# NixOS Configurations

This note covers `21-nixos-configurations/`, which defines a full `nixosConfigurations` output.

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## 1. Module Versus Configuration

`04-nixos-module/` shows a reusable module. This example shows the next layer up: a full system definition built with `nixpkgs.lib.nixosSystem`.

The local `module.nix` still matters, but now it is only one input into a named machine configuration.

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## 2. What the Flake Exposes

The key output is:

- `nixosConfigurations.demo`

That is the shape other commands expect when you build, inspect, or deploy a NixOS system from a flake.

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## 3. Why the Example Sets Boot and File System Options

Even a small NixOS configuration needs a few system-level settings to evaluate cleanly as a full configuration.

This example sets:

- a host name,
- a root file system,
- a boot loader device, and
- `system.stateVersion`.

Those settings are minimal, and they keep the example focused on flake structure rather than real machine setup.

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## 4. What the Check Verifies

The check reads values from the evaluated configuration and compares them with the expected host name and `/etc` file contents.

That proves the configuration merges correctly without turning the example into a deployment workflow.

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## 5. Commands to Try

```bash
cd 21-nixos-configurations

nix flake show
nix eval .#nixosConfigurations.demo.config.networking.hostName --raw
nix flake check
```