nixos-and-flakes-book/docs/nixpkgs/overriding.md

Overriding

In Nix, you can customize Nix packages in pkgs by using the override function, which allows you to define custom build parameters and returns a new derivation with the overridden values. Let's take a look at an example:

pkgs.fcitx5-rime.override { rimeDataPkgs = [ ./rime-data-flypy ]; }

In the above example, we override the rimeDataPkgs parameter of the fcitx5-rime derivation to use a custom package called rime-data-flypy. This creates a new derivation where rimeDataPkgs is overridden, while other parameters remain unchanged.

To find out which parameters of a specific package can be overridden, there are a couple of approaches you can follow:

1. Check the source code of the package in the Nixpkgs repository on GitHub, such as fcitx5-rime.nix. Make sure to select the appropriate branch, such as nixos-unstable, if you are using that branch.
2. Use the nix repl -f '<nixpkgs>' command to open a Nix REPL and then enter :e pkgs.fcitx5-rime. This opens the source code of the package in your default editor, where you can see all the parameters of the package. To learn the basic usage of nix repl, you can type :? to see the help information.

By using these methods, you can discover the input parameters of a package and determine which ones can be modified using override.

For example, let's take a look at the source code of pkgs.hello:

{ callPackage
, lib
, stdenv
, fetchurl
, nixos
, testers
, hello
}:

stdenv.mkDerivation (finalAttrs: {
  pname = "hello";
  version = "2.12.1";

  src = fetchurl {
    url = "mirror://gnu/hello/hello-${finalAttrs.version}.tar.gz";
    sha256 = "sha256-jZkUKv2SV28wsM18tCqNxoCZmLxdYH2Idh9RLibH2yA=";
  };

  doCheck = true;

  # ...
})

In this example, the attributes pname, version, src, and doCheck can all be overridden using overrideAttrs. For instance:

helloWithDebug = pkgs.hello.overrideAttrs (finalAttrs: previousAttrs: {
  doCheck = false;
});

In the above code, we use overrideAttrs to override the doCheck attribute, while leaving other attributes unchanged.

You can also override some default attributes defined in stdenv.mkDerivation using overrideAttrs. For example:

helloWithDebug = pkgs.hello.overrideAttrs (finalAttrs: previousAttrs: {
  separateDebugInfo = true;
});

In this case, we override the separateDebugInfo attribute, which is defined in stdenv.mkDerivation, rather than in the source code of hello.

To see all the attributes defined in stdenv.mkDerivation, you can check its source code by using nix repl -f '<nixpkgs>' and entering :e stdenv.mkDerivation.

This will open the source code in your default editor. If you're new to using nix repl, you can type :? to see the help information.