Overlays
This chapter describes how to extend and change Nixpkgs packages using
overlays. Overlays are used to add layers in the fix-point used by Nixpkgs
to compose the set of all packages.
Installing Overlays
The set of overlays is looked for in the following places. The
first one present is considered, and all the rest are ignored:
As an argument of the imported attribute set. When importing Nixpkgs,
the overlays attribute argument can be set to a list of
functions, which is described in .
In the directory pointed to by the Nix search path entry
<nixpkgs-overlays>.
In the directory ~/.nixpkgs/overlays/.
For the second and third options, the directory should contain Nix expressions defining the
overlays. Each overlay can be a file, a directory containing a
default.nix, or a symlink to one of those. The expressions should follow
the syntax described in .
The order of the overlay layers can influence the recipe of packages if multiple layers override
the same recipe. In the case where overlays are loaded from a directory, they are loaded in
alphabetical order.
To install an overlay using the last option, you can clone the overlay's repository and add
a symbolic link to it in ~/.nixpkgs/overlays/ directory.
Overlays Layout
Overlays are expressed as Nix functions which accept 2 arguments and return a set of
packages.
self: super:
{
boost = super.boost.override {
python = self.python3;
};
rr = super.callPackage ./pkgs/rr {
stdenv = self.stdenv_32bit;
};
}
The first argument, usually named self, corresponds to the final package
set. You should use this set for the dependencies of all packages specified in your
overlay. For example, all the dependencies of rr in the example above come
from self, as well as the overriden dependencies used in the
boost override.
The second argument, usually named super,
corresponds to the result of the evaluation of the previous stages of
Nixpkgs. It does not contain any of the packages added by the current
overlay nor any of the following overlays. This set should be used either
to refer to packages you wish to override, or to access functions defined
in Nixpkgs. For example, the original recipe of boost
in the above example, comes from super, as well as the
callPackage function.
The value returned by this function should be a set similar to
pkgs/top-level/all-packages.nix, which contains
overridden and/or new packages.