yaymukund’s weblog

June 2, 2021

Overriding packages in NixOS

In NixOS, it’s sometimes desirable to override a package in order to extend or modify its behavior. For example, I override my Neovim to add plugins so they get all the benefits of being in the nix store. Here’s how I do it.

# in configuration.nix
nixpkgs.overlays = [
  (import ./overlays.nix)
];

# in overlays.nix
self: super: {
  neovim-mukund = self.callPackage ./packages/neovim-mukund.nix {};
}

# finally, in packages/neovim-mukund.nix
{ pkgs }:
  neovim.override {
    vimAlias = true;
    viAlias = true;
    configure = {
      packages.mukund-plugins = with pkgs.vimPlugins; {
        start = [
          ale
          fzf-vim
          # ...
        ];
      };
    };
  }

# putting it all together
environment.systemPackages = [
  neovim-mukund
];

Bonus: Installing a single package from main

If you need to install a single package from the main branch but keep the rest of your code on your nix channel (usually the main channel or nixos-unstable), then try this:

# in packages/neovim-mukund.nix
let neovim-master = (import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/master.tar.gz") {}).neovim
in
  environment.systemPackage = [
    neovim-master
  ]

This time, I’m fetching and installing from the master.tar.gz file. This is handy if there’s an update upstream that you want to use immediately. For example, I often use this when Discord releases an update. Nixpkgs usually merges the version bump fairly quickly, but it doesn’t reach the release channels for many days during which Discord is unusable.

References