Title: NixOS specific feature: specialisations

	Title: NixOS specific feature: specialisations
	Author: Solène
	Date: 29 August 2022
	Tags: nixos nix tweag
	Description: This blog post explains how to use NixOS' unique feature
	called specialisation

	# Credits

	This blog post is a republication of the article I published on my
	employer's blog under CC BY 4.0. I'm grateful to be allowed to publish
	NixOS related content there, but also to be able to reuse it here!

	License CC by 4.0

	Original publication place: Tweag I/O - NixOS Specialisations

	After the publication of the original post, the NixOS wiki got updated
	to contain most of this content, I added some extra bits for the
	specific use case of "options for the non-specialisation that shouldn't
	be inherited by specialisations" that wasn't convered in this text.

	NixOS wiki: Specialisation

	# Introduction

	I often wished to be able to define different boot entries for
	different uses of my computer, be it for separating professional and
	personal use, testing kernels or using special hardware. NixOS has a
	unique feature that solves this problem in a clever way — NixOS
	specialisations.

	A NixOS specialisation is a mechanism to describe additional boot
	entries when building your system, with specific changes applied on top
	of your non-specialised configuration.

	# When do you need specialisations

	You may have hardware occasionally connected to your computer, and some
	of these devices may require incompatible changes to your day-to-day
	configuration. Specialisations can create a new boot entry you can use
	when starting your computer with your specific hardware connected. This
	is common for people with external GPUs (Graphical Processing Unit),
	and the reason why I first used specialisations.

	With NixOS, when I need my external GPU, I connect it to my computer
	and simply reboot my system. I choose the eGPU specialisation in my
	boot menu, and it just works. My boot menu looks like the following:

	NixOS specialisation shown in Grub

	You can also define a specialisation which will boot into a different
	kernel, giving you a safe opportunity to try a new version while
	keeping a fallback environment with the regular kernel.

	We can push the idea further by using a single computer for
	professional and personal use. Specialisations can have their own
	users, services, packages and requirements. This would create a hard
	separation without using multiple operating systems. However, by
	default, such a setup would be more practical than secure. While your
	users would only exist in one specialisation at a time, both users’
	data are stored on the same partition, so one user could be exploited
	by an attacker to reach the other user’s data.

	In a follow-up blog post, I will describe a secure setup using multiple
	encrypted partitions with different passphrases, all managed using
	specialisations with a single NixOS configuration. This will be quite
	awesome :)

	# How to use specialisations

	As an example, we will create two specialisations, one having the user
	Chani using the desktop environment Plasma, and the other with the user
	Paul using the desktop environment Gnome. Auto login at boot will be
	set for both users in their own specialisations. Our user Paul will
	need an extra system-wide package, for example dune-release.
	Specialisations can use any argument that would work in the top-level
	configuration, so we are not limited in terms of what can be changed.

	NixOS manual: Configuration options

	If you want to try, add the following code to your configuration.nix
	file.

	```nix
	specialisation = {
	chani.configuration = {
	system.nixos.tags = [ "chani" ];
	services.xserver.desktopManager.plasma5.enable = true;
	users.users.chani = {
	isNormalUser = true;
	uid = 1001;
	extraGroups = [ "networkmanager" "video" ];
	};
	services.xserver.displayManager.autoLogin = {
	enable = true;
	user = "chani";
	};
	};

	paul.configuration = {
	system.nixos.tags = [ "paul" ];
	services.xserver.desktopManager.gnome.enable = true;
	users.users.paul = {
	isNormalUser = true;
	uid = 1002;
	extraGroups = [ "networkmanager" "video" ];
	};
	services.xserver.displayManager.autoLogin = {
	enable = true;
	user = "paul";
	};
	environment.systemPackages = with pkgs; [
	dune-release
	];
	};
	};
	```

	After applying the changes, run "nix-rebuild boot" as root. Upon
	reboot, in the GRUB menu, you will notice a two extra boot entries
	named “chani” and “paul” just above the last boot entry for
	your non-specialised system.

	Rebuilding the system will also create scripts to switch from a
	configuration to another, specialisations are no exception.

	Run
	"/nix/var/nix/profiles/system/specialisation/chani/bin/switch-to-config
	uration switch" to switch to the chani specialisation.

	When using the switch scripts, keep in mind that you may not have
	exactly the same environment as if you rebooted into the specialisation
	as some changes may be only applied on boot.

	# Conclusion

	Specialisations are a perfect solution to easily manage multiple boot
	entries with different configurations. It is the way to go when
	experimenting with your system, or when you occasionally need specific
	changes to your regular system.