Hugo - Syntax Highlighting

Hugo provides the ability for you to highlight source code in two different ways — either pre-processed server side from your content, or to defer the processing to the client side, using a JavaScript library.

The advantage of server side is that it doesn’t depend on a JavaScript library and consequently works very well when read from an RSS feed.

The advantage of client side is that it doesn’t cost anything when building your site and some of the highlighting scripts available cover more languages than Pygments does.

Server-side

For the pre-processed approach, highlighting is performed by an external Python-based program called Pygments and is triggered via an embedded Hugo shortcode (see example below). If Pygments is absent from the path, it will silently simply pass the content along unhighlighted.

Pygments

If you have never worked with Pygments before, here is a brief primer:

Install Python from python.org. Version 2.7.x is already sufficient.
Run pip install Pygments in order to install Pygments. Once installed, Pygments gives you a command pygmentize. Make sure it sits in your PATH, otherwise Hugo cannot find it.

On Debian and Ubuntu systems, you may also install Pygments by running sudo apt-get install python3-pygments.

Hugo gives you two options that you can set with the variable pygmentsuseclasses (default false) in config.toml (or config.yaml).

Color-codes for highlighting keywords are directly inserted if pygmentsuseclasses = false (default). See in the example below. The color-codes depend on your choice of the pygmentsstyle (default "monokai"). You can explore the different color styles on pygments.org after inserting some example code.
If you choose pygmentsuseclasses = true, Hugo includes class names in your code instead of color-codes. For class-names to be meaningful, you need to include a .css-file in your website representing your color-scheme. You can either generate this .css-files according to this description or download the standard ones from the GitHub pygments-css repository.

Usage

Highlighting is carried out via the in-built shortcode highlight. highlight takes exactly one required parameter of language, and requires a closing shortcode. Note that highlight is not used for client-side javascript highlighting.

Example

{{< highlight html >}}
<section id="main">
  <div>
    <h1 id="title">{{ .Title }}</h1>
    {{ range .Data.Pages }}
      {{ .Render "summary"}}
    {{ end }}
  </div>
</section>
{{< /highlight >}}

Example Output

<span style="color: #f92672">&lt;section</span> <span style="color: #a6e22e">id=</span><span style="color: #e6db74">&quot;main&quot;</span><span style="color: #f92672">&gt;</span>
  <span style="color: #f92672">&lt;div&gt;</span>
    <span style="color: #f92672">&lt;h1</span> <span style="color: #a6e22e">id=</span><span style="color: #e6db74">&quot;title&quot;</span><span style="color: #f92672">&gt;</span>{{ .Title }}<span style="color: #f92672">&lt;/h1&gt;</span>
    {{ range .Data.Pages }}
      {{ .Render &quot;summary&quot;}}
    {{ end }}
  <span style="color: #f92672">&lt;/div&gt;</span>
<span style="color: #f92672">&lt;/section&gt;</span>

Options

Options to control highlighting can be added as a quoted, comma separated key-value list as the second argument in the shortcode. The example below will highlight as language go with inline line numbers, with line number 2 and 3 highlighted.

{{< highlight go "linenos=inline,hl_lines=2 3" >}}
var a string
var b string
var c string
var d string
{{< / highlight >}}

Supported keywords: style, encoding, noclasses, hl_lines, linenos. Note that style and noclasses will override the similar setting in the global config.

The keywords are the same you would using with Pygments from the command line, see the Pygments doc for more info.

Code fences

It is also possible to add syntax highlighting with GitHub flavoured code fences. To enable this, set the PygmentsCodeFences to true in Hugo’s configuration file.

``` html
<section id="main">
  <div>
    <h1 id="title">{{ .Title }}</h1>
    {{ range .Data.Pages }}
      {{ .Render "summary"}}
    {{ end }}
  </div>
</section>
```

Disclaimers

Pygments is relatively slow and causes a performance hit when building your site, but Hugo has been designed to cache the results to disk.
The caching can be turned off by setting the --ignoreCache flag to true.
Languages available depends on your Pygments installation.

Client-side

Alternatively, code highlighting can be done in client-side JavaScript.

Client-side syntax highlighting is very simple to add. You’ll need to pick a library and a corresponding theme. Some popular libraries are:

Highlight.js example

This example uses the popular Highlight.js library, hosted by Yandex, a popular Russian search engine.

In your ./layouts/partials/ (or ./layouts/chrome/) folder, depending on your specific theme, there will be a snippet that will be included in every generated HTML page, such as header.html or header.includes.html. Simply add the css and js to initialize Highlight.js:

<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.6.0/styles/default.min.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.6.0/highlight.min.js"></script>
<script>hljs.initHighlightingOnLoad();</script>

Prism example

Prism is another popular highlighter library, used on some major sites. Similar to Highlight.js, you simply load prism.css in your <head> via whatever Hugo partial template is creating that part of your pages, like so:

...
<link href="/css/prism.css" rel="stylesheet" />
...

… and add prism.js near the bottom of your <body> tag, again in whatever Hugo partial template is appropriate for your site or theme.

...
<script src="/js/prism.js"></script>
...
</body>

In this example, the local paths indicate that your own copy of these files are being added to the site, typically under ./static/.

Using Client-side highlighting

To use client-side highlighting, most of these javascript libraries expect your code to be wrapped in semantically correct <code> tags, with the language expressed in a class attribute on the <code> tag, such as class="language-abc", where the abc is the code the highlighter script uses to represent that language.

The script would be looking for classes like language-go, language-html, or language-css. If you look at the page’s source, it would be marked up like so:

<pre>
<code class="language-css">
body {
  font-family: "Noto Sans", sans-serif;
}
</code>
</pre>

The markup in your content pages (e.g. my-css-tutorial.md) needs to look like the following, with the name of the language to be highlighted entered directly after the first “fence”, in a fenced code block:

~~~css
body {
  font-family: "Noto Sans", sans-serif;
}
~~~

When passed through the highlighter script, it would yield something like this output when viewed on your rendered page:

body {
  font-family: "Noto Sans", sans-serif;
}

Please see individual libraries’ documentation for how to implement each of the JavaScript-based libraries.