frontmatterkit

A Unix-style CLI for validating, querying, asserting, and minimally updating front matter in Markdown files.

What is frontmatterkit?

frontmatterkit is a command-line tool purpose-built for working with YAML front matter in Markdown files. Whether you’re enforcing content standards in CI, scripting bulk edits across a documentation site, or just querying metadata from a blog post, frontmatterkit gives you precise, composable commands that fit naturally into Unix pipelines.

Key Features

  • Validate — Verify that YAML front matter is well-formed, catching syntax errors before they break your build.
  • Get — Extract front matter values by path, with output in YAML or JSON for easy downstream processing.
  • Set — Add or update front matter fields while preserving the rest of your document exactly as-is.
  • Unset — Remove fields cleanly without touching anything else.
  • Assert — Test conditions on front matter fields with expressive operators — ideal for CI/CD gates and pre-commit hooks.

Why frontmatterkit?

  • Strict validation — Malformed YAML is caught immediately with clear exit codes, not silently ignored.
  • Minimal changes — The set and unset commands modify only what you ask for; the rest of the file is left byte-for-byte identical.
  • Unix-friendly — Commands read from --in or stdin, write to --out or stdout, and use meaningful exit codes. Compose them with grep, jq, xargs, or anything else in your toolkit.
  • CI-ready assertions — Replace fragile grep checks with structured assertions like .draft == false and .tags contains "go".

Quick Start 

# Install
go install github.com/UnitVectorY-Labs/frontmatterkit@latest

# Validate a Markdown file
frontmatterkit validate --in post.md

# Get the title
frontmatterkit get --path .title --in post.md

# Ensure the post is not a draft and has a title
frontmatterkit assert --assert '.draft == false' --assert '.title exists' --in post.md

# Show command-specific help
frontmatterkit set help