.. _ref-pinning-nixpkgs:

Pinning Nixpkgs
===============

Different ways:

- As environment variable: ``$NIX_PATH=URL``

- ``-I`` command line parameter to most of commands like ``nix-build``, ``nix-shell``, etc

- Using `builtins.fetchTarball <https://nixos.org/nix/manual/#ssec-builtins>`_ function that fetches the channel at evaluation time


Possible ``URL`` values
-----------------------

- Local file path. Using just ``.`` means that nixpkgs is located in current folder.

- Pinned to a specific commit: ``https://github.com/NixOS/nixpkgs/archive/addcb0dddf2b7db505dae5c38fceb691c7ed85f9.tar.gz``

- Using latest channel, meaning all tests have passed: ``http://nixos.org/channels/nixos-21.05/nixexprs.tar.xz``

- Using latest channel, but hosted by github: ``https://github.com/NixOS/nixpkgs/archive/nixos-21.05.tar.gz``

- Using latest commit for release branch, but not tested yet: ``https://github.com/NixOS/nixpkgs/archive/release-21.05.tar.gz``


Examples
--------

- ``nix-build -I ~/dev``
- ``nix-build -I ~/dev``
- ``nix-build -I nixpkgs=http://nixos.org/channels/nixos-21.05/nixexprs.tar.xz``
- ``NIX_PATH=nixpkgs=http://nixos.org/channels/nixos-21.05/nixexprs.tar.xz nix-build ...``
- Using just Nix::

    let
      pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/nixos-21.05.tar.gz") {};
    in pkgs.stdenv.mkDerivation { … }

- To make ad-hoc environment available on NixOS: ``nix.nixPath = [ ("nixpkgs=" + toString pkgs.path) ];``