Runar Ovesen Hjerpbakk

Software Philosopher

Jekyll YAML front matter validator

This website 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 my Jekyll YAML front matter validator.


Given the following front matter:

- blog
layout: post
title: Jekyll YAML front matter validator
image: /img/
date: 2019-07-08T12:00:00.0000000+00:00
- 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.

The validator is written in C# using dotnet-script and you can view the source on GitHub.


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" \

The image can be found on Docker Hub.

Continuous integration

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

version: 2
    machine: true
      - checkout
      - run: chmod +x ./
      - run: ./

Where the Bash script contains:

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

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


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

Whitelist files

  1. Add an empty text file named .frontmatterignore to your root Jekyll folder.
  2. 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 during analysis:


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:

- IM0001
- TA0001

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

Validation rules

The available rules are:


  • CA0001 “categories” must contain the value: blog
  • CA0002 When “categories” contains link, a link with an URL must exist in the front matter


  • 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


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


  • IM0001 “image” is missing
  • IM0002 “image” does not exist on disk


  • LA0001 “layout” must have the value: post


  • TA0001 Post must have at least one tag
  • TA0002 Could not find the tag in the subfolder _my_tags


  • TI0001 “title” is missing
  • TI0002 “title” cannot contain: TODO