11ty is a useful static site generator (SSG), and out of the box its easy to set it up with blog posts from your markdown (.md) files.

These by default will create a URL by looking at the blog post title (and 'slugging' it).

If you want to customise your post url in 11ty (eleventy), then you can just add the permalink config to your md files.

Two important notes to adding permalink in eleventy: you have to include a slash at the start and end (otherwise setting permalinks will just try and download a file instead of serving the blog post). And you should include the /post prefix.

So your md file might look like:

---
layout: post
title: Some really long post name that you do not want as a slug
excerpt: Something here
date: 1999-01-01
updatedDate: 1999-01-01
draft: false
permalink: /posts/short-name-here/
tags:
  - post
---
your content here

The important line here to have a custom post url is the permalink: /posts/something/ - and remember the slashes!

Note: remember to redirect your old urls to the new url. If you use netlify, see this guide on setting up redirects