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'''Nix flakes''' は [https://nixos.org/manual/nix/stable/contributing/experimental-Features.html 試験的な機能] で Nix 2.4 で導入されました。 ([https://nixos.org/manual/nix/unstable/release-notes/rl-2.4.html リリースノートを参照してください])。
'''Nix flakes''' は [https://nixos.org/manual/nix/stable/contributing/experimental-Features.html 試験的な機能] で Nix 2.4 で導入されました。 ([https://nixos.org/manual/nix/unstable/release-notes/rl-2.4.html リリースノートを参照してください])。


<div lang="en" dir="ltr" class="mw-content-ltr">
<span id="Introduction"></span>
====Introduction====
====概要====
</div>


Nix flakesはNixのプロジェクトに一定の構造を強制し、ロックファイルを用いて依存するプロジェクトのバージョンを指定することでより便利に再現可能なNix式を記述できるようにします。
Nix flakesはNixのプロジェクトに一定の構造を強制し、ロックファイルを用いて依存するプロジェクトのバージョンを指定することでより便利に再現可能なNix式を記述できるようにします。


<div lang="en" dir="ltr" class="mw-content-ltr">
* [https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-flake.html#description Flake]とは<code>flake.nix</code>というのNixファイルをルートに持つファイルシステムツリーを指します。
* A [https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-flake.html#description flake] refers to a file-system tree whose root directory contains the Nix file specification called <code>flake.nix</code>.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
* <code>flake.nix</code>ファイルの内容は、Nix言語でパッケージとその依存関係を宣言するための統一された命名スキーマに従っています。
* The contents of <code>flake.nix</code> file follow a uniform naming schema for declaring packages and their dependencies in the Nix language.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
Flakeは外部のソースを指定するために[https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-references URLのような構文]を提供しています。
Flakes introduce a [https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-references URL-like syntax] for specifying remote sources.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
* 長いURL構文を簡易にするために[https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-registry.html Flakeはレジストリを使用]して短い記号として登録できます。
* To simplify the long URL syntax with shorter names, [https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-registry.html flakes uses a registry] of symbolic identifiers.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
* また、Flakesは参照(Gitのrefsの事)とバージョンを固定することができ、それらをプログラムによって照会したり更新したりすることができます。
* Flakes also allow for locking references and versions that can then be queried and updated programmatically.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
* [https://nixos.org/manual/nix/stable/command-ref/new-cli/nix.html 実験的なコマンドラインインタフェース]はFlakeの参照を受け取りパッケージのビルド、実行やべプロ医ができます。
* An [https://nixos.org/manual/nix/stable/command-ref/new-cli/nix.html experimental command-line interface] accepts flake references for expressions that build, run, and deploy packages.
</div>


<span id="Enable_flakes_temporarily"></span>
<span id="Enable_flakes_temporarily"></span>
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====NixOSでflakesを永続的に有効にする====
====NixOSでflakesを永続的に有効にする====


<div lang="en" dir="ltr" class="mw-content-ltr">
[[Overview_of_the_NixOS_Linux_distribution#Declarative_Configuration system configuration |NixOS configuration]]に以下を追加します
Add the following to the [[Overview_of_the_NixOS_Linux_distribution#Declarative_Configuration system configuration |NixOS configuration]]:
</div>


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<syntaxHighlight lang=nix>
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=====その他ディストリビューション、Home-Managerなし=====
=====その他ディストリビューション、Home-Managerなし=====


<div lang="en" dir="ltr" class="mw-content-ltr">
{{注記: | [https://github.com/DeterminateSystems/nix-installer Determinate Nix Installer]ではデフォルトでflakesを有効化しています。}}
{{Note | The  [https://github.com/DeterminateSystems/nix-installer Determinate Nix Installer] enables flakes by default.}}
</div>


次の内容を<code>~/.config/nix/nix.conf</code>または<code>/etc/nix/nix.conf</code>に追記してください:
次の内容を<code>~/.config/nix/nix.conf</code>または<code>/etc/nix/nix.conf</code>に追記してください:
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</syntaxHighlight>
</syntaxHighlight>


<div lang="en" dir="ltr" class="mw-content-ltr">
<span id="Basic_Usage_of_Flake"></span>
===Basic Usage of Flake===
===基本的なFlakeの使用方法===
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
この時点でnixコマンドを実行する前に、以下の2つの警告に注意してください。1つは暗号化に関するもの、もう1つはgitに関するものです。
Before running any nix commands at this point, please note the two warnings below: one for encryption and the other for git.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
<span id="Encryption_WARNING"></span>
====Encryption WARNING====
====暗号化に関する警告====
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
{{Warning | flakeファイルの内容は世界的に読み取り可能なNix storeフォルダにコピーされるため、暗号化されていない機密情報はflakeファイルに置かないでください。代わりに[[Comparison of secret managing schemes|secret managing scheme]]を使うべきです。}}
{{Warning | Since contents of flake files are copied to the world-readable Nix store folder, do not put any unencrypted secrets in flake files. You should instead use a [[Comparison of secret managing schemes|secret managing scheme]].}}
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
<span id="Git_WARNING"></span>
====Git WARNING====
====Gitに関する警告====
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
Gitリポジトリ内のフレークでは、作業ツリー内のファイルのみがNix storeにコピーされます。
For flakes in git repos, only files in the working tree will be copied to the store.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
したがって、flakeに<code>git</code>を使用する場合は、プロジェクトファイルを作成した後、必ず<code>git add</code>を実行してください。
Therefore, if you use <code>git</code> for your flake, ensure to <code>git add</code> any project files after you first create them.
</div>


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</div>
</div>


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<span id="Generate_flake.nix_file"></span>
====Generate flake.nix file====
====flake.nixファイルを生成する====
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
基本的な方法でFlakeを使い始めるにはプロジェクトディレクトリでflakeコマンドを実行します:
To start the basic usage of flake, run the flake command in the project directory:
</div>


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</syntaxHighlight>
</syntaxHighlight>


<div lang="en" dir="ltr" class="mw-content-ltr">
== Flake schema ==
== Flake schema ==
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
flake.nixファイルはNixファイルですが、特殊な制限が設けられています(これについては後述します)。
The flake.nix file is a Nix file but that has special restrictions (more on that later).
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
Flakeは4つのトップレベルアトリビュートを持ちます:
It has 4 top-level attributes:
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
* <code>description</code>はflakeを説明する文字列です。
* <code>description</code> is a string describing the flake.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
* <code>inputs</code>はflakeの依存関係をアトリビュートセットとして記述したものです。下にスキーマを記述しています。
* <code>inputs</code> is an attribute set of all the dependencies of the flake. The schema is described below.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
* <code>outputs</code>はすべてのrealizeされたinputを受け取り、下に記述されたスキーマのようにアトリビュートセットを返す関数です。
* <code>outputs</code> is a function of one argument that takes an attribute set of all the realized inputs, and outputs another attribute set whose schema is described below.
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
* <code>nixConfig</code>[https://nixos.org/manual/nix/stable/command-ref/conf-file.html nix.confに使用できる値]を反映しています。これを用いてこのflake特有の設定を反映させることができます。例としてバイナリキャッシュを設定できます。
* <code>nixConfig</code> is an attribute set of values which reflect the [https://nixos.org/manual/nix/stable/command-ref/conf-file.html values given to nix.conf]. This can extend the normal behavior of a user's nix experience by adding flake-specific configuration, such as a binary cache.
</div>


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<span id="Input_schema"></span>
=== Input schema ===
=== Input スキーマ ===
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
[https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-inputs Nixのflake inputsのマニュアル].
[https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-inputs The nix flake inputs manual].
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
[https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-references Nix flake referencesのマニュアル].
[https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-references The nix flake references manual].
</div>


<div lang="en" dir="ltr" class="mw-content-ltr">
inputsアトリビュートはflakeの依存関係を定義します。例えば、システムflakeではシステムをビルドするためにnixpkgsに依存しています。
The inputs attribute defines the dependencies of the flake. For example, nixpkgs has to be defined as a dependency for a system flake in order for the system to build properly.
</div>


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Nixpkgsは以下のコードで定義できます:
Nixpkgs can be defined using the following code:
</div>


<code>inputs.nixpkgs.url = "github:NixOS/nixpkgs/<branch name>";</code>
<code>inputs.nixpkgs.url = "github:NixOS/nixpkgs/<branch name>";</code>

Latest revision as of 03:12, 19 November 2024

Nix flakes試験的な機能 で Nix 2.4 で導入されました。 (リリースノートを参照してください)。

概要

Nix flakesはNixのプロジェクトに一定の構造を強制し、ロックファイルを用いて依存するプロジェクトのバージョンを指定することでより便利に再現可能なNix式を記述できるようにします。

  • Flakeとはflake.nixというのNixファイルをルートに持つファイルシステムツリーを指します。
  • flake.nixファイルの内容は、Nix言語でパッケージとその依存関係を宣言するための統一された命名スキーマに従っています。
  • また、Flakesは参照(Gitのrefsの事)とバージョンを固定することができ、それらをプログラムによって照会したり更新したりすることができます。

flakesを一時的に有効にする

任意のnixコマンドを使用する場合は、次のコマンドラインオプションを追加します:

 --experimental-features 'nix-command flakes'

NixOSでflakesを永続的に有効にする

NixOS configurationに以下を追加します

  nix.settings.experimental-features = [ "nix-command" "flakes" ];

その他ディストリビューション、Home-Managerあり

home-managerの設定に以下を追加します:

  nix.settings.experimental-features = [ "nix-command" "flakes" ];

その他ディストリビューション、Home-Managerなし

Template:注記:

次の内容を~/.config/nix/nix.confまたは/etc/nix/nix.confに追記してください:

experimental-features = nix-command flakes

基本的なFlakeの使用方法

この時点でnixコマンドを実行する前に、以下の2つの警告に注意してください。1つは暗号化に関するもの、もう1つはgitに関するものです。

暗号化に関する警告

Warning: flakeファイルの内容は世界的に読み取り可能なNix storeフォルダにコピーされるため、暗号化されていない機密情報はflakeファイルに置かないでください。代わりにsecret managing schemeを使うべきです。

Gitに関する警告

Gitリポジトリ内のフレークでは、作業ツリー内のファイルのみがNix storeにコピーされます。

したがって、flakeにgitを使用する場合は、プロジェクトファイルを作成した後、必ずgit addを実行してください。

flake.nixファイルを生成する

基本的な方法でFlakeを使い始めるにはプロジェクトディレクトリでflakeコマンドを実行します:

nix flake init

Flake schema

flake.nixファイルはNixファイルですが、特殊な制限が設けられています(これについては後述します)。

Flakeは4つのトップレベルアトリビュートを持ちます:

  • descriptionはflakeを説明する文字列です。
  • inputsはflakeの依存関係をアトリビュートセットとして記述したものです。下にスキーマを記述しています。
  • outputsはすべてのrealizeされたinputを受け取り、下に記述されたスキーマのようにアトリビュートセットを返す関数です。
  • nixConfignix.confに使用できる値を反映しています。これを用いてこのflake特有の設定を反映させることができます。例としてバイナリキャッシュを設定できます。

Input スキーマ

Nixのflake inputsのマニュアル.

Nix flake referencesのマニュアル.

inputsアトリビュートはflakeの依存関係を定義します。例えば、システムflakeではシステムをビルドするためにnixpkgsに依存しています。

Nixpkgsは以下のコードで定義できます:

inputs.nixpkgs.url = "github:NixOS/nixpkgs/<branch name>";

For any repository with its own flake.nix file, the website must also be defined. Nix knows where the nixpkgs repository is, so stating that it's on GitHub is unnecessary.

For example, adding Hyprland as an input would look something like this:

inputs.hyprland.url = "github:hyprwm/Hyprland";

If you want to make Hyprland follow the nixpkgs input to avoid having multiple versions of nixpkgs, this can be done using the following code:

inputs.hyprland.inputs.nixpkgs.follows = "nixpkgs";

Using curly brackets({}), we can shorten all of this and put it in a table. The code will look something like this:

inputs = {
  nixpkgs.url = "github:NixOS/nixpkgs/<branch name>";
  hyprland = {
    url = "github:hyprwm/Hyprland";
    inputs.nixpkgs.follows = "nixpkgs";
  };
};

Output schema

This is described in the nix package manager src/nix/flake-check.md.

Once the inputs are resolved, they're passed to the function `outputs` along with with `self`, which is the directory of this flake in the store. `outputs` returns the outputs of the flake, according to the following schema.

Where:

  • <system> is something like "x86_64-linux", "aarch64-linux", "i686-linux", "x86_64-darwin"
  • <name> is an attribute name like "hello".
  • <flake> is a flake name like "nixpkgs".
  • <store-path> is a /nix/store.. path
{ self, ... }@inputs:
{
  # Executed by `nix flake check`
  checks."<system>"."<name>" = derivation;
  # Executed by `nix build .#<name>`
  packages."<system>"."<name>" = derivation;
  # Executed by `nix build .`
  packages."<system>".default = derivation;
  # Executed by `nix run .#<name>`
  apps."<system>"."<name>" = {
    type = "app";
    program = "<store-path>";
  };
  # Executed by `nix run . -- <args?>`
  apps."<system>".default = { type = "app"; program = "..."; };

  # Formatter (alejandra, nixfmt or nixpkgs-fmt)
  formatter."<system>" = derivation;
  # Used for nixpkgs packages, also accessible via `nix build .#<name>`
  legacyPackages."<system>"."<name>" = derivation;
  # Overlay, consumed by other flakes
  overlays."<name>" = final: prev: { };
  # Default overlay
  overlays.default = final: prev: { };
  # Nixos module, consumed by other flakes
  nixosModules."<name>" = { config, ... }: { options = {}; config = {}; };
  # Default module
  nixosModules.default = { config, ... }: { options = {}; config = {}; };
  # Used with `nixos-rebuild switch --flake .#<hostname>`
  # nixosConfigurations."<hostname>".config.system.build.toplevel must be a derivation
  nixosConfigurations."<hostname>" = {};
  # Used by `nix develop .#<name>`
  devShells."<system>"."<name>" = derivation;
  # Used by `nix develop`
  devShells."<system>".default = derivation;
  # Hydra build jobs
  hydraJobs."<attr>"."<system>" = derivation;
  # Used by `nix flake init -t <flake>#<name>`
  templates."<name>" = {
    path = "<store-path>";
    description = "template description goes here?";
  };
  # Used by `nix flake init -t <flake>`
  templates.default = { path = "<store-path>"; description = ""; };
}

You can also define additional arbitrary attributes, but these are the outputs that Nix knows about.

nix run

When output apps.<system>.myapp is not defined, nix run myapp runs <packages or legacyPackages.<system>.myapp>/bin/<myapp.meta.mainProgram or myapp.pname or myapp.name (the non-version part)>

Using flakes with stable Nix

There exists the flake-compat library that you can use to shim default.nix and shell.nix files. It will download the inputs of the flake, pass them to the flake’s outputs function and return an attribute set containing defaultNix and shellNix attributes. The attributes will contain the output attribute set with an extra default attribute pointing to current platform’s defaultPackage (resp. devShell for shellNix).

Place the following into default.nix (for shell.nix, replace defaultNix with shellNix) to use the shim:

(import (
  fetchTarball {
    url = "https://github.com/edolstra/flake-compat/archive/12c64ca55c1014cdc1b16ed5a804aa8576601ff2.tar.gz";
    sha256 = "0jm6nzb83wa6ai17ly9fzpqc40wg1viib8klq8lby54agpl213w5"; }
) {
  src =  ./.;
}).defaultNix

You can also use the lockfile to make updating the hashes easier using nix flake lock --update-input flake-compat. Add the following to your flake.nix:

  inputs.flake-compat = {
    url = "github:edolstra/flake-compat";
    flake = false;
  };

and add flake-compat to the arguments of outputs attribute. Then you will be able to use default.nix like the following:

(import (
  let
    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
    nodeName = lock.nodes.root.inputs.flake-compat;
  in
  fetchTarball {
    url =
      lock.nodes.${nodeName}.locked.url
        or "https://github.com/edolstra/flake-compat/archive/${lock.nodes.${nodeName}.locked.rev}.tar.gz";
    sha256 = lock.nodes.${nodeName}.locked.narHash;
  }
) { src = ./.; }).defaultNix

Accessing flakes from Nix expressions

If you want to access a flake from within a regular Nix expression on a system that has flakes enabled, you can use something like (builtins.getFlake "/path/to/directory").packages.x86_64-linux.default, where 'directory' is the directory that contains your flake.nix.

Making your evaluations pure

Nix flakes run in pure evaluation mode, which is underdocumented. Some tips for now:

  • fetchurl and fetchtar require a sha256 argument to be considered pure.
  • builtins.currentSystem is non-hermetic and impure. This can usually be avoided by passing the system (i.e., x86_64-linux) explicitly to derivations requiring it.
  • Imports from channels like <nixpkgs> can be made pure by instead importing from the output function in flake.nix, where the arguments provide the store path to the flake's inputs:
 outputs = { self, nixpkgs, ... }:
  {
    nixosConfigurations.machine = nixpkgs.lib.nixosSystem {
      modules = [
        "${nixpkgs}/nixos/modules/<some-module>.nix"
        ./machine.nix
      ];
    };
  };

The nix flakes command

The nix flake subcommand is described in command reference page of the unstable manual.

Install packages with `nix profile`

Using nix flakes with NixOS

nixos-rebuild switch will read its configuration from /etc/nixos/flake.nix if it is present.

A basic nixos flake.nix could look like this:

{
  inputs.nixpkgs.url = github:NixOS/nixpkgs/nixos-unstable;
  outputs = { self, nixpkgs }: {
    # replace 'joes-desktop' with your hostname here.
    nixosConfigurations.joes-desktop = nixpkgs.lib.nixosSystem {
      modules = [ ./configuration.nix ];
    };
  };
}

If you want to pass on the flake inputs to external configuration files, you can use the specialArgs attribute:

{
  inputs.nixpkgs.url = github:NixOS/nixpkgs/nixos-unstable;
  inputs.home-manager.url = github:nix-community/home-manager;

  outputs = { self, nixpkgs, ... }@inputs: {
    nixosConfigurations.fnord = nixpkgs.lib.nixosSystem {
      specialArgs = { inherit inputs; };
      modules = [ ./configuration.nix ];
    };
  };
}

Then, you can access the flake inputs from the file configuration.nix like this:

{ config, lib, inputs, ... }: {
  # do something with home-manager here, for instance:
  imports = [ inputs.home-manager.nixosModules.default ];
  ...
}


nixos-rebuild also allows to specify different flake using the --flake flag (# is optional):

$ sudo nixos-rebuild switch --flake .

By default nixos-rebuild will use the currents system hostname to lookup the right nixos configuration in nixosConfigurations. You can also override this by using appending it to the flake parameter:

$ sudo nixos-rebuild switch --flake /etc/nixos#joes-desktop

To switch a remote host you can use:

$ nixos-rebuild --flake .#mymachine \
  --target-host mymachine-hostname \
  --build-host mymachine-hostname --fast \
  switch
Warning: Remote building seems to have an issue that's resolved by setting the --fast flag.

Pinning the registry on NixOS

{ inputs, ... }:
{
 nix.registry = {
    nixpkgs.flake = inputs.nixpkgs;
  };
}

To make sure the registry entry is "locked", use the following:

  nix.registry = {
    nixpkgs.to = {
      type = "path";
      path = pkgs.path;
      narHash = builtins.readFile
          (pkgs.runCommandLocal "get-nixpkgs-hash"
            { nativeBuildInputs = [ pkgs.nix ]; }
            "nix-hash --type sha256 --sri ${pkgs.path} > $out");
    };
  };

This has the unfortunate side-effect of requiring import-from-derivation and slowing down build times, however it may greatly speed up almost every eval. Full-time flakes users may be able to just use narHash = pkgs.narHash.

Super fast nix-shell

A feature of the nix Flake edition is that Nix evaluations are cached.

Let’s say that your project has a shell.nix file that looks like this:

{
  pkgs ? import <nixpkgs> { },
}:
pkgs.mkShell {
  packages = [ pkgs.nixfmt ];

  shellHook = ''
    # ...
  '';
}

Running nix-shell can be a bit slow and take 1-3 seconds.

Now create a flake.nix file in the same repository:

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";

  outputs =
    { nixpkgs, ... }:
    {
      /*
        This example assumes your system is x86_64-linux
        change as neccesary
      */
      devShells.x86_64-linux =
        let
          pkgs = nixpkgs.legacyPackages.x86_64-linux;
        in
        {
          default = pkgs.mkShell {
            packages = [ pkgs.hello ];
          };
        };
    };
}
}

( If you're in a git repository run `git add flake.nix` so that Nix recognizes it. )

And finally, run nix develop. This is what replaces the old nix-shell invocation.

Exit and run again, this command should now be super fast.

Warning: TODO: there is an alternative version where the defaultPackage is a pkgs.buildEnv that contains all the dependencies. And then nix shell is used to open the environment.

Automatically switch nix shells with nix-direnv

You can easily switch nix shells when you cd into different projects with nix-direnv.

Pushing Flakes to Cachix

https://docs.cachix.org/pushing#flakes

To push all flake outputs automatically, checkout devour-flake.

Build specific attributes in a flake repository

When in the repository top-level, run nix build .#<attr>. It will look in the legacyPackages and packages output attributes for the corresponding derivation.

Eg, in nixpkgs:

$ nix build .#hello

Building flakes from a Git repo url with submodules

As per nix 2.9.1, git submodules in package src's won't get copied to the nix store, this may cause the build to fail. To workaround this, use:

nix build '.?submodules=1#hello'

Importing packages from multiple nixpkgs branches

A NixOS config flake could be as follows:

{
  description = "NixOS configuration with two or more channels";

 inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
    nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
  };

  outputs =
    { nixpkgs, nixpkgs-unstable, ... }:
    {
      nixosConfigurations."<hostname>" = nixpkgs.lib.nixosSystem {
        modules = [
          {
            nixpkgs.overlays = [
              (final: prev: {
                unstable = nixpkgs-unstable.legacyPackages.${prev.system};
                # use this variant if unfree packages are needed:
                # unstable = import nixpkgs-unstable {
                #   inherit system;
                #   config.allowUnfree = true;
                # };
              })
            ];
          }
          ./configuration.nix
        ];
      };
    };
}
# NixOS configuration.nix, can now use "pkgs.package" or "pkgs.unstable.package"
{ pkgs, ... }:
{
  environment.systemPackages = [
    pkgs.firefox
    pkgs.unstable.chromium
  ];
  # ...
}

If the variable nixpkgs points to the flake, you can also define pkgs with overlays with:

pkgs = import nixpkgs { system = "x86_64-linux"; overlays = [ /*the overlay in question*/ ]; };

Getting Instant System Flakes Repl

How to get a nix repl out of your system flake:

$ nix repl

nix-repl> :lf /path/to/flake
Added 18 variables.

nix-repl> nixosConfigurations.myHost.config.networking.hostName
"myHost"

However, this won't be instant upon evaluation if any file changes have been done since your last configuration rebuild. Instead, if one puts:

nix.nixPath = let path = toString ./.; in [ "repl=${path}/repl.nix" "nixpkgs=${inputs.nixpkgs}" ];

In their system flake.nix configuration file, and includes the following file in their root directory flake as repl.nix:

let
  flake = builtins.getFlake (toString ./.);
  nixpkgs = import <nixpkgs> { };
in
{ inherit flake; }
// flake
// builtins
// nixpkgs
// nixpkgs.lib
// flake.nixosConfigurations

(Don't forget to git add repl.nix && nixos-rebuild switch --flake "/etc/nixos") Then one can run (or bind a shell alias):

source /etc/set-environment && nix repl $(echo $NIX_PATH | perl -pe 's|.*(/nix/store/.*-source/repl.nix).*|\1|')

This will launch a repl with access to nixpkgs, lib, and the flake options in a split of a second.

An alternative approach to the above shell alias is omitting repl from nix.nixPath and creating a shell script:

nix.nixPath = [ "nixpkgs=${inputs.nixpkgs}" ];
environment.systemPackages = let
  repl_path = toString ./.;
  my-nix-fast-repl = pkgs.writeShellScriptBin "my-nix-fast-repl" ''
    source /etc/set-environment
    nix repl "${repl_path}/repl.nix" "$@"
  '';
in [
  my-nix-fast-repl
];

Enable unfree software

Refer to Unfree Software.

Development tricks

Build a package added in a PR

nix build github:nixos/nixpkgs?ref=pull/<PR_NUMBER>/head#<PACKAGE>

this allows building a package that has not yet been added to nixpkgs.

note that this will download a full source tarball of nixpkgs. if you already have a local clone, using that may be faster due to delta compression:

git fetch upstream pull/<PR_NUMBER>/head && git checkout FETCH_HEAD && nix build .#PACKAGE

this allows building a package that has not yet been added to nixpkgs.

How to add a file locally in git but not include it in commits

When a git folder exists, flake will only copy files added in git to maximize reproducibility (this way if you forgot to add a local file in your repo, you will directly get an error when you try to compile it). However, for development purpose you may want to create an alternative flake file, for instance containing configuration for your preferred editors as described here… of course without committing this file since it contains only your own preferred tools. You can do so by doing something like that (say for a file called extra/flake.nix):

git add --intent-to-add extra/flake.nix
git update-index --skip-worktree --assume-unchanged extra/flake.nix

Rapid iteration of a direct dependency

One common pain point with using Nix as a development environment is the need to completely rebuild dependencies and re-enter the dev shell every time they are updated. The nix develop --redirect <flake> <directory> command allows you to provide a mutable dependency to your shell as if it were built by Nix.

Consider a situation where your executable, consumexe, depends on a library, libdep. You're trying to work on both at the same time, where changes to libdep are reflected in real time for consumexe. This workflow can be achieved like so:

cd ~/libdep-src-checkout/
nix develop # Or `nix-shell` if applicable.
export prefix="./install" # configure nix to install it here
buildPhase   # build it like nix does
installPhase # install it like nix does

Now that you've built the dependency, consumexe can take it as an input. In another terminal:

cd ~/consumexe-src-checkout/
nix develop --redirect libdep ~/libdep-src-checkout/install
echo $buildInputs | tr " " "\n" | grep libdep
# Output should show ~/libdep-src-checkout/ so you know it worked

If Nix warns you that your redirected flake isn't actually used as an input to the evaluated flake, try using the --inputs-from . flag. If all worked well you should be able to buildPhase && installPhase when the dependency changes and rebuild your consumer with the new version without exiting the development shell.

See also

  • RFC 49 (2019) - Original flakes specification
  • NixOS & Flakes Book(Ryan4yin, 2023) - 🛠️ ❤️ An unofficial NixOS & Flakes book for beginners.