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

Simply put, all Nix packages in nixpkgs can be customized with <pkg>.override {} to define some build parameters, which returns a new Derivation that uses custom parameters. For example:

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

The result of this Nix expression is a new Derivation, where rimeDataPkgs is overridden as [./rime-data-flypy], while other parameters remain their original values.

How to know which parameters of fcitx5-rime can be overridden? There are several ways:

1. Try to find the source code of the package in the nixpkgs repository on GitHub, such as fcitx5-rime.nix
   1. Note: Be sure to select the correct branch, for example, if you are using the nixos-unstable branch, you need to find it in the nixos-unstable branch.
2. Check by using nix repl '<nixpkgs>', then enter :e pkgs.fcitx5-rime, which will open the source code of this package through the default editor, and then you can see all the parameters of this package.
   1. Note: To learn the basic usage of nix repl, just type :? to see the help information

Through these two methods, you can see that the fcitx5-rime package has the following input parameters, which can all be modified by override:

{ lib, stdenv
, fetchFromGitHub
, pkg-config
, cmake
, extra-cmake-modules
, gettext
, fcitx5
, librime
, rime-data
, symlinkJoin
, rimeDataPkgs ? [ rime-data ]
}:

stdenv.mkDerivation rec {
  ...
}

Instead of override the function's parameters, we can also override the attributes of the Derivation created by stdenv.mkDerivation.

Take pkgs.hello as an example, first check the source code of this package through the method we mentioned earlier:

# https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/applications/misc/hello/default.nix
{ 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;

  # ......
})

The attributes showed above, such as pname version src doCheck, can all be overridden by overrideAttrs, for example:

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

Here we use overrideAttrs to override doCheck, while other attributes remain their original values.

Some default attributes defined in stdenv.mkDerivation can also be overridden by overrideAttrs, for example:

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

The attribute we override here, separateDebugInfo, is defined in stdenv.mkDerivation, not in the source code of hello. We can check the source code of stdenv.mkDerivation to see all the attributes defined in it by using nix repl '<nixpkgs>' and then enter :e stdenv.mkDerivation(To learn the basic usage of nix repl, just type :? to see the help information).