NixOS
NixOS is an independently developed GNU/Linux distribution that aims to improve the state of the art in system configuration management. In NixOS, the entire operating system, including the kernel, applications, system packages and configuration files, are built by the Nix package manager.
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Vanilla OS
Vanilla OS, a fixed-release distribution based on Debian (formerly Ubuntu, prior to version 2) with the GNOME desktop, is an immutable operating system. The core parts of the system are locked down to prevent unwanted changes and corruption from third-party applications or a faulty update.