Nixpkgs
Nixpkgs is the massive collection of Nix expressions that define essentially all software available for Nix. It's the standard library for Nix, containing tens of thousands of packages. When you interact with nixpkgs in your Nix expressions, you're using this vast resource.
How to Get Packages from Nixpkgs
The simplest way to use a package from Nixpkgs is to import it:
This evaluates an expression that imports Nixpkgs and makes its contents available as pkgs. The result will be a Nix store path for the hello package derivation.
You can also use nix build:
This will build and symlink hello into your current directory as result.
And nix run:
You could also run
./result/bin/hello.
This command tells Nix to run the hello executable from the hello package in Nixpkgs.
Searching Nixpkgs
You can search for packages directly from your terminal:
This command will list all packages in your Nixpkgs channel that contain "firefox" in their name or description. You'll likely see results like firefox and firefox-bin.
However, the CLI is slow and not convenient to use. You should use the Nixpkgs search instead.
This is the entry of Firefox in Nixpkgs:
How to install firefox?
You can use Nix as a traditional package manager:
This is not recommended as packages installed this way must be updated and maintained by the user in the same way as with a traditional package manager. To temporarily install a package for testing purposes, use nix-shell instead:
This will spawn a shell with firefox available. To permanently install a package with Nix, add it to your NixOS or Home Manager configuration. NixOS and Home Manager will be covered later in this guide.
pkgs.lib utility functions
The pkgs argument (or nixpkgs itself) isn't just a list of applications; it also provides a powerful utility library called pkgs.lib. This library contains helper functions for working with Nix expressions, strings, lists, and more.
Many of these functions are used extensively within Nixpkgs itself to define packages and modules. You can browse the full pkgs.lib documentation online for more details.
The Nixpkgs GitHub Repository
Nixpkgs is an open-source project hosted on GitHub: github.com/NixOS/nixpkgs. You can explore its source code to see how packages are defined. Every package definition is a Nix expression!
For example, you could find the definition for hello at pkgs/by-name/he/hello/package.nix. It uses stdenv.mkDerivation just like our example.
Binary Caches
Building everything from source every time can be slow. Nix solves this with binary caches. When someone builds a derivation, if that exact derivation (with its exact hash) has already been built and uploaded to a binary cache (like cache.nixos.org), Nix will simply download the pre-built binaries from the cache instead of building it locally.
This is possible because of the unique hashing of store paths. If the hash matches, the content must be identical, so a downloaded binary is guaranteed to be the same as one built locally. This significantly speeds up package installation and system updates.