Overlays
This chapter describes how to extend and change Nixpkgs using overlays.
Overlays are used to add layers in the fixed-point used by Nixpkgs to compose
the set of all packages.
Nixpkgs can be configured with a list of overlays, which are applied in
order. This means that the order of the overlays can be significant if
multiple layers override the same package.
Installing overlays
The list of overlays can be set either explicitly in a Nix expression, or
through <nixpkgs-overlays> or user configuration
files.
Set overlays in NixOS or Nix expressions
On a NixOS system the value of the nixpkgs.overlays
option, if present, is passed to the system Nixpkgs directly as an
argument. Note that this does not affect the overlays for non-NixOS
operations (e.g. nix-env), which are
looked up independently.
The list of overlays can be passed explicitly when importing nixpkgs, for
example import <nixpkgs> { overlays = [ overlay1 overlay2 ];
}.
Further overlays can be added by calling the pkgs.extend
or pkgs.appendOverlays, although it is often preferable
to avoid these functions, because they recompute the Nixpkgs fixpoint,
which is somewhat expensive to do.
Install overlays via configuration lookup
The list of overlays is determined as follows.
First, if an
overlays
argument to the Nixpkgs function itself is given, then that is
used and no path lookup will be performed.
Otherwise, if the Nix path entry
<nixpkgs-overlays> exists, we look for overlays at
that path, as described below.
See the section on NIX_PATH in the Nix manual for
more details on how to set a value for
<nixpkgs-overlays>.
If one of ~/.config/nixpkgs/overlays.nix and
~/.config/nixpkgs/overlays/ exists, then we look
for overlays at that path, as described below. It is an error if both
exist.
If we are looking for overlays at a path, then there are two cases:
If the path is a file, then the file is imported as a Nix expression and
used as the list of overlays.
If the path is a directory, then we take the content of the directory,
order it lexicographically, and attempt to interpret each as an overlay
by:
Importing the file, if it is a .nix file.
Importing a top-level default.nix file, if it is
a directory.
Because overlays that are set in NixOS configuration do not affect
non-NixOS operations such as nix-env, the
overlays.nix option provides a convenient way to use
the same overlays for a NixOS system configuration and user configuration:
the same file can be used as overlays.nix and imported
as the value of nixpkgs.overlays.
Defining overlays
Overlays are Nix functions which accept two arguments, conventionally called
self and super, and return a set of
packages. For example, the following is a valid overlay.
self: super:
{
boost = super.boost.override {
python = self.python3;
};
rr = super.callPackage ./pkgs/rr {
stdenv = self.stdenv_32bit;
};
}
The first argument (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 overridden dependencies used in the
boost override.
The second argument (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, containing overridden
and/or new packages.
Overlays are similar to other methods for customizing Nixpkgs, in particular
the packageOverrides attribute described in
. Indeed,
packageOverrides acts as an overlay with only the
super argument. It is therefore appropriate for basic
use, but overlays are more powerful and easier to distribute.