NixOS

NixOS is a Linux distribution based upon the Nix package manager. The idea is to build a immutable system from a modular and purely functional specification. Software, configuration files, scripts, etc. are generated from a specification. This method of creating a system ensures that everything comes up deterministically and reproducibly. Alterations to configuration files are also defined in this specification, allowing for easy tracability of configuration file changes. Upgrades involve updating the specification file and then rebuilding the system which in essence allows software to update atomically.

This is in contrast to the conventional way of managing a system: an imperative way where where packages are installed and updated in place and configuration files are created and edited after installation.

Nix

Central to the idea of NixOS is the Nix package manager. Packages (also known as derivations) are stored under /nix/store/hash-name-version, where a cryptographic hash uniquely identifies the derivation. All files under the Nix store are immutable. For example, bash would be in /nix/store/s4zia7hhqkin1di0f187b79sa2srhv6k-bash-4.2-p45/, under its own bin/bash directory. NIX will arrange binaries in your PATH appropriately.

This method of handling software packages allows for multiple versions of the same software package to be installed. Any dependencies are handled by installing the exact version of the dependency and then ensuring the proper link. This means that you could run mysql 5.2 with glibc-2.18, and mysql 5.5 with glibc-2.19 all in the same system.

Package upgrades are handled by installing the newer version on the system and then switching to that newer version.

The Nix database is stored at /nix/var/nix/db

Profile

A profile in Nix gathers separate components spread among different paths into a unified path. They are versioned (known as a generation) that allows for easy roll back.

Profiles are stored under ~/.nix-profile which is a symlink to /nix/var/nix/profiles/default. Different generations of the profile are stored /nix/var/nix/profiles/.

nix-env handles the profiles and all the symlinks in this location. paths are also merged by nix-env.

Introduction

Various commands that were introduced in Chapter 3 of the Nix Pills.

Command	Description
nix-env	Manages environments, profiles, and their generations
nix-env -i hello	installs the hello environment
nix-env --list-generations	lists all environment generations
nix-env -q	lists all derivations
nix-env -q --references `which hello`	shows all derivations that are required by the given binary
nix-env -q --referrers `which hello`	shows all derivations that are dependent on the given binary
nix-env --rollback	rolls back the generation by one
nix-env -G 3	sets the environment generation to the given generation (3)
nix-env -u	upgrades all packages in the environment
nix-env -e '*'	Uninstalls everything from the current environment. You can recover this by running nix-env from the nix store and reinstalling the nix-env derivative or roll back the environment generation.

Closures of a derivation is a recursive list of all dependencies that are required. nix-store -qR `which man` or nix-store -q --tree `which man`

Channels are basically like repos.

Command	Description
nix-channel --list	lists all channels. This is configured in ~/.nix-channels
nix-channel --update	Synchronizes the channel repo data