NixOS
also: Nix
A Linux distribution built on the Nix package manager, where the entire system configuration is declared in a single reproducible configuration file, enabling atomic upgrades and rollbacks.
NixOS is a unique Linux distribution that treats your entire system—kernel, packages, services, and configuration—as reproducible code. Instead of imperative package installation and configuration files scattered across the filesystem, NixOS uses declarative configuration written in the Nix language, typically in /etc/nixos/configuration.nix.
When you modify your configuration and run nixos-rebuild switch, NixOS builds a new system generation without affecting your current one. This enables powerful features like atomic upgrades (the system flips to the new version instantly) and instant rollbacks if something breaks—you can boot into a previous generation from the GRUB menu.
For example, instead of manually installing PostgreSQL and editing config files, you simply add services.postgresql.enable = true; to your configuration, rebuild, and the entire database service is reproducibly configured. This approach also makes system configurations portable and shareable across machines.