Q:

Change site.url to localhost during jekyll local development

My jekyll blog template have links to resources and pages like so:
{{ site.url }}/my-page.html

This works well in deployment, but when I run jekyll serve in development, all of the links point to the live page instead of the development page.
my-site-url/my-page.html

# But I want this in development
localhost:4000/my-page.html

Is there a way to make jekyll use a different {{ site.url }} in development?

A:

This is a common problem between different Jekyll environments.
Some explanations
We need to understand site.url and site.baseurl and in which situation we need them.
Those variables don't serve the same purpose.
site.url
By default, this variable is only used in page head for the canonical header and the RSS link. It's also used in the xml feed to point to site resources as the software that will manage this feed doesn't know resource's urls.
This variable is only necessary for external systems.
site.baseurl
This variable indicates the root folder of your Jekyll site. By default it is set to "" (empty string). That means that your Jekyll site is at the root of http://example.com.
If your Jekyll site lives in http://example.com/blog, you have to set site.baseurl to /blog (note the slash). This will allow assets (css, js) to load correctly.
See how assets are loaded in you head :
<link rel="stylesheet" href="{{ "/css/main.css" | prepend: site.baseurl }}">

that can also be :
<link rel="stylesheet" href="{{ site.baseurl }}/css/main.css">

Working in different environments
Now you have to test your site locally and to deploy it in production. Sometimes, the baseurl is different and the jekyll build may not work out of the box in one of those environment.
Here we have two solutions :
Use jekyll serve
Let's imagine that your site lives in a github repository and is served at https://username.github.io/myProject.
You can setup the baseurl to /myProject. and test your site locally with jekyll serve, your site will be served at http://127.0.0.1:4000/myProject/
Use multiple configuration files
If, for one reason or another, you cannot use jekyll serve, you can set a configuration file for both environment and jekyll build depending on where you are deploying.
Let's say we have the local site served at http://localhost and the production site served at https://username.github.io/myProject.
We leave the _config.yml with url: https://username.github.io and baseurl: /myProject
We create a new _config_dev.yml with only url: https://localhost and baseurl: ""
Now to test locally :
jekyll build --config _config.yml,_config_dev.yml

or
jekyll build --config _config.yml,_config_dev.yml --watch

When pushed on production, the jekyll build command will use the default _config.yml.

