NixOS

NixOS is a declarative, reproducible Linux distribution built on top of the Nix package manager, created by Eelco Dolstra as part of his PhD research at Utrecht University. The latest release is version 26.05, and the project maintains Nixpkgs, the largest package repository in the world with over 140,000 packages spanning programming languages, system utilities, desktop applications, and development tools. The core innovation is a purely functional package management model where every package is built in an isolated environment with declared inputs, stored in a content-addressed store keyed by a cryptographic hash of all build inputs and the build recipe. This guarantees reproducibility, meaning a package that builds successfully on one machine will produce byte-for-byte identical output on any other machine. The entire system configuration is declared in a single functional configuration file using the Nix expression language, specifying packages, services, users, filesystem mounts, boot configuration, and network settings. Changes to this file produce a new system generation rather than modifying the running system, enabling atomic upgrades that cannot leave the system in a broken state and trivial rollback to any previous generation by selecting it from the boot menu. The multi-user mode allows non-privileged users to install packages without root access, and multiple versions of the same package can coexist without conflicts. Nix Flakes provide reproducible evaluation and hermetic builds with locked input versions. Nix Shell creates temporary development environments with specific tool versions per project. Used by CERN for scientific computing, Mozilla for build infrastructure, Replit for reproducible development environments, and Antithesis for deterministic testing. Open source under the LGPL license.