Nix common issues

13.09.2023

This is collection of common problems/issues I faced working with Nix packages or on NixOS. I find it quite difficult to find necessary information in Nix reference as it missing or hidden too deep in the text.

I just want to build a derivation

From time to time I google for an expression to put into default.nix to build a derivation defined in derivation.nix. Here is oneliner:

nix-build -E 'with import <nixpkgs> { }; callPackage ./derivation.nix { }'

You can also put this expression into default.nix and run with just nix-build. The -K will create nix-build-derivation-ver directory in /tmp so you can debug it.

shell.nix example

I'm always looking for shell.nix example which I create almost in every project:

{ pkgs ? import <nixpkgs> {} }:
pkgs.mkShell {
  # runtime deps
  buildInputs = [
    hello
  ];
}

Why src = ./.; fails but src = fetchFromGithub {...} not?

This is because your local copy is probably dirty; nix does cp which copies all stuff to /nix/store. In some cases running make clean will save you. But I would suggest using git to fetch your local branch:

src = fetchgit {
    url = ./.;
    hash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
}

Note that you will need to commit all your changes.

error: cycle detected in build of '/nix/store/xxx.drv' in the references of output 'bin' from output 'out'

Don't know what this error is about but I solved it by removing "bin" from the outputs:

{ pkgs ? import <nixpkgs> {} }:
with pkgs; stdenv.mkDerivation rec {

  ...

  #          "bin" was here
  #          v
  outputs = [ "dev" "out" "doc" ];

  ...

}

Flake input is not the latest commit

From time to time results of the nix build .# didn't have the latest inputs. This probably happens because use specified your input as a git branch and did some changes to the branch. But forgot to tell nix that the branch was changed (with nix flake update):

...
inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    #                                        vvvvvvvvvvvvvvvvvv
    xfsprogs.url = "github:alberand/xfsprogs?branch=fsverity-v2";
    xfsprogs.flake = false;
}
...

The solution to this is always use rev=<commit hash> to pinpoint flake's inputs.

Force rebuild (download sources)

When running something like nix run github:alberand/nix-kernel-vm nix will download the source code. Unfortunately, if repository is updated right after that, nix will not re-download new version if command is run again. I haven't found a way to force nix do it except asking garbage collector to clean the whole /nix/store:

nix-store -gc

Note that this command remove only unused packages (ones which are not installed into the system).