Textbook of Linux

NixOS is a Linux distribution built around the Nix package manager, taking its reproducible-build principles to their logical conclusion: the entire operating system is described by a single declarative configuration file, and every state of the system (packages installed, services running, kernel version) is reproducible from that configuration. Rolling back to a previous system state is as easy as selecting an older generation at the bootloader.

NixOS makes several radical choices:

• Packages live in /nix/store/<hash>-name-version/, never in /usr/bin or /usr/lib. Binaries are symlinked into user profiles or service environments.
• System configuration lives in /etc/nixos/configuration.nix as a single declarative file in the Nix expression language.
• nixos-rebuild switch applies changes atomically; failures roll back automatically.
• Multiple versions of the same package coexist without conflict.

# /etc/nixos/configuration.nix excerpt
{ config, pkgs, ... }: {
  environment.systemPackages = with pkgs; [ vim git htop ];
  services.nginx.enable = true;
  networking.firewall.allowedTCPPorts = [ 80 443 ];
}

The payoff is reproducibility: if your Nix configuration builds on one machine, it builds the same way on another. The cost is a steep learning curve—Nix's language and mental model are genuinely different. NixOS attracts users who value reproducibility highly: research computing, immutable infrastructure, developer environments.