# Preface {#preface}

The Nix Packages collection (Nixpkgs) is a set of thousands of packages for the
[Nix package manager](https://nixos.org/nix/), released under a
[permissive MIT license](https://github.com/NixOS/nixpkgs/blob/master/COPYING).
Packages are available for several platforms, and can be used with the Nix
package manager on most GNU/Linux distributions as well as [NixOS](https://nixos.org/nixos).

This document is the user [_reference_](https://nix.dev/contributing/documentation/diataxis#reference) manual for Nixpkgs.
It describes the entire public interface of Nixpkgs in a concise and orderly manner, and all relevant behaviors, with examples and cross-references.

To discover other kinds of documentation:
- [nix.dev](https://nix.dev/): Tutorials and guides for getting things done with Nix
- [NixOS **Option Search**](https://search.nixos.org/options) and reference documentation
- [Nixpkgs **Package Search**](https://search.nixos.org/packages)
- [**NixOS** manual](https://nixos.org/manual/nixos/stable/): Reference documentation for the NixOS Linux distribution
- [`CONTRIBUTING.md`](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md): Contributing to Nixpkgs, including this manual

## Overview of Nixpkgs {#overview-of-nixpkgs}

Nix expressions describe how to build packages from source and are collected in
the [Nixpkgs repository](https://github.com/NixOS/nixpkgs). Also included in the
collection are Nix expressions for
[NixOS modules](https://nixos.org/nixos/manual/index.html#sec-writing-modules).
With these expressions the Nix package manager can build binary packages.

Packages, including the Nix packages collection, are distributed through
[channels](https://nixos.org/nix/manual/#sec-channels). The collection is
distributed for users of Nix on non-NixOS distributions through the channel
`nixpkgs-unstable`. Users of NixOS generally use one of the `nixos-*` channels,
e.g., `nixos-22.11`, which includes all packages and modules for the stable NixOS
22.11. Stable NixOS releases are generally only given
security updates. More up-to-date packages and modules are available via the
`nixos-unstable` channel.

Both `nixos-unstable` and `nixpkgs-unstable` follow the `master` branch of the
Nixpkgs repository, although both do lag the `master` branch by generally
[a couple of days](https://status.nixos.org/). Updates to a channel are
distributed as soon as all tests for that channel pass, e.g.,
[this table](https://hydra.nixos.org/job/nixpkgs/trunk/unstable#tabs-constituents)
shows the status of tests for the `nixpkgs-unstable` channel.

The tests are conducted by a cluster called [Hydra](https://nixos.org/hydra/),
which also builds binary packages from the Nix expressions in Nixpkgs for
`x86_64-linux`, `aarch64-linux`, and `aarch64-darwin`.
The binaries are made available via a [binary cache](https://cache.nixos.org).

The current Nix expressions of the channels are available in the
[Nixpkgs repository](https://github.com/NixOS/nixpkgs) in branches
that correspond to the channel names (e.g., `nixos-22.11-small`).
