Examples

Table of Contents

• Validate
  • Reject malformed front matter
• Get
  • Extract an array as JSON for another tool
• Set
  • Update front matter from stdin and emit the rewritten document
• Unset
  • Remove a publishing flag and write a clean output file
• Assert
  • Fail fast when required front matter is missing

---

These examples are intentionally few and complete. Each one is backed by a data-driven CLI case under testdata/, and each one shows the Markdown input, the generated output, or both.

For the full flag reference, see the Usage page.

Validate

Reject malformed front matter

Backed by testdata/validate/invalid-yaml. Use this example when you want to see what a real validation failure looks like.

frontmatterkit validate --in invalid-yaml.md

Input Markdown

---
title: [broken
---
Body content.

Error Output

Error: validation failed: invalid YAML in front matter: yaml: line 1: did not find expected ',' or ']'

This command exits with code 1.

Get

Extract an array as JSON for another tool

Backed by testdata/get/json-tags. This is a good example of using get as a structured data extractor instead of just printing YAML back out.

frontmatterkit get --format json --path .tags --in json-tags.md

Input Markdown

---
title: Hello World
draft: false
count: 42
tags:
  - go
  - yaml
author:
  name: Test User
---
This is the body content.

Output

[
  "go",
  "yaml"
]

Set

Update front matter from stdin and emit the rewritten document

Backed by testdata/set/stdin-set. This shows the Unix-friendly stdin workflow while still giving a complete before-and-after example.

cat stdin-set.md | frontmatterkit set --set '.title=Stdin Title'

Input Markdown

---
title: Hello World
draft: false
count: 42
tags:
  - go
  - yaml
author:
  name: Test User
---
This is the body content.

Output Markdown

---
title: Stdin Title
draft: false
count: 42
tags:
  - go
  - yaml
author:
  name: Test User
---
This is the body content.

Unset

Remove a publishing flag and write a clean output file

Backed by testdata/unset/out-file. This is the useful unset workflow: remove one field, preserve everything else, and write the result to a separate file.

frontmatterkit unset --path .draft --in out-file.md --out out-file.out.md

Input Markdown

---
title: Hello World
draft: false
count: 42
tags:
  - go
  - yaml
author:
  name: Test User
---
This is the body content.

Output Markdown

---
title: Hello World
count: 42
tags:
  - go
  - yaml
author:
  name: Test User
---
This is the body content.

Assert

Fail fast when required front matter is missing

Backed by testdata/assert/exists-fail. This is the most useful assert pattern for CI: enforce a required field and stop the pipeline with a clear failure.

frontmatterkit assert --assert '.missing exists' --in exists-fail.md

Input Markdown

---
title: Hello World
draft: false
count: 42
tags:
  - go
  - yaml
author:
  name: Test User
---
This is the body content.

Error Output

Error: assertion ".missing exists" failed: assertion failed: .missing does not exist

This command exits with code 1.