Overlays

Overlays provide a method to extend and change nixpkgs. They replace constructs like packageOverride and overridePackages.

Consider a simple example of setting the default proxy in Google Chrome:

let overlay1 = self: super:
{
   google-chrome = super.google-chrome.override {
     commandLineArgs =
       "--proxy-server='https=127.0.0.1:3128;http=127.0.0.1:3128'";
   };
};

Applying overlays manually

import <nixpkgs> { overlays = [ overlay1 overlay2 ]; }

Applying overlays automatically

On the user level

A list of overlays placed into ~/.config/nixpkgs/overlays.nix will be automatically loaded by all nix tools.

Alternatively, you can put each overlay in its own .nix file under your ~/.config/nixpkgs/overlays directory.

On the system level

If you want your overlays to be accessible by nix tools and also in the system-wide configuration, add nixpkgs-overlays to your NIX_PATH:

NIX_PATH="$NIX_PATH:nixpkgs-overlays=/cfg/overlays"

Currently nixos-rebuild only works with a <nixpkgs-overlays> path that is a directory.

There is a configuration option nixpkgs.overlays. Overlays set here will not be automatically applied by nix tools.

External Documentation

• Overlays in nixpkgs manual
• Blog post "The DOs and DON’Ts of nixpkgs overlays"