Using front matter is one way that you can specify configuration in the pages and posts for your site. Setting things like a default layout, or customizing the title, or specifying a more precise date/time for the post can all be added to your page or post front matter.

Often times, you will find that you are repeating a lot of configuration options. Setting the same layout in each file, adding the same category to a post, etc. You can even add custom variables like author names, which might be the same for the majority of posts on your blog.

There are two ways to accomplish this: the data cascade, and via your site’s configuration file.

The Data Cascade

New in Bridgetown 0.17, you can add _defaults.yml (also .yaml or .json) files anywhere in your source tree, which will then cause a “data cascade”. In other words, any pages/documents in that folder or in a subfolder will use the front matter data contained in the defaults file. Defaults files in subfolders can also potentially overwrite values contained in parent folders (hence the term “cascade”).

For example, if you want all posts to have the layout “post” without having to write layout: post in each post’s front matter, simply add _defaults.yml to the src/_posts folder:

layout: post

Now, if you have some posts in a subfolder (let’s say fancy_posts) and you want those posts to use the “fancy_post” layout, you could add a second _defaults.yml file in that subfolder like so:

layout: fancy_post

Now all the fancy_posts posts will use the fancy_post layout. If you had other front matter variables in the parent _defaults.yml in src/_posts, those would carry over to the fancy_posts defaults unless you decide to override them explicitly.

Also, keep in mind these are “default” values, so if you were to add layout: some_other_layout to a post, it would overwrite either layout: post or layout: fancy_post. This is what makes front matter defaults so powerful!

Configuration-based Front Matter Defaults

Instead of (or in addition to) the data cascade, you can set front matter defaults in your configuration file using a special rules-based syntax.

To do this, add a defaults key to the bridgetown.config.yml file in your project’s root folder. The defaults key holds an array of scope/values pairs that define what defaults should be set for a particular file path, and optionally, a file type in that path.

Let’s say that you want to add a default layout to all pages and posts in your site. You would add this to your bridgetown.config.yml file:

defaults:
  -
    scope:
      path: "" # an empty string here means all files in the project
    values:
      layout: "default"

Here, we are scoping the values to any file that exists in the path scope. Since the path is set as an empty string, it will apply to all files in your project. You probably don’t want to set a layout on every file in your project - like css files, for example - so you can also specify a type value under the scope key.

defaults:
  -
    scope:
      path: "" # an empty string here means all files in the project
      type: "posts"
    values:
      layout: "default"

Now, this will only set the layout for files where the type is posts. The different types that are available to you are pages, posts, drafts or any collection in your site. While type is optional, you must specify a value for path when creating a scope/values pair.

As mentioned earlier, you can set multiple scope/values pairs for defaults.

defaults:
  -
    scope:
      path: ""
      type: "pages"
    values:
      layout: "my-site"
  -
    scope:
      path: "projects"
      type: "pages"
    values:
      layout: "project" # overrides previous default layout
      author: "Ursula K. Le Guin"

With these defaults, all pages would use the my-site layout. Any html files that exist in the projects/ folder will use the project layout, if it exists. Those files will also have the page.author liquid variable set to Ursula K. Le Guin.

collections:
  my_collection:
    output: true

defaults:
  -
    scope:
      path: ""
      type: "my_collection" # a collection in your site, in plural form
    values:
      layout: "default"

In this example, the layout is set to default inside the collection with the name my_collection.

Glob patterns in Front Matter defaults

It is also possible to use glob patterns (currently limited to patterns that contain *) when matching defaults. For example, it is possible to set specific layout for each special-page.html in any subfolder of section folder.

collections:
  my_collection:
    output: true

defaults:
  -
    scope:
      path: "section/*/special-page.html"
    values:
      layout: "specific-layout"

Precedence

Bridgetown will apply all of the configuration settings you specify in the defaults section of your bridgetown.config.yml file. You can choose to override settings from other scope/values pair by specifying a more specific path for the scope.

You can see that in the second to last example above. First, we set the default page layout to my-site. Then, using a more specific path, we set the default layout for pages in the projects/ path to project. This can be done with any value that you would set in the page or post front matter.

Finally, if you set defaults in the site configuration by adding a defaults section to your bridgetown.config.yml file, you can override those settings in a post or page file. All you need to do is specify the settings in the post or page front matter. For example:

# In bridgetown.config.yml
...
defaults:
  -
    scope:
      path: "projects"
      type: "pages"
    values:
      layout: "project"
      author: "Ursula K. Le Guin"
      category: "project"
...

# In projects/foo_project.md
---
author: "John Smith"
layout: "foobar"
---
The post text goes here...

The projects/foo_project.md would have the layout set to foobar instead of project and the author set to John Smith instead of Ursula K. Le Guin when the site is built.