Jekyll YAML front matter validator

My blog is built using the excellent static site generator Jekyll. I write my posts in Markdown with a YAML front matter block that tells Jekyll these files should be processed according to the values specified. It works great but has one drawback.

What if you specify invalid values? Jekyll doesn't care so long as the values are of the type expected by the variable. The result might not be would you'd like, but Jekyll will bravely try to build most of what you throw at it and you'll need to visually inspect the site or the source to find errors. As a software engineer, this is no good. I need to fail fast, and if compilation does succeed, the resulting artifacts need to be verified by tests.

Enter this project.

Example

Given the following front matter:

---
categories:
- blog
layout: post
title: Jekyll YAML front matter validator
meta_description:
image: /img/
date: 2019-07-08T12:00:00.0000000+00:00
tags:
- jekyll
- dotnet
- dotnet-script
- docker
---

My validator will report the following errors:

meta_description is empty.
image contains an incomplete path that will not exist on the published site.
For this example, the date was also set in the future and Jekyll would not have generated the post to be published.

Usage

Running through dotnet-script

If dotnet-script is installed on your local machine, download the script and run it thusly:

dotnet script main.csx -- [path_to_root_jekyll_folder]

Where [path_to_root_jekyll_folder] is the path to the root Jekyll folder. The script will look for posts in the _posts subfolder.

Running using Docker

Using Docker, navigate to your root Jekyll folder and run this command in the Terminal:

docker run --rm -it --volume="$PWD:/scripts:ro" hjerpbakk/jekyll-front-matter-analyser

The image can be found on Docker Hub.

Continuous integration

I run the validator as part of my blog's CI pipeline using CircleCI. CircleCI is configured using a .circleci/config.yml:

version: 2
jobs:
  build:
    machine: true
    steps:
      - checkout
      - run: chmod +x ./test.sh
      - run: ./test.sh

Where the test.sh Bash script contains:

#!/usr/bin/env bash
set -e

docker pull hjerpbakk/jekyll-front-matter-analyser
docker run --rm -it --volume="$PWD:/scripts:ro" hjerpbakk/jekyll-front-matter-analyser

Configuration

You can configure the validator by either whitelisting specific files or ignore specific rules for a single file.

Whitelist files

Add an empty text file named .frontmatterignore to your root Jekyll folder.
Add filenames to be ignored during analysis separated by newlines.

The following .frontmatterignore will ignore the posts 2014-1-16-os-x-script-for-fetching-app-store-icons.html and 2018-06-29-beautiful-code-fira-code.md during analysis:

2014-1-16-os-x-script-for-fetching-app-store-icons.html
2018-06-29-beautiful-code-fira-code.md

Ignore specific rules for a single file

To ignore specific rules for a given post, create an ignore list in the front matter of the post:

---
ignore:
- IM0001
- TA0001
---

Where IM0001 and TA0001 are validation errors. See below for a complete list.

Validation rules

The available rules are:

Date

DA0001 "date" is missing
DA0002 "date" is in the future
DA0003 "last_modified_at" is in the future
DA0004 "last_modified_at" in index.html or archives.html is not the same as "last_modified_at" or "date" in the newest post

Description

DE0001 "meta_description" is missing
DE0002 "meta_description" cannot contain: TODO
DE0003 "meta_description" must be between 25 and 160 characters of length

Image

IM0001 "image" is missing
IM0002 "image" does not exist on disk

Layout

LA0001 "layout" must have the value: post

Title

TI0001 "title" is missing
TI0002 "title" cannot contain: TODO
TI0003 "title" is too short, < 30 characters
TI0004 "title" is too long, > 60 characters

Jekyll

JE0001 Path to Jekyll site not specified
JE0002 _posts subfolder must exist
JE0003 _posts subfolder must contain at least one post

hjerpbakk/jekyll-yaml-front-matter-analyser