NixOS 15.09: declarative and reproducible Linux distribution

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An OS as code

Nix — package manager developed by Eelco Dolstra during his PhD (2003-2006, Utrecht University) — introduces a radical paradigm: every package lives in /nix/store/<hash>-name-version/, immutable, identified by the hash of its transitive dependencies. Two installations with the same inputs produce the same hash → reproducible builds.

NixOS, Linux distribution built on Nix, evolves from 2005. Release 15.09 (October 2015) consolidates the YY.MM naming scheme and production maturity. MIT licence.

Features

• Declarative configuration in /etc/nixos/configuration.nix:

{
    services.nginx.enable = true;
    services.postgresql.enable = true;
    users.users.alice = {
        isNormalUser = true;
        extraGroups = [ "wheel" ];
    };
}

nixos-rebuild switch applies the configuration. The entire system is deterministic: same configuration, bit-identical system.

• Atomic rollbacks — each system generation is a link; one can go back with nixos-rebuild --rollback
• Multiple coexisting versions — python-3.9 and python-3.11 on the same system, no conflicts
• User profiles — different users with different packages, no sudo
• Development shells — nix-shell -p foo bar creates temporary environment with packages

Reproducibility

Nix is reproducible builds-compliant: a package built from identical Nix files gives bit-identical output. Value for:

• Supply chain security — build integrity verification
• Scientific computing — analysis reproducibility
• DevOps — bit-identical dev/staging/prod
• Legacy bug fixing — exact reproduction of old build environments

NixOS, Guix, Nix standalone

• NixOS — complete distribution based on Nix
• Nix (standalone package manager) — installable on macOS, other Linux, WSL
• Guix (GNU) — Nix philosophy fork with Guile Scheme and FSF-pure focus

In the Italian context

Limited but growing adoption:

• Scientific HPC — for reproducibility
• Sophisticated DevOps — small Nix-managed clusters
• Individual developers — per-project dev environments
• Academic research — Universities of Turin, Trento, Pisa

Steep learning curve (functional Nix language is peculiar) but significant architectural impact.

References: NixOS 15.09 (October 2015). Eelco Dolstra, Utrecht University (PhD thesis 2006). Nix package manager. MIT licence. Content-addressed /nix/store. Declarative configuration.nix. Determinism and reproducible builds.