 64-64 64c-11.5 0-22.3-3-31.6-8.4c-.2 2.8-.4 5.5-.4 8.4c0 53 43 96 96 96s96-43 96-96s-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6z"/></svg></span></template>
|
||
<template id=fa-eye-slash><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M38.8 5.1C28.4-3.1 13.3-1.2 5.1 9.2S-1.2 34.7 9.2 42.9l592 464c10.4 8.2 25.5 6.3 33.7-4.1s6.3-25.5-4.1-33.7L525.6 386.7c39.6-40.6 66.4-86.1 79.9-118.4c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C465.5 68.8 400.8 32 320 32c-68.2 0-125 26.3-169.3 60.8L38.8 5.1zM223.1 149.5C248.6 126.2 282.7 112 320 112c79.5 0 144 64.5 144 144c0 24.9-6.3 48.3-17.4 68.7L408 294.5c5.2-11.8 8-24.8 8-38.5c0-53-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6c0 10.2-2.4 19.8-6.6 28.3l-90.3-70.8zm223.1 298L373 389.9c-16.4 6.5-34.3 10.1-53 10.1c-79.5 0-144-64.5-144-144c0-6.9 .5-13.6 1.4-20.2L83.1 161.5C60.3 191.2 44 220.8 34.5 243.7c-3.3 7.9-3.3 16.7 0 24.6c14.9 35.7 46.2 87.7 93 131.1C174.5 443.2 239.2 480 320 480c47.8 0 89.9-12.9 126.2-32.5z"/></svg></span></template>
|
||
<template id=fa-copy><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M502.6 70.63l-61.25-61.25C435.4 3.371 427.2 0 418.7 0H255.1c-35.35 0-64 28.66-64 64l.0195 256C192 355.4 220.7 384 256 384h192c35.2 0 64-28.8 64-64V93.25C512 84.77 508.6 76.63 502.6 70.63zM464 320c0 8.836-7.164 16-16 16H255.1c-8.838 0-16-7.164-16-16L239.1 64.13c0-8.836 7.164-16 16-16h128L384 96c0 17.67 14.33 32 32 32h47.1V320zM272 448c0 8.836-7.164 16-16 16H63.1c-8.838 0-16-7.164-16-16L47.98 192.1c0-8.836 7.164-16 16-16H160V128H63.99c-35.35 0-64 28.65-64 64l.0098 256C.002 483.3 28.66 512 64 512h192c35.2 0 64-28.8 64-64v-32h-47.1L272 448z"/></svg></span></template>
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