Automatically managing remote sources with niv#

The Nix language can be used to describe dependencies between files managed by Nix. Nix expressions themselves can depend on remote sources, and there are multiple ways to specify their origin, as shown in Towards reproducibility: pinning Nixpkgs.

For more automation around handling remote sources, set up niv in your project:

$ nix-shell -p niv --run "niv init --nixpkgs nixos/nixpkgs --nixpkgs-branch nixos-23.05"

This command will fetch the latest revision of the Nixpkgs 23.05 release branch. In the current directory it will generate nix/sources.json, which will contain a pinned reference to the obtained revision. It will also create nix/sources.nix, which exposes those dependencies as an attribute set.

Import the generated nix/sources.nix as the default value for the argument to the function in default.nix and use it to refer to the Nixpkgs source directory:

1{ sources ? import ./nix/sources.nix }:
2let
3  pkgs = import sources.nixpkgs {};
4  build = pkgs.hello;
5in {
6  inherit build;
7}

nix-build will call the top-level function with the empty attribute set {}, or with the attributes passed via --arg or --argstr. This pattern allows overriding remote sources programmatically.

Add niv to the development environment for your project to have it readily available:

 { sources ? import ./nix/sources.nix }:
 let
   pkgs = import sources.nixpkgs {};
   build = pkgs.hello;
 in {
   inherit build;
+  shell = pkgs.mkShell {
+    inputsFrom = [ build ];
+    packages = with pkgs; [
+      niv
+    ];
+  };
 }

Also add a shell.nix to enter that environment more conveniently:

1(import ./. {}).shell

See Dependencies in the development shell for details, and note that here you have to pass an empty attribute set to the imported expression, since default.nix now contains a function.

Overriding sources#

As an example, we will use the previously created expression with an older version of Nixpkgs.

Enter the development environment, create a new directory, and set up niv with a different version of Nixpkgs:

$ nix-shell
[nix-shell]$ mkdir old
[nix-shell]$ cd old
[nix-shell]$ niv init --nixpkgs nixos/nixpkgs --nixpkgs-branch 18.09

Create a file default.nix in the new directory, and import the original one with the sources just created.

1import ../default.nix { sources = import ./nix/sources.nix; }

This will result in a different version being built:

$ nix-build -A build
$ ./result/bin/hello --version | head -1
hello (GNU Hello) 2.10

Sources can also be overridden on the command line:

nix-build .. -A build --arg sources 'import ./nix/sources.nix'

Check the built-in help for details:

niv --help

Next steps#