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While one can rollback NixOS build generations easily, this won't give back the corresponding NixOS configuration in /etc/nixos/configuration.nix. Thus, it seems to be common to track /etc/nixos/ in a version control system (VCS) e.g. in Git.

When doing so, should the /etc/nixos/result symlink also be tracked? Why?/Why not?

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I can't see any significant advantage in doing it, but maybe my use cases have been too simple in this respect.

I prefer to watch ls -l /nix/var/nix/profiles/ and/or ls -l /nix/var/nix/profiles/system-profiles, based on whether you name your system profile(s), and I typically delete by hand the symlinks that I want garbage-collected.

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You can keep track of your configuration.nix in it's corresponding generation. You just need to create a reference to it (within itself) so that it becomes part of the closure. Here's an example:

configuration.nix

{ config, lib, pkgs, ... }:
{
  # yada, yada

  environment.etc."configuration.nix".source = /etc/nixos/configuration.nix;
};

The environment.etc option allows you to declarative-ly create files in /etc. Nix will detect /etc/nixos/configuration.nix as a path type (paths are first-class types in Nix) and thus copy it's contents into the Nix store. The result is a symbolic link /etc/configuration.nix pointing (indirectly) to /nix/store/blah-blah-configuration.nix, which means each generation will have a /etc/configuration.nix symbolic link pointing to it's corresponding configuration. You can then do something similar with additional files, such as hardware-configuration.nix.

The catch

Although the above will store your configuration, it will not make it usable by nix-env, nix-shell, nixos-rebuild, etc, because those commands use $NIX_PATH to locate configs. While you may be able to address this (see here) I don't recommend it because when you run nixos-rebuild it will use the wrong config file (/etc/nixos/configuration.nix rather than the copy in the Nix store, which is read-only anyway). Things would get wonky real quick.

And of course the Nix store is world-readable.

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