Flakes/en: Difference between revisions

== 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";

     nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";

   };

   };

}</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.}}

=== 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>{

   ...

     ];

   }

== Setup ==

When using any [[Nix command|<code>nix</code> command]], add the following command-line options:

<syntaxhighlight lang="shell">

  --experimental-features 'nix-command flakes'

</syntaxhighlight>

=== Enabling flakes permanently ===

experimental-features = nix-command flakes

</syntaxHighlight>

== Usage ==

=== The nix flakes command ===

{{Main|Nix (command)}}

   description = "Example flake with a devShell";

     let

       system = "x86_64-linux";

In this example the input would point to the `nixpkgs-unstable` channel.

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.

   description = "A flake targeting multiple architectures";

     nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";

   };

     systems = [ "x86_64-linux" "aarch64-linux" ];

     forAllSystems = f: builtins.listToAttrs (map (system: {

# Output should show ~/libdep-src-checkout/ so you know it worked

</syntaxHighlight>

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.

{{references}}