yq can process files with yaml front matter (e.g. jekyll, assemble and others) - this is done via the --front-matter/-f flag.

Process front matter

Use --front-matter=process to process the front matter, that is run the expression against the yaml content, and output back the entire file, included the non-yaml content block. For example:

File:

---
a: apple
b: bannana
---
<h1>I like {{a}} and {{b}} </h1>

The running

yq e --front-matter=process '.a="chocolate"' file.jekyll

Will yield:

---
a: chocolate
b: bannana
---
<h1>I like {{a}} and {{b}} </h1>

Extract front matter

Running with --front-matter=extract will only output the yaml contents and ignore the rest. From the previous example, if you were to instead run:

yq e --front-matter=extract '.a="chocolate"' file.jekyll

Then this would yield:

---
a: chocolate
b: bannana

​