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NixOS

A purely functional Linux distribution built on Nix.

NixOS treats the entire system configuration as a build product of a Nix expression: every package, the kernel, services, networking, users, and firewall rules are reproducibly defined in code (~/etc/nixos/configuration.nix), and "nixos-rebuild switch" applies a new configuration as a fresh generation that can be rolled back atomically. There is no concept of imperative state drift — running a command on a NixOS system to install a package is not how you install a package.

The Nix package manager underneath stores each package in /nix/store keyed by a hash of its build inputs. Multiple versions of the same package coexist trivially; environments expose a chosen subset of those packages on PATH. Nix Flakes (experimental but widely used) add reproducible inputs and cleaner dependency management.

Eelco Dolstra developed Nix and NixOS during his PhD at TU Delft, with the first public release around 2003. The community has grown substantially in the last few years; the Nixpkgs collection has more than 100,000 packages, and NixOS is widely used in DevOps and reproducible-build settings.

Download an ISO from https://nixos.org/download/
Or install Nix on another distribution:
  sh <(curl -L https://nixos.org/nix/install) --daemon