Is specifying nixpkgs necessary to build sd-image?

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1

currently I am using such a script to build an sd image using a nixpkgs version pinned in sources.json is there something i can simplify about this?

sources="$(pwd)/sources.json"
nixpkgs=$(nix-instantiate --eval --expr "(builtins.fetchTarball (builtins.fromJSON (builtins.readFile $sources)).nixpkgs)"| tr -d \")
nix-build '<nixpkgs/nixos>' -A config.system.build.sdImage --argstr system aarch64-linux -I nixos-config=./sd-image.nix -I nixpkgs=$nixpkgs
2

You can inline the variable. But I am not sure whether that counts as “simplification” though.

What would definitely help reading the snippet are line breaks in the final invocation.

Of course you can remove the -I nixpkgs=..., but that would resolve through system state and not use the pin anymore.

3

If you’re comfortable using flakes and changing your sources file, you can simplify it to one command

nix-build '<nixpkgs/nixos>' -A config.system.build.sdImage --argstr system aarch64-linux -I nixos-config=./sd-image.nix -I nixpkgs=flake:$(<./nixpkgs-ref)

You could also use a default.nix file and call nixos from that file, e.g.

{
  nixpkgs ? builtins.fetchTarball (builtins.fromJSON (builtins.readFile ./sources.json)).nixpkgs,
  system ? builtins.currentSystem
}:

import "${nixpkgs}/nixos" {
  configuration = ./sd-image.nix;
  inherit system;
}

This would mean that you would only have to run

nix-build --argstr system aarch64-linux -A config.system.build.sdImage
4

Once you have a nix file that yields a nixos configuration, like in this example, you can also use nixos-rebuild build-image. e.g.

nixos-rebuild build-image --image-variant sd-card -f nixos.nix in this case. The sd-image module import is implied here. So a minimal nixos.nix here could be just

{ pkgs, ...}:
  pkgs.nixosSystem {
    # whatever nixos options you need
  }

Everything else is handled by defaults, including the target system and nixpkgs commit. The latter defaults to your nix path - so you can just prepend NIX_PATH="nixpkgs=$path-to-nixpkgs" to pin it, if desired (there’s also various other ways, build-image supports the --flake, --file and --attr flags of nixos-rebuild)

2 Likes
5

At that point, why not make it into a package output, that you can nix build?

1 Like
6

how would one do that?

...
(import "${nixpkgs}/nixos"
  {
    inherit configuration system;
  }).config.system.build.sdImage
1 Like