In this directory I’ll store all the configuration files that can be used to add or remove some feature from a system. Every module will add or remove one feature. So the gaming module will only enable gaming.
The most important module is the common/ module, as this module has all the settings that each of the systems is going to use. This module will also be imported by every system. This is the only module that cannot be excluded from the loading process.
The modules directory is where all the modules get stored. Each module is supposed to have a setting named modules.moduleName that can be either true, or false. One important note is that the module must have the same name as it’s setting to prevent confusion. So the setting modules.moduleName implies there is a file called default.nix at /modules/moduleName/.
Which brings us to our next point, how are those modules being loaded and ignored?
NixOS/
├── modules/
│ ├── moduleName/
│ │ ├── ...
│ │ └── default.nix
│ │ ...
│ └── default.nix
│ ...
This is why every system at /systems/category/machine/ imports /modules/ the file that gets loaded is /modules/default.nix. Which in turn takes the machine-settings argument, and loads the modules as needed. The /modules/default.nix loads the /modules/moduleName/default.nix file if the settings match. Do not load modules individually.
There are modules that are system level, meaning that if enabled, they’re enabled for the entire system. You can read more about it here.
The same goes for user level modules. If enabled, they’re enabled for the only this user. This does not mean that it won’t effect the system. A good example is the docker module. This will be enabled on the system, however only users who have it enabled have access. You can read more about it here.