NixOS describes your computer in configuration files. This makes a setup reproducible, predictable and versioned: keep the files in Git, review changes, and reuse them elsewhere. Each rebuild also creates a bootable generation, so a broken change is easy to roll back.

Start with the system

The installer generates /etc/nixos/hardware-configuration.nix. Keep that hardware-specific file and put your choices in /etc/nixos/configuration.nix:

{ config, pkgs, ... }:

{
  imports = [ ./hardware-configuration.nix ];

  boot.loader.systemd-boot.enable = true;
  boot.loader.efi.canTouchEfiVariables = true;

  networking.hostName = "my-pc";
  services.desktopManager.cosmic.enable = true;
  services.displayManager.cosmic-greeter.enable = true;

  environment.systemPackages = with pkgs; [
    git
    ripgrep
  ];

  services.libinput.touchpad.naturalScrolling = true;

  users.users.alex = {
    isNormalUser = true;
    extraGroups = [ "networkmanager" "wheel" ];
  };

  system.stateVersion = "25.11"; # Keep the value created by the installer.
}

Apply it with:

sudo nixos-rebuild switch

On the next boot, systemd-boot lets you select an older NixOS generation if needed. If the machine also has Ubuntu installed, its bootloader entry is separate: select Ubuntu from the firmware or boot menu. COSMIC, by contrast, is a desktop environment enabled by the NixOS configuration above.

Manage Firefox with Home Manager

Home Manager applies the same declarative idea to your user account. Once it is installed, a minimal ~/.config/home-manager/home.nix can contain:

{ config, pkgs, ... }:

{
  home.username = "alex";
  home.homeDirectory = "/home/alex";

  programs.firefox = {
    enable = true;
    profiles.default.settings = {
      "browser.newtabpage.activity-stream.feeds.section.topstories" = false;
      "browser.newtabpage.activity-stream.showSponsored" = false;
      "browser.newtabpage.activity-stream.showSponsoredTopSites" = false;
    };
  };

  programs.home-manager.enable = true;
  home.stateVersion = "25.11"; # Keep your original value when upgrading.
}

Apply it with:

home-manager switch

Move to another PC

Commit configuration.nix and home.nix to Git, but generate a fresh hardware-configuration.nix on the new machine. Then copy or clone the shared files, adjust the hostname, username and any machine-specific options, and run:

sudo nixos-rebuild switch
home-manager switch

The new PC now has the same desktop, tools, browser preferences and system settings. If a rebuild causes trouble, reboot into the previous generation or run sudo nixos-rebuild switch --rollback.