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:

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.