When copying configuration from one machine to another, what to do about boot loader config options?

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I’ve heard it said that one nice thing about NixOS is that one can copy a configuration from one machine to another, but then I notice a line like this in configuration.nix:

boot.loader.grub.device = "/dev/nvme0n1";

I’m a bit surprised that this isn’t in hardware-configuration.nix, since it seems that it’s specific to a given machine.

Do you typically just copy parts of the old configuration.nix into the new one, piecemeal, being mindful of what’s hardware-specific? Or is there a fancier way of copying a config from one machine to another?

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I classify NixOS modules in two categories: modules that declare new bespoke options (and possibly use those bespoke options to control other config), and modules that merely define (set) options.

The former are put in a directory structure called modules/ and I import them all unconditionally in all configs.

The latter are host-specific because hosts are never identical. These modules live under hosts/. Each module is a different host’s entry-point. (In my case, I store all my hosts’ configs in the same repo.)

Hence I don’t believe the “copy config” propaganda, I define essentially different configs by host, and only import common config where it makes sense.

It doesn’t really matter what filenames you assign to a module, as long as you know what the code’s doing. hardware-configuration.nix is not a special filename, I assimilated it into my host-specific config and deleted that file.

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Would I be correct in guessing that you (1) have a flake-based config, and (2), that config does more than just import configuration.nix?

I’ve been working on flakifying my config on my laptop, with the goal of getting something that I can mostly copy to my desktop once I install NixOS on it.

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I happen to use flakes for NixOS config, but that’s not a requirement for this setup. After observing the divergences in flakes behavior in the (at least 3) nix implementations and what a mess lazy trees turned out to be, I’m probably going to get rid of flakes from the config…

Anyway, you could easily use e.g. nixos-rebuild boot --sudo -I nixos-config=./hosts/myhost.nix to achieve the same effect of having individual entry-points per host.

It really is as simple as adding configuration.nix (or whatever you named your entry point) to modules/imports, though.

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Oh, I know that much. I’ve already done it.