Where does Apache read $domain variable from? - apache

Question

In my operational Wordpress enabled Ubuntu instance, I have a 000-default.conf file with the settings:
./apache2/sites-available/000-default.conf: ServerName $domain
./apache2/sites-available/000-default.conf: ServerAlias www.$domain
grepping for \$domain inside etc gives me basically only this file.
Where does Apache get this value when running my site?
full 000-default.conf:
UseCanonicalName On
<VirtualHost *:80>
ServerAdmin webmaster#localhost
ServerName $domain
ServerAlias www.$domain
DocumentRoot /var/www/html
<Directory /var/www/html/>
Options FollowSymLinks
AllowOverride All
Require all granted
</Directory>
<FilesMatch \.php$>
# 2.4.10+ can proxy to unix socket
SetHandler "proxy:unix:/var/run/php/php7.4-fpm.sock|fcgi://localhost"
</FilesMatch>
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>

Answer 1

$domain is not a pre-defined variable in Apache HTTPD. The list of variables in the documentation does not have it.
As Environment variables are accessed using a %{VAR}-notation in Apache, so it is not read from the environment either.
It's also not in my 000-default.conf - which means it must be something specific to your setup.
And indeed it is! Wordpress includes that file in sites-available with $domain as a placeholder. During setup, the file gets copied into sites-enabled and the placeholder gets replaced. Look in sites-enabled/000-default.conf and instead of $domain, there should be your domain.