Modularize Your NixOS Configuration
At this point, the skeleton of the entire system is configured. The current configuration structure in /etc/nixos should be as follows:
$ tree
.
├── flake.lock
├── flake.nix
├── home.nix
└── configuration.nix$ tree
.
├── flake.lock
├── flake.nix
├── home.nix
└── configuration.nixThe functions of these four files are:
flake.lock: An automatically generated version-lock file that records all input sources, hash values, and version numbers of the entire flake to ensure reproducibility.flake.nix: The entry file that will be recognized and deployed when executingsudo nixos-rebuild switch. See Flakes - NixOS Wiki for all options of flake.nix.configuration.nix: Imported as a Nix module in flake.nix, all system-level configuration is currently written here. See Configuration - NixOS Manual for all options of configuration.nix.home.nix: Imported by Home-Manager as the configuration of the userryanin flake.nix, containing all ofryan's configuration and managingryan's home folder. See Appendix A. Configuration Options - Home-Manager for all options of home.nix.
By modifying these files, you can declaratively change the system and home directory status.
However, as the configuration grows, relying solely on configuration.nix and home.nix can lead to bloated and difficult-to-maintain files. A better solution is to use the Nix module system to split the configuration into multiple Nix modules and write them in a classified manner.
The Nix module system provides a parameter, imports, which accepts a list of .nix files and merges all the configuration defined in these files into the current Nix module. Note that imports will not simply overwrite duplicate configuration but handle it more reasonably. For example, if program.packages = [...] is defined in multiple modules, then imports will merge all program.packages defined in all Nix modules into one list. Attribute sets can also be merged correctly. The specific behavior can be explored by yourself.
I only found a description of
importsin Nixpkgs-Unstable Official Manual - evalModules Parameters:A list of modules. These are merged together to form the final configuration.It's a bit ambiguous...
With the help of imports, we can split home.nix and configuration.nix into multiple Nix modules defined in different .nix files.
Here is a nice starter example of modularizing the configuration, Highly recommended:
A more complicated example, ryan4yin/nix-config/i3-kickstarter is the configuration of my previous NixOS system with the i3 window manager. Its structure is as follows:
├── flake.lock
├── flake.nix
├── home
│ ├── default.nix # here we import all submodules by imports = [...]
│ ├── fcitx5 # fcitx5 input method's configuration
│ │ ├── default.nix
│ │ └── rime-data-flypy
│ ├── i3 # i3 window manager's configuration
│ │ ├── config
│ │ ├── default.nix
│ │ ├── i3blocks.conf
│ │ ├── keybindings
│ │ └── scripts
│ ├── programs
│ │ ├── browsers.nix
│ │ ├── common.nix
│ │ ├── default.nix # here we import all modules in programs folder by imports = [...]
│ │ ├── git.nix
│ │ ├── media.nix
│ │ ├── vscode.nix
│ │ └── xdg.nix
│ ├── rofi # rofi launcher's configuration
│ │ ├── configs
│ │ │ ├── arc_dark_colors.rasi
│ │ │ ├── arc_dark_transparent_colors.rasi
│ │ │ ├── power-profiles.rasi
│ │ │ ├── powermenu.rasi
│ │ │ ├── rofidmenu.rasi
│ │ │ └── rofikeyhint.rasi
│ │ └── default.nix
│ └── shell # shell/terminal related configuration
│ ├── common.nix
│ ├── default.nix
│ ├── nushell
│ │ ├── config.nu
│ │ ├── default.nix
│ │ └── env.nu
│ ├── starship.nix
│ └── terminals.nix
├── hosts
│ ├── msi-rtx4090 # My main machine's configuration
│ │ ├── default.nix # This is the old configuration.nix, but most of the content has been split out to modules.
│ │ └── hardware-configuration.nix # hardware & disk related configuration, autogenerated by nixos
│ └── nixos-test # my test machine's configuration
│ ├── default.nix
│ └── hardware-configuration.nix
├── modules # some common NixOS modules that can be reused
│ ├── i3.nix
│ └── system.nix
└── wallpaper.jpg # wallpaper├── flake.lock
├── flake.nix
├── home
│ ├── default.nix # here we import all submodules by imports = [...]
│ ├── fcitx5 # fcitx5 input method's configuration
│ │ ├── default.nix
│ │ └── rime-data-flypy
│ ├── i3 # i3 window manager's configuration
│ │ ├── config
│ │ ├── default.nix
│ │ ├── i3blocks.conf
│ │ ├── keybindings
│ │ └── scripts
│ ├── programs
│ │ ├── browsers.nix
│ │ ├── common.nix
│ │ ├── default.nix # here we import all modules in programs folder by imports = [...]
│ │ ├── git.nix
│ │ ├── media.nix
│ │ ├── vscode.nix
│ │ └── xdg.nix
│ ├── rofi # rofi launcher's configuration
│ │ ├── configs
│ │ │ ├── arc_dark_colors.rasi
│ │ │ ├── arc_dark_transparent_colors.rasi
│ │ │ ├── power-profiles.rasi
│ │ │ ├── powermenu.rasi
│ │ │ ├── rofidmenu.rasi
│ │ │ └── rofikeyhint.rasi
│ │ └── default.nix
│ └── shell # shell/terminal related configuration
│ ├── common.nix
│ ├── default.nix
│ ├── nushell
│ │ ├── config.nu
│ │ ├── default.nix
│ │ └── env.nu
│ ├── starship.nix
│ └── terminals.nix
├── hosts
│ ├── msi-rtx4090 # My main machine's configuration
│ │ ├── default.nix # This is the old configuration.nix, but most of the content has been split out to modules.
│ │ └── hardware-configuration.nix # hardware & disk related configuration, autogenerated by nixos
│ └── nixos-test # my test machine's configuration
│ ├── default.nix
│ └── hardware-configuration.nix
├── modules # some common NixOS modules that can be reused
│ ├── i3.nix
│ └── system.nix
└── wallpaper.jpg # wallpaperThere is no need to follow the above structure, you can organize your configuration in any way you like. The key is to use imports to import all the submodules into the main module.
lib.mkOverride, lib.mkDefault, and lib.mkForce
In Nix, some people use lib.mkDefault and lib.mkForce to define values. These functions are designed to set default values or force values of options.
You can explore the source code of lib.mkDefault and lib.mkForce by running nix repl -f '<nixpkgs>' and then entering :e lib.mkDefault. To learn more about nix repl, type :? for the help information.
Here's the source code:
# ......
mkOverride = priority: content:
{ _type = "override";
inherit priority content;
};
mkOptionDefault = mkOverride 1500; # priority of option defaults
mkDefault = mkOverride 1000; # used in config sections of non-user modules to set a default
mkImageMediaOverride = mkOverride 60; # image media profiles can be derived by inclusion into host config, hence needing to override host config, but do allow user to mkForce
mkForce = mkOverride 50;
mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’
# ...... # ......
mkOverride = priority: content:
{ _type = "override";
inherit priority content;
};
mkOptionDefault = mkOverride 1500; # priority of option defaults
mkDefault = mkOverride 1000; # used in config sections of non-user modules to set a default
mkImageMediaOverride = mkOverride 60; # image media profiles can be derived by inclusion into host config, hence needing to override host config, but do allow user to mkForce
mkForce = mkOverride 50;
mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’
# ......In summary, lib.mkDefault is used to set default values of options with a priority of 1000 internally, and lib.mkForce is used to force values of options with a priority of 50 internally. If you set a value of an option directly, it will be set with a default priority of 1000, the same as lib.mkDefault.
The lower the priority value, the higher the actual priority. As a result, lib.mkForce has a higher priority than lib.mkDefault. If you define multiple values with the same priority, Nix will throw an error.
Using these functions can be very helpful for modularizing the configuration. You can set default values in a low-level module (base module) and force values in a high-level module.
For example, in my configuration at ryan4yin/nix-config/blob/c515ea9/modules/nixos/core-server.nix, I define default values like this:
{ lib, pkgs, ... }:
{
# ......
nixpkgs.config.allowUnfree = lib.mkDefault false;
# ......
}{ lib, pkgs, ... }:
{
# ......
nixpkgs.config.allowUnfree = lib.mkDefault false;
# ......
}Then, for my desktop machine, I override the value in ryan4yin/nix-config/blob/c515ea9/modules/nixos/core-desktop.nix like this:
{ lib, pkgs, ... }:
{
# import the base module
imports = [
./core-server.nix
];
# override the default value defined in the base module
nixpkgs.config.allowUnfree = lib.mkForce true;
# ......
}{ lib, pkgs, ... }:
{
# import the base module
imports = [
./core-server.nix
];
# override the default value defined in the base module
nixpkgs.config.allowUnfree = lib.mkForce true;
# ......
}lib.mkOrder, lib.mkBefore, and lib.mkAfter
In addition to lib.mkDefault and lib.mkForce, there are also lib.mkBefore and lib.mkAfter, which are used to set the merge order of list-type options. These functions further contribute to the modularization of the configuration.
As mentioned earlier, when you define multiple values with the same override priority, Nix will throw an error. However, by using lib.mkOrder, lib.mkBefore, or lib.mkAfter, you can define multiple values with the same override priority, and they will be merged in the order you specify.
To examine the source code of lib.mkBefore, you can run nix repl -f '<nixpkgs>' and then enter :e lib.mkBefore. To learn more about nix repl, type :? for the help information:
# ......
mkOrder = priority: content:
{ _type = "order";
inherit priority content;
};
mkBefore = mkOrder 500;
mkAfter = mkOrder 1500;
# The default priority for things that don't have a priority specified.
defaultPriority = 100;
# ...... # ......
mkOrder = priority: content:
{ _type = "order";
inherit priority content;
};
mkBefore = mkOrder 500;
mkAfter = mkOrder 1500;
# The default priority for things that don't have a priority specified.
defaultPriority = 100;
# ......Therefore, lib.mkBefore is a shorthand for lib.mkOrder 500, and lib.mkAfter is a shorthand for lib.mkOrder 1500.
To test the usage of lib.mkBefore and lib.mkAfter, let's create a simple Flake project:
# Create flake.nix with the following content
› cat <<EOF | sudo tee flake.nix
{
description = "Ryan's NixOS Flake";
inputs = {
nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05";
};
outputs = { self, nixpkgs, ... }@inputs: {
nixosConfigurations = {
"nixos-test" = nixpkgs.lib.nixosSystem {
system = "x86_64-linux";
modules = [
# Demo module 1: insert 'git' at the head of the list
({lib, pkgs, ...}: {
environment.systemPackages = lib.mkBefore [pkgs.git];
})
# Create flake.nix with the following content
› cat <<EOF | sudo tee flake.nix
{
description = "Ryan's NixOS Flake";
inputs = {
nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05";
};
outputs = { self, nixpkgs, ... }@inputs: {
nixosConfigurations = {
"nixos-test" = nixpkgs.lib.nixosSystem {
system = "x86_64-linux";
modules = [
# Demo module 1: insert 'git' at the head of the list
({lib, pkgs, ...}: {
environment.systemPackages = lib.mkBefore [pkgs.git];
})
As you can see, the order of systemPackages is git -> curl -> default packages -> vim, which matches the order we defined in flake.nix.
Although adjusting the order of
systemPackagesmay not be useful in practice, it can be helpful in other scenarios.