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https://github.com/ryan4yin/nixos-and-flakes-book
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249 lines
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11 KiB
Markdown
249 lines
No EOL
11 KiB
Markdown
# Modularize Your NixOS Configuration
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At this point, the skeleton of the entire system is configured. The current configuration structure in `/etc/nixos` should be as follows:
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```
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$ tree
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.
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├── flake.lock
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├── flake.nix
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├── home.nix
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└── configuration.nix
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```
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The functions of these four files are:
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- `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.
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- `flake.nix`: The entry file that will be recognized and deployed when executing `sudo nixos-rebuild switch`. See [Flakes - NixOS Wiki](https://nixos.wiki/wiki/Flakes) for all options of flake.nix.
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- `configuration.nix`: Imported as a Nix module in flake.nix, all system-level configuration is currently written here. See [Configuration - NixOS Manual](https://nixos.org/manual/nixos/unstable/index.html#ch-configuration) for all options of configuration.nix.
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- `home.nix`: Imported by Home-Manager as the configuration of the user `ryan` in flake.nix, containing all of `ryan`'s configuration and managing `ryan`'s home folder. See [Appendix A. Configuration Options - Home-Manager](https://nix-community.github.io/home-manager/options.html) for all options of home.nix.
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By modifying these files, you can declaratively change the system and home directory status.
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As the configuration increases, it becomes difficult to maintain the configuration by relying solely on `configuration.nix` and `home.nix`. Therefore, a better solution is to use the Nix module system to split the configuration into multiple modules and write them in a classified manner.
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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.
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> I only found a description of `imports` in [Nixpkgs-Unstable Official Manual - evalModules Parameters](https://nixos.org/manual/nixpkgs/unstable/#module-system-lib-evalModules-parameters): `A list of modules. These are merged together to form the final configuration.` It's a bit ambiguous...
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With the help of `imports`, we can split `home.nix` and `configuration.nix` into multiple Nix modules defined in different `.nix` files.
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For example, [ryan4yin/nix-config/v0.0.2](https://github.com/ryan4yin/nix-config/tree/v0.0.2) is the configuration of my previous NixOS system with the i3 window manager. Its structure is as follows:
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```shell
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├── flake.lock
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├── flake.nix
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├── home
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│ ├── default.nix # here we import all submodules by imports = [...]
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│ ├── fcitx5 # fcitx5 input method's configuration
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│ │ ├── default.nix
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│ │ └── rime-data-flypy
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│ ├── i3 # i3 window manager's configuration
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│ │ ├── config
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│ │ ├── default.nix
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│ │ ├── i3blocks.conf
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│ │ ├── keybindings
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│ │ └── scripts
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│ ├── programs
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│ │ ├── browsers.nix
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│ │ ├── common.nix
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│ │ ├── default.nix # here we import all modules in programs folder by imports = [...]
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│ │ ├── git.nix
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│ │ ├── media.nix
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│ │ ├── vscode.nix
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│ │ └── xdg.nix
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│ ├── rofi # rofi launcher's configuration
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│ │ ├── configs
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│ │ │ ├── arc_dark_colors.rasi
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│ │ │ ├── arc_dark_transparent_colors.rasi
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│ │ │ ├── power-profiles.rasi
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│ │ │ ├── powermenu.rasi
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│ │ │ ├── rofidmenu.rasi
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│ │ │ └── rofikeyhint.rasi
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│ │ └── default.nix
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│ └── shell # shell/terminal related configuration
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│ ├── common.nix
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│ ├── default.nix
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│ ├── nushell
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│ │ ├── config.nu
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│ │ ├── default.nix
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│ │ └── env.nu
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│ ├── starship.nix
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│ └── terminals.nix
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├── hosts
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│ ├── msi-rtx4090 # My main machine's configuration
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│ │ ├── default.nix # This is the old configuration.nix, but most of the content has been split out to modules.
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│ │ └── hardware-configuration.nix # hardware & disk related configuration, autogenerated by nixos
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│ └── nixos-test # my test machine's configuration
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│ ├── default.nix
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│ └── hardware-configuration.nix
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├── modules # some common NixOS modules that can be reused
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│ ├── i3.nix
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│ └── system.nix
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└── wallpaper.jpg # wallpaper
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```
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For more details, you can refer to the [ryan4yin/nix-config/v0.0.2](https://github.com/ryan4yin/nix-config/tree/v0.0.2) repository.
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## `lib.mkOverride`, `lib.mkDefault`, and `lib.mkForce`
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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.
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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.
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Here's the source code:
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```nix
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# ......
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mkOverride = priority: content:
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{ _type = "override";
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inherit priority content;
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};
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mkOptionDefault = mkOverride 1500; # priority of option defaults
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mkDefault = mkOverride 1000; # used in config sections of non-user modules to set a default
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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
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mkForce = mkOverride 50;
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mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’
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# ......
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```
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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`.
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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.
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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.
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For example, in my configuration at [ryan4yin/nix-config/blob/main/modules/nixos/core-server.nix#L30](https://github.com/ryan4yin/nix-config/blob/main/modules/nixos/core-server.nix#L30), I define default values like this:
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```nix
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{ lib, pkgs, ... }:
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{
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# ......
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nixpkgs.config.allowUnfree = lib.mkDefault false;
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# ......
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}
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```
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Then, for my desktop machine, I override the value in [ryan4yin/nix-config/blob/main/modules/nixos/core-desktop.nix#L15](https://github.com/ryan4yin/nix-config/blob/main/modules/nixos/core-desktop.nix#L15) like this:
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```nix
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{ lib, pkgs, ... }:
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{
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# import the base module
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imports = [
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./core-server.nix
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];
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# override the default value defined in the base module
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nixpkgs.config.allowUnfree = lib.mkForce true;
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# ......
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}
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```
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## `lib.mkOrder`, `lib.mkBefore`, and `lib.mkAfter`
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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
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-type options**. These functions further contribute to the modularization of the configuration.
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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.
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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:
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```nix
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# ......
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mkOrder = priority: content:
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{ _type = "order";
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inherit priority content;
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};
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mkBefore = mkOrder 500;
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mkAfter = mkOrder 1500;
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# The default priority for things that don't have a priority specified.
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defaultPriority = 100;
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# ......
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```
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Therefore, `lib.mkBefore` is a shorthand for `lib.mkOrder 500`, and `lib.mkAfter` is a shorthand for `lib.mkOrder 1500`.
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To test the usage of `lib.mkBefore` and `lib.mkAfter`, let's create a simple Flake project:
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```shell
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# Create flake.nix with the following content
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› cat <<EOF | sudo tee flake.nix
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{
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description = "Ryan's NixOS Flake";
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inputs = {
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nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05";
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};
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outputs = { self, nixpkgs, ... }@inputs: {
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nixosConfigurations = {
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"nixos-test" = nixpkgs.lib.nixosSystem {
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system = "x86_64-linux";
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modules = [
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# Demo module 1: insert 'git' at the head of the list
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({lib, pkgs, ...}: {
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environment.systemPackages = lib.mkBefore [pkgs.git];
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})
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# Demo module 2: insert 'vim' at the tail of the list
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({lib, pkgs, ...}: {
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environment.systemPackages = lib.mkAfter [pkgs.vim];
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})
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# Demo module 3: simply add 'curl' to the list
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({lib, pkgs, ...}: {
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environment.systemPackages = with pkgs; [curl];
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})
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];
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};
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};
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};
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}
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EOF
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# Create flake.lock
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› nix flake update
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# Enter the nix repl environment
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› nix repl
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Welcome to Nix 2.13.3. Type :? for help.
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# Load the flake we just created
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nix-repl> :lf .
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Added 9 variables.
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# Check the order of systemPackages
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nix-repl> outputs.nixosConfigurations.nixos-test.config.environment.systemPackages
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[ «derivation /nix/store/0xvn7ssrwa0ax646gl4hwn8cpi05zl9j-git-2.40.1.drv»
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«derivation /nix/store/7x8qmbvfai68sf73zq9szs5q78mc0kny-curl-8.1.1.drv»
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«derivation /nix/store/bly81l03kh0dfly9ix2ysps6kyn1hrjl-nixos-container.drv»
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......
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......
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«derivation /nix/store/qpmpv
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q5azka70lvamsca4g4sf55j8994-vim-9.0.1441.drv» ]
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```
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As you can see, the order of `systemPackages` is `git -> curl -> default packages -> vim`, which matches the order we defined in `flake.nix`.
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> Although adjusting the order of `systemPackages` may not be useful in practice, it can be helpful in other scenarios.
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## References
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- [Nix modules: Improving Nix's discoverability and usability](https://cfp.nixcon.org/nixcon2020/talk/K89WJY/)
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- [Module System - Nixpkgs](https://github.com/NixOS/nixpkgs/blob/nixos-unstable/doc/module-system/module-system.chapter.md) |