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'''Nix Flakes''' are an upcoming feature of the Nix package manager.
<languages />


== Introduction ==
<translate>
<!--T:1-->
{{Cleanup}}


Flakes allow you to specify your code's dependencies (e.g. remote Git repositories) in a declarative way, simply by listing them inside a <code>flake.nix</code> file:
<!--T:182-->
'''Nix flakes''' are an [[Experimental Nix features|experimental feature]] first introduced in the 2.4 [[Nix]] release,{{Cite manual|nix|development/experimental-features|number=13.8|title=Experimental Features|subsection=xp-feature-flakes|subtitle=flakes}}{{Cite manual|nix|release-notes/rl-2.4|number=14.27|title=Release 2.4 (2021-11-01)}} aiming to address a number of areas of improvement for the Nix ecosystem: they provide a uniform structure for Nix projects, allow for pinning specific versions of each dependencies, and sharing these dependencies via lock files, and overall make it more convenient to write reproducible Nix expressions.


<syntaxHighlight lang=nix>
<!--T:183-->
{
A flake is a directory which directly contains a Nix file called <code>flake.nix</code>, that follows a very specific structure. Flakes introduce a URL-like syntax{{Cite manual|nix|command-ref/new-cli/nix3-flake|number=8.5.17|title=nix flake|subsection=url-like-syntax|subtitle=URL-like syntax}} for specifying remote resources. To simplify the URL syntax, flakes use a registry of symbolic identifiers,{{Cite manual|nix|command-ref/new-cli/nix3-registry|number=8.5.62|title=nix registry}} allowing the direct specification of resources through syntax such as <code>github:NixOS/nixpkgs</code>.
   inputs = {
 
     home-manager.url = "github:nix-community/home-manager";
<!--T:184-->
Flakes also allow for locking references and versions, which can then be queried and updated programatically via the inputs {{cite manual|nix|command-ref/new-cli/nix3-flake-lock|number=7.5.19|title=nix flake lock}}{{cite manual|nix|command-ref/new-cli/nix3-flake-info|number=7.5.17|title=nix flake info}}. Additionally, an experimental CLI utility accepts flake references for expressions that build, run, and deploy packages.{{Cite manual|nix|command-ref/new-cli/nix|number=8.5.1|title=nix}}
 
<!--T:185-->
== Flake file structure ==
Minimally, a flake file contains a description of the flake, a set of input dependencies and an output. You can generate a very basic flake file at any time using nix flake init. This will populate the current directory with a file called flake.nix that will contain something akin to:
{{File|3=<nowiki>{
   description = "A very basic flake";
 
  <!--T:186-->
inputs = {
     nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
   };
   };
}
</syntaxHighlight>


Each dependency gets pinned, that is: its commit hash gets automatically stored into a file - named <code>flake.lock</code> - making it easy to, say, upgrade it:
  <!--T:187-->
outputs = { self, nixpkgs }: {
 
    <!--T:188-->
packages.x86_64-linux.hello = nixpkgs.legacyPackages.x86_64-linux.hello;
 
    <!--T:189-->
packages.x86_64-linux.default = self.packages.x86_64-linux.hello;
 
  <!--T:190-->
};
}</nowiki>|name=flake.nix|lang=nix}}
In the example above, you can see the description, the input specified as a GitHub repository with a specific branch (here <code>nixos/nixpkgs</code> on the <code>nixos-unstable</code> branch), and an output that makes use of the input. The output simply specifies that the flake contains one package for the x86_64 architecture called <code>hello</code>. Even if your flake's output wouldn't use its input (however, in practice, that is highly unlikely), the output still needs to be a Nix function.
{{Note|Flakes require you to specify its outputs for each architecture separately. For more information, read the related section below.}}
 
<!--T:191-->
=== Nix configuration ===
It is possible to override the global Nix configuration set in your <code>nix.conf</code> file for the purposes of evaluating a flake. This can be useful, for example, for setting up binary caches specific to certain projects, while keeping the global configuration untouched. The flake file can contain a nixConfig attribute with any relevant configuration settings supplied. For example, enabling the nix-community binary cache would be achieved by:
{{File|3=<nowiki>{
  ...
  nixConfig = {
    extra-substituters = [
      "https://nix-community.cachix.org"
    ];
    extra-trusted-public-keys = [
      "nix-community.cachix.org-1:...="
    ];
  }
}</nowiki>|name=flake.nix|lang=nix}}{{Note|If you are used to configuring your Nix settings via the NixOS configuration, these options are under <code>nix.settings</code> and not <code>nix</code>. For example, you cannot specify the automatic storage optimisation under <code>nix.optimisation.enable</code>.}}
 
== Setup == <!--T:192-->
 
=== Enabling flakes temporarily === <!--T:5-->


<syntaxHighlight lang=console>
<!--T:6-->
$ nix flake lock --update-input home-manager
When using any [[Nix command|<code>nix</code> command]], add the following command-line options:
</syntaxHighlight>
</translate>
<syntaxhighlight lang="shell">
--experimental-features 'nix-command flakes'
</syntaxhighlight>
<translate>
=== Enabling flakes permanently === <!--T:193-->


''(if you're familiar with modern packages managers like <code>cargo</code> or <code>npm</code>, then the overall mechanism shouldn't surprise you - Nix works in a similar way, although without a centralized repository.)''
==== NixOS ==== <!--T:7-->


Flakes replace the nix-channels command and things like ad-hoc invocations of <code>builtins.fetchgit</code> - no more worrying about keeping your channels in sync, no more worrying about forgetting about a dependency deep down in your tree: everything's at hand right inside <code>flake.lock</code>.
<!--T:8-->
Add the following to the [[Overview_of_the_NixOS_Linux_distribution#Declarative_Configuration system configuration |NixOS configuration]]:


== Enable flakes ==
</translate>
<syntaxHighlight lang=nix>
  nix.settings.experimental-features = [ "nix-command" "flakes" ];
</syntaxHighlight>
<translate>


Flakes have been introduced with Nix 2.4 ([https://nixos.org/manual/nix/unstable/release-notes/rl-2.4.html release notes]).
====Home Manager==== <!--T:10-->


See [https://lazamar.co.uk/nix-versions/?channel=nixpkgs-unstable&package=nix Nix package versions] to find a channel or <code>nixpkgs</code> revision providing the required Nix version.
<!--T:11-->
Add the following to your [[Home Manager|home manager]] config:


===Temporary===
</translate>
<syntaxhighlight lang="nix">
  nix.settings.experimental-features = [ "nix-command" "flakes" ];
</syntaxhighlight>
<translate>


Add <code>--experimental-features 'nix-command flakes'</code> when calling the <code>nix</code> command.
====Nix standalone==== <!--T:13-->


=== Permanent ===
<!--T:14-->
{{Note | The  [https://github.com/DeterminateSystems/nix-installer Determinate Nix Installer] enables flakes by default.}}


Edit either <code>~/.config/nix/nix.conf</code> or <code>/etc/nix/nix.conf</code> and add:
<!--T:15-->
Add the following to <code>~/.config/nix/nix.conf</code> or <code>/etc/nix/nix.conf</code>:


</translate>
<syntaxHighlight lang=text>
<syntaxHighlight lang=text>
experimental-features = nix-command flakes  
experimental-features = nix-command flakes
</syntaxHighlight>
</syntaxHighlight>
<translate>
== Usage == <!--T:17-->
<!--T:20-->
{{Warning | Since contents of flake files are copied to the world-readable [[Nix_package_manager#Nix_store|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]].}}
<!--T:146-->
{{Note | For flakes in [[git]] repositories, only files in the working tree will be copied to the store.
<!--T:22-->
Therefore, if you use <code>git</code> for your flake, ensure to <code>git add</code> any project files after you first create them.}}
<!--T:64-->
=== The nix flakes command ===
{{Main|Nix (command)}}
<!--T:65-->
The {{ic|nix flake}} subcommand is described in {{Nix Manual|name=command reference page of the Nix manual|anchor=command-ref/new-cli/nix3-flake}}.
<!--T:194-->
This flake produces a single flake output <code>packages</code>. And within that, <code>x86_64-linux</code> is a system-specifc attribute set. And within that, two package [[derivations]] <code>default</code> and <code>hello</code>. You can find outputs with the {{Nix Manual|name=show command|anchor=command-ref/new-cli/nix3-flake-show}} of a flake as shown below:
<!--T:195-->
<syntaxhighlight lang="console">
$ nix flake show
└───packages
    └───x86_64-linux
        ├───default: package 'hello-2.12.2'
        └───hello: package 'hello-2.12.2'
</syntaxhighlight>
==== Development shells ==== <!--T:196-->


If the Nix installation is in multi-user mode, don’t forget to restart the <code>nix-daemon</code>.
<!--T:197-->
A <code>devShell</code> is a Nix-provided [[Development_environment_with_nix-shell#nix develop|development environment]] defined within a flake. It lets you declare a reproducible shell environment with the tools, libraries, and environment variables you need for the development of a specific project. This is flake equivalent to defining a <code>nix-shell</code>.


=== NixOS ===
<!--T:198-->
<syntaxhighlight lang="nix">
{
  description = "Example flake with a devShell";


On NixOS set the following options in <code>configuration.nix</code> and run <code>nixos-rebuild</code>.
  <!--T:199-->
inputs.nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";


<syntaxHighlight lang=nix>
  <!--T:200-->
{ pkgs, ... }: {
outputs = { self, nixpkgs}:
  nix.settings.experimental-features = [ "nix-command" "flakes" ];
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs { inherit system; };
    in {
      devShells.x86_64-linux.default = pkgs.mkShell {
        buildInputs = with pkgs; [
          hello
        ];
        shellHook = ''
          echo "Welcome to the devShell!"
        '';
      };
    };
}
}
</syntaxHighlight>
</syntaxhighlight>


== Basic project usage ==
<!--T:201-->
To enter the development shell environment:


{{warning | All files tracked by the version control system (e.g. git or mercurial) will be copied to the nix store when the flake is evaluated. So be careful when putting secrets in version control (which is not optimal by itself) around a flake.}}
<!--T:202-->
<syntaxhighlight lang="console">
$ nix develop
</syntaxhighlight>


In your repo, run <code>nix flake init</code> to generate the flake.nix file. Then run <code>git add flake.nix</code> to add it to the git staging area, otherwise nix will not recognize that the file exists.
<!--T:203-->
{{note|You don’t need to define a devShell to enter a development shell using nix develop.
If no devShell is defined, nix develop will drop you into an environment containing the default build dependencies of the flake (if any).}}


See also https://www.tweag.io/blog/2020-05-25-flakes/
==== Build specific attributes in a flake repository ==== <!--T:102-->


To install a flake when using nix outside of nixOS, use <code>nix profile install /path/to/flake</code>. The path may also be an URL (e.g.: <code>nix profile install git+https://example.com/my-repo?dir=subdirectory</code>).
<!--T:103-->
Running <code>nix build</code> will look in the <code>legacyPackages</code> and <code>packages</code> output attributes for the corresponding [[derivation]] and then your system architecture and build the default output. If you want to specify a build attribute in a flake repository, you can run <code>nix build .#<attr></code>. In the example above, if you wanted to build the <code>packages.x86_64-linux.hello</code> attribute, run:


== Flake schema ==
<!--T:204-->
<syntaxHighlight lang=console>
$ nix build .#hello
</syntaxHighlight>
 
<!--T:205-->
Likewise, you can specify an attribute with the run command: <code>nix run .#hello</code> and the develop command: <code>nix develop .#hello</code>.
 
== Flake schema == <!--T:27-->


<!--T:28-->
The flake.nix file is a Nix file but that has special restrictions (more on that later).
The flake.nix file is a Nix file but that has special restrictions (more on that later).


<!--T:29-->
It has 4 top-level attributes:
It has 4 top-level attributes:


<!--T:30-->
* <code>description</code> is a string describing the flake.
* <code>description</code> is a string describing the flake.
<!--T:147-->
* <code>inputs</code> is an attribute set of all the dependencies of the flake. The schema is described below.
* <code>inputs</code> is an attribute set of all the dependencies of the flake. The schema is described below.
* <code>outputs</code> is a function of one argument that takes an attribute set of all the realized inputs, and outputs another attribute set which schema is described below.
* <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.


=== Input schema ===
<!--T:148-->
* <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.
 
<!--T:149-->
* <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|binary cache]].
 
=== Input schema === <!--T:31-->
 
<!--T:32-->
[https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-inputs The nix flake inputs manual].
 
<!--T:150-->
[https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-references The nix flake references manual].
 
<!--T:33-->
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.
 
<!--T:34-->
[[Nixpkgs]] can be defined using the following code:
 
</translate>
<code>inputs.nixpkgs.url = "github:NixOS/nixpkgs/<branch name>";</code>
 
Nixpkgs can alternatively also point to an url cached by the NixOS organization:
 
<code>inputs.nixpkgs.url = "<nowiki>https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz</nowiki>";</code>
 
In this example the input would point to the `nixpkgs-unstable` channel.
 
<translate>
 
<!--T:36-->
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.
 
<!--T:37-->
For example, adding [[Hyprland]] as an input would look something like this:
 
</translate>
<code>inputs.hyprland.url = "github:hyprwm/Hyprland";</code>
<translate>
 
<!--T:39-->
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:
 
</translate>
<code>inputs.hyprland.inputs.nixpkgs.follows = "nixpkgs";</code>
<translate>
 
<!--T:41-->
Using curly brackets (<code>{}</code>), we can shorten all of this and put it in a table. The code will look something like this:
 
</translate>
<syntaxhighlight lang="nix">
inputs = {
  nixpkgs.url = "github:NixOS/nixpkgs/<branch name>";
  hyprland = {
    url = "github:hyprwm/Hyprland";
    inputs.nixpkgs.follows = "nixpkgs";
  };
};
</syntaxhighlight>
<translate>


This is not a complete schema but should be enough to get you started:
<!--T:206-->
By default, Git submodules in package <code>src</code>'s won't get copied to the nix store, this may cause the build to fail. Flakes in Git repositories can declare that they need Git submodules to be enabled. Since Nix version [https://discourse.nixos.org/t/nix-2-27-0-released/62003 2.27], you can enable submodules by:


<syntaxHighlight lang=nix>
<!--T:207-->
{
<syntaxhighlight lang="nix">
  inputs = {
  inputs.self.submodules = true;
    # github example, also supported gitlab:
</syntaxhighlight>
    nixpkgs.url = "github:Mic92/nixpkgs/master";
    # git urls
    git-example.url = "git+https://git.somehost.tld/user/path?ref=branch&rev=fdc8ef970de2b4634e1b3dca296e1ed918459a9e";
    # local directories (for absolute paths you can omit 'path:')
    directory-example.url = "path:/path/to/repo";
    # Use this for non-flakes
    bar.url = "github:foo/bar/branch";
    bar.flake = false;
    # Overwrite inputs in a flake
    # This is useful to use the same nixpkgs version in both flakes
    sops-nix.url = "github:Mic92/sops-nix";
    sops-nix.inputs.nixpkgs.follows = "nixpkgs";
    # Pin flakes to a specific revision
    nix-doom-emacs.url = "github:vlaci/nix-doom-emacs?rev=238b18d7b2c8239f676358634bfb32693d3706f3";
    nix-doom-emacs.flake = false;
    # To use a subdirectory of a repo, pass dir=
    nixpkgs.url = "github:foo/bar?dir=shu";
  }
}
</syntaxHighlight>
Also see [https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-references the nix flake manual].


The bar input is then passed to the output schema
=== Output schema === <!--T:42-->


=== Output schema ===
<!--T:151-->
This is described in the nix package manager [https://github.com/NixOS/nix/blob/master/src/nix/flake-check.md src/nix/flake-check.md].


This is described in the nix package manager [https://github.com/NixOS/nix/blob/master/src/nix/flake.cc src/nix/flake.cc] in CmdFlakeCheck.
<!--T:43-->
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.


<!--T:44-->
Where:
Where:
 
<!--T:45-->
* <code><system></code> is something like "x86_64-linux", "aarch64-linux", "i686-linux", "x86_64-darwin"
* <code><system></code> is something like "x86_64-linux", "aarch64-linux", "i686-linux", "x86_64-darwin"
* <code><attr></code> is an attribute name like "hello".
 
<!--T:152-->
* <code><name></code> is an attribute name like "hello".
 
<!--T:153-->
* <code><flake></code> is a flake name like "nixpkgs".
* <code><flake></code> is a flake name like "nixpkgs".
<!--T:154-->
* <code><store-path></code> is a <code>/nix/store..</code> path
* <code><store-path></code> is a <code>/nix/store..</code> path


</translate>
<syntaxHighlight lang=nix>
<syntaxHighlight lang=nix>
{ self, ... }@inputs:
{ self, ... }@inputs:
Line 134: Line 315:
   apps."<system>".default = { type = "app"; program = "..."; };
   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>`
   # Used for nixpkgs packages, also accessible via `nix build .#<name>`
   legacyPackages."<system>"."<name>" = derivation;
   legacyPackages."<system>"."<name>" = derivation;
Line 139: Line 322:
   overlays."<name>" = final: prev: { };
   overlays."<name>" = final: prev: { };
   # Default overlay
   # Default overlay
   overlays.default = {};
   overlays.default = final: prev: { };
   # Nixos module, consumed by other flakes
   # Nixos module, consumed by other flakes
   nixosModules."<name>" = { config }: { options = {}; config = {}; };
   nixosModules."<name>" = { config, ... }: { options = {}; config = {}; };
   # Default module
   # Default module
   nixosModules.default = {};
   nixosModules.default = { config, ... }: { options = {}; config = {}; };
   # Used with `nixos-rebuild --flake .#<hostname>`
   # Used with `nixos-rebuild switch --flake .#<hostname>`
   # nixosConfigurations."<hostname>".config.system.build.toplevel must be a derivation
   # nixosConfigurations."<hostname>".config.system.build.toplevel must be a derivation
   nixosConfigurations."<hostname>" = {};
   nixosConfigurations."<hostname>" = {};
Line 162: Line 345:
}
}
</syntaxHighlight>
</syntaxHighlight>
<translate>


<!--T:48-->
You can also define additional arbitrary attributes, but these are the outputs that Nix knows about.
You can also define additional arbitrary attributes, but these are the outputs that Nix knows about.


==== nix run ====
== Core usage patterns == <!--T:208-->
When output <code>apps.<system>.myapp</code> is not defined, <code>nix run myapp</code> runs <code><packages or legacyPackages.<system>.myapp>/bin/<myapp.meta.mainProgram or myapp.pname or myapp.name (the non-version part)></code>


== Using flakes project from a legacy Nix ==
=== Making your evaluations pure === <!--T:60-->
There is a [https://github.com/edolstra/flake-compat flake-compat] library you can use to shim legacy <code>default.nix</code> and <code>shell.nix</code> files. It will download the inputs of the flake, pass them to the flake’s <code>outputs</code> function and return an attribute set containing <code>defaultNix</code> and <code>shellNix</code> attributes. The attributes will contain the output attribute set with an extra <code>default</code> attribute pointing to current platform’s <code>defaultPackage</code> (resp. <code>devShell</code> for <code>shellNix</code>).


Place the following into <code>default.nix</code> (for <code>shell.nix</code>, replace <code>defaultNix</code> with <code>shellNix</code>) to use the shim:
<!--T:61-->
Nix flakes are evaluated in a pure evaluation mode, meaning that access to the external environment is restricted to ensure reproducibility. To maintain purity when working with flakes, consider the following:


<syntaxHighlight lang=nix>
<!--T:62-->
(import (
* {{Nixpkgs Manual|name=fetchurl|anchor=#sec-pkgs-fetchers-fetchurl-inputs}} and {{Nixpkgs Manual|name=fetchzip|anchor=#sec-pkgs-fetchers-fetchzip-inputs}} require a <code>sha256</code> argument to be considered pure.
  fetchTarball {
    url = "https://github.com/edolstra/flake-compat/archive/12c64ca55c1014cdc1b16ed5a804aa8576601ff2.tar.gz";
    sha256 = "0jm6nzb83wa6ai17ly9fzpqc40wg1viib8klq8lby54agpl213w5"; }
) {
  src = ./.;
}).defaultNix
</syntaxHighlight>


You can also use the lockfile to make updating the hashes easier using <code>nix flake lock --update-input flake-compat</code>. Add the following to your <code>flake.nix</code>:
<!--T:156-->
* <code>builtins.currentSystem</code> is non-hermetic and impure as it reflects the host system performing the evauluation. This can usually be avoided by passing the system (i.e., x86_64-linux) explicitly to derivations requiring it.


<syntaxHighlight lang=nix>
<!--T:209-->
  inputs.flake-compat = {
*  <code>builtins.getEnv</code> is also impure. Avoid reading from environment variables and likewise, do not reference files outside of the flake's directory.
    url = "github:edolstra/flake-compat";
    flake = false;
  };
</syntaxHighlight>


and add <code>flake-compat</code> to the arguments of <code>outputs</code> attribute. Then you will be able to use <code>default.nix</code> like the following:
=== Defining a flake for multiple architectures === <!--T:210-->


<syntaxHighlight lang=nix>
<!--T:211-->
(import (
Flakes force you to specify a program for each supported architecture. An example below shows how to write a flake that targets multiple architectures.
  let
    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
  in fetchTarball {
    url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
    sha256 = lock.nodes.flake-compat.locked.narHash; }
) {
  src =  ./.;
}).defaultNix
</syntaxHighlight>


== Making your evaluations pure ==
<!--T:212-->
<syntaxhighlight lang="nix">
{
  description = "A flake targeting multiple architectures";


Nix flakes run in pure evaluation mode, which is underdocumented. Some tips for now:
  <!--T:213-->
inputs = {
    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
  };


* fetchurl and fetchtar [https://github.com/NixOS/nix/blob/36c4d6f59247826dde32ad2e6b5a9471a9a1c911/src/libexpr/primops/fetchTree.cc#L201 require] a sha256 argument to be considered pure.
  <!--T:214-->
* 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.
outputs = { self, nixpkgs }: let
* Imports from channels like <code><nixpkgs></code> can be made pure by instead importing from the <code>output</code> function in <code>flake.nix</code>, where the arguments provide the store path to the flake's inputs:
    systems = [ "x86_64-linux" "aarch64-linux" ];
 
    forAllSystems = f: builtins.listToAttrs (map (system: {
<syntaxHighlight lang=nix>
      name = system;
outputs = { self, nixpkgs, ... }:
      value = f system;
   {
    }) systems);
     nixosConfigurations.machine = nixpkgs.lib.nixosSystem {
   in {
      # Note that you cannot put arbitrary configuration here: the configuration must be placed in the files loaded via modules
     packages = forAllSystems (system: let
       system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};
       modules = [
    in {
        (nixpkgs + "/nixos/modules/<some-module>.nix")
       hello = pkgs.hello;
        ./machine.nix
       default = pkgs.hello;
      ];
     });
     };
   };
   };
</syntaxHighlight>
}
</syntaxhighlight>


== The nix flakes command ==
<!--T:215-->
You can also use third-parties projects like [[Flake Utils|flake-utils]] or [[Flake Parts|flake-parts]] that automatically provide code to avoid this boilerplate. To avoid re-defining the program multiple times, refer to [[Flake Utils#Defining a flake for multiple architectures]]


The {{ic|nix flake}} subcommand is described in [https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-flake.html command reference page of the unstable manual] and here in the [[Nix command/flake]] article.
=== Using overlays === <!--T:216-->


== Using nix flakes with NixOS ==
<!--T:217-->
To use [[Overlays]] with flakes, refer to [[Overlays#In a Nix flake]] page.


nixos-rebuild switch will read its configuration from <code>/etc/nixos/flake.nix</code> if it is present.
=== Enable unfree software === <!--T:129-->


A basic nixos flake.nix could look like this:
<!--T:218-->
To allow for [[Unfree software|unfree software]] in a flake project, you need to explicitly allow it by setting <code>config.allowUnree = true;</code> when importing Nixpkgs.


<syntaxHighlight lang=nix>
<!--T:219-->
<syntaxhighlight lang="nix">
{
{
   outputs = { self, nixpkgs }: {
  inputs.nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
     # replace 'joes-desktop' with your hostname here.
   outputs = { self, nixpkgs, flake-compat }:
    nixosConfigurations.joes-desktop = nixpkgs.lib.nixosSystem {
     let
       system = "x86_64-linux";
       system = "x86_64-linux";
       modules = [ ./configuration.nix ];
       pkgs = import nixpkgs { inherit system; config.allowUnfree = true;};
    in {
      ...
     };
     };
  };
}
</syntaxHighlight>
If you want to pass on the flake inputs to external configuration files, you can use the <code>specialArgs</code> attribute:
<syntaxHighlight lang=nix>
{
  inputs.nixpkgs.url = github:NixOS/nixpkgs;
  inputs.home-manager.url = github:nix-community/home-manager;
 
  outputs = { self, nixpkgs, ... }@attrs: {
    nixosConfigurations.fnord = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      specialArgs = attrs;
      modules = [ ./configuration.nix ];
    };
  };
}
</syntaxHighlight>
Then, you can access the flake inputs from the file <code>configuration.nix</code> like this:
<syntaxHighlight lang=nix>
{ config, lib, nixpkgs, home-manager, ... }: {
  # do something with home-manager here, for instance:
  imports = [ home-manager.nixosModule ];
  ...
}
</syntaxHighlight>
nixos-rebuild also allows to specify different flake using the <code>--flake</code> flag (# is optional):
<syntaxHighlight lang=console>
$ sudo nixos-rebuild switch --flake '.#'
</syntaxHighlight>
By default nixos-rebuild will use the currents system hostname to lookup the right nixos configuration in <code>nixosConfigurations</code>. You can also override this by using appending it to the flake parameter:
<syntaxHighlight lang=console>
$ sudo nixos-rebuild switch --flake '/etc/nixos#joes-desktop'
</syntaxHighlight>
To switch a remote configuration, use:
<syntaxHighlight lang=console>
$ nixos-rebuild --flake .#mymachine \
  --target-host mymachine-hostname --build-host localhost \
  switch
</syntaxHighlight>
{{warning|Remote building seems to be broken at the moment, which is why the build host is set to “localhost”.}}
== Super fast nix-shell ==
One of the nix feature of the Flake edition is that Nix evaluations are cached.
Let’s say that your project has a <code>shell.nix</code> file that looks like this:
<syntaxHighlight lang=nix>
{ pkgs ? import <nixpkgs> { } }:
with pkgs;
mkShell {
  buildInputs = [
    nixpkgs-fmt
  ];
  shellHook = ''
    # ...
  '';
}
}
</syntaxHighlight>
</syntaxhighlight>  


Running nix-shell can be a bit slow and take 1-3 seconds.
== NixOS configuration with flakes == <!--T:220-->


Now create a <code>flake.nix</code> file in the same repository:
<!--T:221-->
It is possible to manage a [[NixOS]] system configuration using flakes, gaining the benefits of reproducible, declarative inputs and streamlined updates.


<syntaxHighlight lang=nix>
<!--T:222-->
{
For details and examples, see [[NixOS system configuration#Defining NixOS as a flake]].
  description = "my project description";


  inputs.flake-utils.url = "github:numtide/flake-utils";
== Development tricks == <!--T:131-->


  outputs = { self, nixpkgs, flake-utils }:
=== Automatically switch nix shells with direnv === <!--T:97-->
    flake-utils.lib.eachDefaultSystem
      (system:
        let pkgs = nixpkgs.legacyPackages.${system}; in
        {
          devShells.default = import ./shell.nix { inherit pkgs; };
        }
      );
}
</syntaxHighlight>


Run git add flake.nix so that Nix recognizes it.
<!--T:98-->
It is possible to automatically activate different Nix shells when navigating between project directories by using [[Direnv]]. Additional Nix integration with Direnv can be achieved with [https://github.com/nix-community/nix-direnv nix-direnv].


And finally, run <code>nix develop</code>. This is what replaces the old nix-shell invocation.
=== Pushing Flakes to Cachix === <!--T:99-->


Exit and run again, this command should now be super fast.
</translate>
https://docs.cachix.org/pushing#flakes
<translate>


{{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.}}
=== Flake support in projects without flakes === <!--T:50-->


=== Direnv integration ===
<!--T:51-->
The [https://github.com/edolstra/flake-compat flake-compat] library provides a compatibility layer that allows projects using traditional <code>default.nix</code> and <code>shell.nix</code> files to operate with flakes. For more details and usage examples, see the [[Flake Compat]] page.


Assuming that the flake defines a <code>devShell</code> output attribute and that you are using direnv. Here is how to replace the old use nix stdlib function with the faster flake version:
<!--T:223-->
Another project that allows consuming flakes from non-flake projects is [https://github.com/fricklerhandwerk/flake-inputs flake-inputs].


<syntaxHighlight lang=sh>
=== Accessing flakes from Nix expressions === <!--T:58-->
use_flake() {
  watch_file flake.nix
  watch_file flake.lock
  eval "$(nix print-dev-env --profile "$(direnv_layout_dir)/flake-profile")"
}
</syntaxHighlight>
 
Copy this in <code>~/.config/direnv/lib/use_flake.sh</code> or in <code>~/.config/direnv/direnvrc</code>
or directly in your project specific <code>.envrc</code>.


Note: You may not need to create <code>use_flake()</code> yourself; as of [https://github.com/direnv/direnv/releases/tag/v2.29.0#:~:text=add%20use_flake%20function direnv 2.29,] <code>use flake</code> is part of direnv's standard library.
<!--T:59-->
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 <code>(builtins.getFlake "/path/to/directory").packages.x86_64-linux.default</code>, where 'directory' is the directory that contains your <code>flake.nix</code>.


With this in place, you can now replace the use nix invocation in the <code>.envrc</code> file with <code>use flake</code>:
=== Efficiently build multiple flake outputs === <!--T:224-->


<syntaxHighlight lang=text>
<!--T:101-->
# .envrc
To push ''all'' flake outputs automatically, checkout [https://github.com/srid/devour-flake#usage devour-flake].
use flake
</syntaxHighlight>


The nice thing about this approach is that evaluation is cached.
=== Build a package added in a PR === <!--T:161-->


==== Optimize the reloads ====
</translate>
 
<syntaxHighlight>
Nix Flakes has a Nix evaluation caching mechanism. Is it possible to expose that somehow to automatically trigger direnv reloads?
nix build github:nixos/nixpkgs?ref=pull/<PR_NUMBER>/head#<PACKAGE>
 
With the previous solution, direnv would only reload if the flake.nix or flake.lock files have changed. This is not completely precise as the flake.nix file might import other files in the repository.
 
==== Setting the bash prompt like nix-shell ====
 
A [https://github.com/NixOS/nix/pull/4189 new experimental feature of flakes] allow to setup a bash-prompt per flake:
<syntaxHighlight lang=nix>
{
  description = "...";
  nixConfig.bash-prompt = "\[nix-develop\]$ ";
  ...
}
</syntaxHighlight>
</syntaxHighlight>
<translate>


Otherwise it's also possible to set the <code>nix develop</code> bash prompt system wide using the [https://nixos.org/manual/nix/unstable/command-ref/conf-file.html nix.conf option bash-prompt]. (On nixos I think it is set in <code>nix.extraOptions</code>)
<!--T:162-->
this allows building a package that has not yet been added to nixpkgs.


== Pushing Flakes to Cachix ==
<!--T:132-->
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:


https://docs.cachix.org/pushing#flakes
</translate>
 
<syntaxHighlight>
== Build specific attributes in a flake repository ==
git fetch upstream pull/<PR_NUMBER>/head && git checkout FETCH_HEAD && nix build .#PACKAGE
 
When in the repository top-level, run <code>nix build .#<attr></code>. It will look in the <code>legacyPackages</code> and <code>packages</code> output attributes for the corresponding derivation.
 
Eg, in nixpkgs:
 
<syntaxHighlight lang=console>
$ nix build .#hello
</syntaxHighlight>
</syntaxHighlight>
<translate>


=== Building flakes from a Git repo url with submodules ===
<!--T:163-->
this allows building a package that has not yet been added to nixpkgs.


As per nix 2.9.1, git submodules in package <code>src</code>s won't get copied to the nix store, this may cause the build to fail.  To workaround this, use:
=== How to add a file locally in git but not include it in commits === <!--T:164-->


<syntaxHighlight lang=console>
<!--T:133-->
$ nix build .?submodules=1#hello
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 [https://discourse.nixos.org/t/local-personal-development-tools-with-flakes/22714/8 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 <code>extra/flake.nix</code>):
</syntaxHighlight>


See: https://github.com/NixOS/nix/pull/5434
</translate>
 
<syntaxHighlight>
== Importing packages from multiple channels ==
git add --intent-to-add extra/flake.nix
 
git update-index --skip-worktree --assume-unchanged extra/flake.nix
You can import packages from different channels by creating an overlay on the ''pkgs'' attribute :
<syntaxHighlight lang=nix>
let
  overlay-unstable = final: prev: {
    unstable = nixpkgs-unstable.legacyPackages.${prev.system}; # considering nixpkgs-unstable is an input registered before.
  };
in nixpkgs.overlays = [ overlay-unstable ]; # we assign the overlay created before to the overlays of nixpkgs.
</syntaxHighlight>
should make a package accessible through <code>pkgs.unstable.package</code>
For example, a NixOS config flake skeleton could be as follows:
<syntaxHighlight lang=nix>
{
  description = "NixOS configuration with two or more channels";
 
  inputs = {
    nixpkgs.url = "nixpkgs/nixos-21.11";
    nixpkgs-unstable.url = "nixpkgs/nixos-unstable";
  };
 
  outputs = { self, nixpkgs, nixpkgs-unstable }:
    let
      system = "x86_64-linux";
      overlay-unstable = 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;
        # };
 
      };
    in {
      nixosConfigurations."<hostname>" = nixpkgs.lib.nixosSystem {
        inherit system;
        modules = [
          # Overlays-module makes "pkgs.unstable" available in configuration.nix
          ({ config, pkgs, ... }: { nixpkgs.overlays = [ overlay-unstable ]; })
          ./configuration.nix
        ];
      };
    };
}
</syntaxHighlight>
</syntaxHighlight>
<translate>


<syntaxHighlight lang=nix>
=== Rapid iteration of a direct dependency === <!--T:135-->
# NixOS configuration.nix, can now use "pkgs.package" or "pkgs.unstable.package"
{ config, pkgs, ... }: {
  environment.systemPackages = [pkgs.firefox pkgs.unstable.chromium];
  # ...
}
</syntaxHighlight>
Same can be done with the NURs, as it already has an ''overlay'' attribute in the flake.nix of the project, you can just add <syntaxHighlight lang=nix>nixpkgs.overlays = [ nur.overlay ];</syntaxHighlight>


If the variable <code>nixpkgs</code> points to the flake, you can also define <code>pkgs</code> with overlays with:
<!--T:165-->
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 <code>nix develop --redirect <flake> <directory></code> command allows you to provide a mutable dependency to your shell as if it were built by Nix.


<syntaxHighlight lang=nix>
<!--T:136-->
pkgs = import nixpkgs { overlays = [ /*the overlay in question*/ ]; };
Consider a situation where your executable, <code>consumexe</code>, depends on a library, <code>libdep</code>. You're trying to work on both at the same time, where changes to <code>libdep</code> are reflected in real time for <code>consumexe</code>. This workflow can be achieved like so:
</syntaxHighlight>


== Getting ''Instant'' System Flakes Repl ==
</translate>
 
<syntaxHighlight lang=bash>
How to get a nix repl out of your system flake:
cd ~/libdep-src-checkout/
 
nix develop # Or `nix-shell` if applicable.
<syntaxHighlight lang=text>
export prefix="./install" # configure nix to install it here
# nix repl
buildPhase  # build it like nix does
>> :lf /etc/nixos
installPhase # install it like nix does
>> nixosConfigurations.myhost.config
{ ... }
</syntaxHighlight>
</syntaxHighlight>
<translate>


Or out of your current flake:
<!--T:166-->
<syntaxHighlight lang=text>
Now that you've built the dependency, <code>consumexe</code> can take it as an input. '''In another terminal''':
# nix repl
>> :lf .#
</syntaxHighlight>


You can then access to the inputs, outputs… For instance if you would like to check the default version of the kernel present in nixpgs:
</translate>
<syntaxHighlight lang=text>
<syntaxHighlight lang=bash>
nix-repl> inputs.nixpkgs.legacyPackages.x86_64-linux.linuxPackages.kernel.version       
cd ~/consumexe-src-checkout/
"5.15.74"
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
</syntaxHighlight>
</syntaxHighlight>
<translate>


However, this won't be instant upon evaluation if any file changes have been done since your last configuration rebuild. Instead, if one puts:
<!--T:167-->
If Nix warns you that your redirected flake isn't actually used as an input to the evaluated flake, try using the <code>--inputs-from .</code> flag. If all worked well you should be able to <code>buildPhase && installPhase</code> when the dependency changes and rebuild your consumer with the new version ''without'' exiting the development shell.


<syntaxHighlight lang=nix>
== See also == <!--T:138-->
nix.nixPath = let path = toString ./.; in [ "repl=${path}/repl.nix" "nixpkgs=${inputs.nixpkgs}" ];
</syntaxHighlight>


In their system <code>flake.nix</code> configuration file, and includes the following file in their root directory flake as <code>repl.nix</code>:
=== Official sources === <!--T:225-->


<syntaxHighlight lang=nix>
<!--T:139-->
let
* [https://nix.dev/concepts/flakes Flakes] - nix.dev
  flake = builtins.getFlake (toString ./.);
  nixpkgs = import <nixpkgs> { };
in
{ inherit flake; }
// flake
// builtins
// nixpkgs
// nixpkgs.lib
// flake.nixosConfigurations
</syntaxHighlight>


(Don't forget to <code>git add repl.nix && nixos-rebuild  switch --flake "/etc/nixos"</code>)
<!--T:176-->
Then one can run (or bind a shell alias):
* [https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-flake.html Nix flake command reference manual] - Many additional details about flakes, and their parts.


<syntaxHighlight lang=bash>
<!--T:178-->
source /etc/set-environment && nix repl $(echo $NIX_PATH | perl -pe 's|.*(/nix/store/.*-source/repl.nix).*|\1|')</syntaxHighlight>
* [https://github.com/NixOS/nix/blob/master/src/nix/flake.md spec describing flake inputs in more detail]


This will launch a repl with access to <code>nixpkgs</code>, <code>lib</code>, and the <code>flake</code> options in a split of a second.
<!--T:168-->
* [https://github.com/NixOS/rfcs/pull/49 RFC 49] (2019) - Original flakes specification


An alternative approach to the above shell alias is omitting <code>repl</code> from <code>nix.nixPath</code> and creating a shell script:
=== Guides === <!--T:226-->
<syntaxHighlight lang=nix>
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
];
</syntaxHighlight>


== Enable unfree software ==
<!--T:169-->
* [https://jade.fyi/blog/flakes-arent-real/ Flakes aren't real and can't hurt you] (Jade Lovelace, 2024)


Because flake evalutations are hermetic, they will ignore the system configuration on nonfree software and the <code>NIXPKGS_ALLOW_UNFREE</code> environment variable by default.
<!--T:170-->
* [https://github.com/ryan4yin/nixos-and-flakes-book NixOS & Flakes Book](Ryan4yin, 2023) - 🛠️ ❤️ An unofficial NixOS & Flakes book for beginners.


To use nonfree software with CLI tools like <code>nix shell</code> or <code>nix run</code>, the <code>--impure</code> flag must be used for Nixpkgs to access the current environment variables:
<!--T:171-->
<syntaxHighlight lang=console>
* [https://xeiaso.net/blog/nix-flakes-1-2022-02-21 Nix Flakes: an Introduction] (Xe Iaso, 2022)
$ NIXPKGS_ALLOW_UNFREE=1 nix run --impure nixpkgs#discord
</syntaxHighlight>


To use nonfree software in a flake, add <code>nixpkgs</code> as an input in your flake and import it with the <code>allowUnfree</code> option:
<!--T:172-->
<syntaxHighlight lang=nix>
* [https://serokell.io/blog/practical-nix-flakes Practical Nix Flakes] (Alexander Bantyev, 2021) - Intro article on working with Nix and Flakes
pkgs = import nixpkgs { config = { allowUnfree = true; }; };
</syntaxHighlight>


=== Enable unfree software in home-manager ===
<!--T:173-->
* [https://www.tweag.io/blog/2020-05-25-flakes/ Nix Flakes, Part 1: An introduction and tutorial] (Eelco Dolstra, 2020)


If you want to install software using home-manager via nix flakes in non NixOS systems (like darwin) you can use the home-manager <code>nixpkgs.config</code> option for example
<!--T:174-->
* [https://www.tweag.io/blog/2020-06-25-eval-cache/ Nix Flakes, Part 2: Evaluation caching] (Eelco Dolstra, 2020)


<syntaxHighlight lang=nix>
<!--T:175-->
nixpkgs.config.allowUnfree = true;
* [https://www.tweag.io/blog/2020-07-31-nixos-flakes/ Nix Flakes, Part 3: Managing NixOS systems] (Eelco Dolstra, 2020)
</syntaxHighlight>


== Development tricks ==
<!--T:177-->
=== How to add a file locally in git but not include it in commits ===
* [https://www.youtube.com/watch?v=QXUlhnhuRX4&list=PLgknCdxP89RcGPTjngfNR9WmBgvD_xW0l Nix flakes 101: Introduction to nix flakes] (Jörg Thalheim, 2020) YouTube video


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 [https://discourse.nixos.org/t/local-personal-development-tools-with-flakes/22714/8 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 <code>extra/flake.nix</code>):
=== Useful flake modules === <!--T:227-->  


<syntaxHighlight>
<!--T:179-->
git add --intent-to-add extra/flake.nix
* [[Flake Utils|flake-utils]]: Library to avoid some boiler-code when writing flakes
git update-index --assume-unchanged extra/flake.nix
</syntaxHighlight>


<!--T:228-->
* [[Flake Parts|flake-parts]]: Library to help write modular and organized flakes


== See also ==
<!--T:229-->
* [[Flake Compat|flake-compat]]: A compatibility layer for flakes


* [https://www.tweag.io/blog/2020-05-25-flakes/ Nix Flakes, Part 1: An introduction and tutorial] (Eelco Dolstra, 2020)
<!--T:181-->
* [https://www.tweag.io/blog/2020-06-25-eval-cache/ Nix Flakes, Part 2: Evaluation caching] (Eelco Dolstra, 2020)
* [https://www.tweag.io/blog/2020-07-31-nixos-flakes/ Nix Flakes, Part 3: Managing NixOS systems] (Eelco Dolstra, 2020)
* [https://xeiaso.net/blog/nix-flakes-1-2022-02-21 Nix Flakes: an Introduction] (Xe Iaso, 2022)
* [https://serokell.io/blog/practical-nix-flakes Practical Nix Flakes] (Alexander Bantyev, 2021) - Intro article on working with Nix and Flakes
* [https://www.youtube.com/watch?v=QXUlhnhuRX4&list=PLgknCdxP89RcGPTjngfNR9WmBgvD_xW0l Nix flakes 101: Introduction to nix flakes] (Jörg Thalheim, 2020)
* [https://github.com/NixOS/rfcs/pull/49 RFC 49] (2019) - Original flakes specification
* [https://github.com/NixOS/nix/blob/master/src/nix/flake.md spec describing flake inputs in more detail]
* [https://github.com/numtide/flake-utils flake-utils: Library to avoid some boiler-code when writing flakes]
* [https://zimbatm.com/NixFlakes/#direnv-integration zimbat's direnv article]
* [https://github.com/nix-community/todomvc-nix building Rust and Haskell flakes]
* [https://github.com/nix-community/todomvc-nix building Rust and Haskell flakes]


<!--T:230-->
{{references}}


</translate>
[[Category:Software]]
[[Category:Software]]
[[Category:Nix]]
[[Category:Nix]]
[[Category:Nix Language]]
[[Category:Flakes]]
[[Category:Flakes]]
Retrieved from "https://wiki.nixos.org/wiki/Flakes"