flake.nix Configuration Explained

Above, we created a flake.nix file to manage system configurations, but you might still be unclear about its structure. Let's explain the content of this file in detail.

1. Flake Inputs

First, let's look at the inputs attribute. It is an attribute set that defines all the dependencies of this flake. These dependencies will be passed as arguments to the outputs function after they are fetched:

{
  inputs = {
    # NixOS official package source, using the nixos-23.11 branch here
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
  };

  outputs = { self, nixpkgs, ... }@inputs: {
    # Omitting previous configurations......
  };
}

Dependencies in inputs has many types and definitions. It can be another flake, a regular Git repository, or a local path. The section Other Usage of Flakes - Flake Inputs describes common types of dependencies and their definitions in detail.

Here we only define a dependency named nixpkgs, which is the most common way to reference in a flake, i.e., github:owner/name/reference. The reference here can be a branch name, commit-id, or tag.

After nixpkgs is defined in inputs, you can use it in the parameters of the subsequent outputs function, which is exactly what our example does.

2. Flake Outputs

Now let's look at outputs. It is a function that takes the dependencies from inputs as its parameters, and its return value is an attribute set, which represents the build results of the flake:

{
  description = "A simple NixOS flake";

  inputs = {
    # NixOS official package source, here using the nixos-23.11 branch
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
  };

  outputs = { self, nixpkgs, ... }@inputs: {
    # The host with the hostname `my-nixos` will use this configuration
    nixosConfigurations.my-nixos = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ./configuration.nix
      ];
    };
  };
}

Flakes can have various purposes and can have different types of outputs. The section Flake Outputs provides a more detailed introduction. Here, we are only using the nixosConfigurations type of outputs, which is used to configure NixOS systems.

When we run the sudo nixos-rebuild switch command, it looks for the nixosConfigurations.my-nixos attribute (where my-nixos will be the hostname of your current system) in the attribute set returned by the outputs function of /etc/nixos/flake.nix and uses the definition there to configure your NixOS system.

Actually, we can also customize the location of the flake and the name of the NixOS configuration instead of using the defaults. This can be done by adding the --flake parameter to the nixos-rebuild command. Here's an example:

sudo nixos-rebuild switch --flake /path/to/your/flake#your-hostname

A brief explanation of the --flake /path/to/your/flake#your-hostname parameter:

1. /path/to/your/flake is the location of the target flake. The default path is /etc/nixos/.
2. # is a separator, and your-hostname is the name of the NixOS configuration. nixos-rebuild will default to using the hostname of your current system as the configuration name to look for.

You can even directly reference a remote GitHub repository as your flake source, for example:

sudo nixos-rebuild switch --flake github:owner/repo#your-hostname

3. The Special Parameter self of the outputs Function

Although we have not mentioned it before, all the example code in the previous sections has one more special parameter in the outputs function, and we will briefly introduce its purpose here.

The description of it in the nix flake - Nix Manual is:

The special input named self refers to the outputs and source tree of this flake.

This means that self is the return value of the current flake's outputs function and also the path to the current flake's source code folder (source tree).

We are not using the self parameter here, but in some more complex examples (or configurations you may find online) later, you will see the usage of self.

Note: You might come across some code where people use self.outputs to reference the outputs of the current flake, which is indeed possible. However, the Nix Manual does not provide any explanation for this, and it is considered an internal implementation detail of flakes. It is not recommended to use this in your own code!

4. Simple Introduction to nixpkgs.lib.nixosSystem Function

A Flake can depend on other Flakes to utilize the features they provide.

By default, a flake searches for a flake.nix file in the root directory of each of its dependencies (i.e., each item in inputs) and lazily evaluates their outputs functions. It then passes the attribute set returned by these functions as arguments to its own outputs function, enabling us to use the features provided by the other flakes within our current flake.

More precisely, the evaluation of the outputs function for each dependency is lazy. This means that a flake's outputs function is only evaluated when it is actually used, thereby avoiding unnecessary calculations and improving efficiency.

The description above may be a bit confusing, so let's take a look at the process with the flake.nix example used in this section. Our flake.nix declares the inputs.nixpkgs dependency, so that nixpkgs/flake.nix will be evaluated when we run the sudo nixos-rebuild switch command.

From the source code of the Nixpkgs repository, we can see that its flake outputs definition includes the lib attribute, and in our example, we use the lib attribute's nixosSystem function to configure our NixOS system:

{
  inputs = {
    # NixOS official package source, here using the nixos-23.11 branch
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
  };

  outputs = { self, nixpkgs, ... }@inputs: {
    nixosConfigurations.my-nixos = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ./configuration.nix
      ];
    };
  };
}

The attribute set following nixpkgs.lib.nixosSystem is the function's parameter. We have only set two parameters here:

1. system: This is straightforward, it's the system architecture parameter.
2. modules: This is a list of modules, where the actual NixOS system configuration is defined. The /etc/nixos/configuration.nix configuration file itself is a Nixpkgs Module, so it can be directly added to the modules list for use.

Understanding these basics is sufficient for beginners. Exploring the nixpkgs.lib.nixosSystem function in detail requires a grasp of the Nixpkgs module system. Readers who have completed the Modularizing NixOS Configuration section can return to nixpkgs/flake.nix to find the definition of nixpkgs.lib.nixosSystem, trace its source code, and study its implementation.