Title: Make nix flakes commands using the same nixpkgs as NixOS does

	Title: Make nix flakes commands using the same nixpkgs as NixOS does
	Author: Solène
	Date: 20 July 2022
	Tags: nixos linux nix
	Description: This explains how to make the command nix shell
	nixpkgs#foobar to reuse nixpkgs from your NixOS system to save time and
	bandwidth

	# Introduction

	This article will explain how to make the flakes enabled nix commands
	reusing the nixpkgs repository used as input to build your NixOS
	system. This will regularly save you time and bandwidth.

	# Flakes and registries

	By default, nix commands using flakes such as nix shell or nix run are
	pulling a tarball of the development version of nixpkgs. This is the
	default value set in the nix registry for nixpkgs.

	```shell
	$ nix registry list \| grep nixpkgs
	global flake:nixpkgs github:NixOS/nixpkgs/nixpkgs-unstable
	```

	Because of this, when you run a command, you are likely to download a
	tarball of the nixpkgs repository including the latest commit every
	time you use flakes, this is particularly annoying because the tarball
	is currently around 30 MB. There is a simple way to automatically set
	your registry to define the nixpkgs repository to the local archive
	used by your NixOS configuration.

	To your `flake.nix` file describing your system configuration, you
	should have something similar to this:

	```nix
	inputs.nixpkgs.url = "nixpkgs/nixos-unstable";

	[...]
	nixosConfiguration = {
	my-computer =lib.nixosSystem {
	specialArgs = { inherit inputs; };
	[...]
	};
	};
	```

	Edit /etc/nixos/configuration.nix and make sure you have "inputs"
	listed in the first line, such as:

	```nix
	{ lib, config, pkgs, inputs, ... }:
	```


	```nix
	nix.registry.nixpkgs.flake = inputs.nixpkgs;
	```

	After this change, running a command such as "nix shell
	nixpkgs#gnumake" will reuse the same nixpkgs from your nix store used
	by NixOS, otherwise it would have been fetching the latest archive from
	GitHub.

	# nix-shell vs nix shell

	If you started using flakes, you may wonder why there are commands
	named "nix-shell" and "nix shell", they work totally differently.

	nix-shell and non flakes commands use the nixpkgs offered in the
	NIX_PATH environment variable, which should be set to a directory
	managed by nix-channel, but the channels are obsoleted by flakes...

	Fortunately, in the same way we synchronized the system flakes with the
	commands flakes, you can add this code to use the system nixpkgs with
	your nix-shell:

	```nix
	nix.nixPath = [ "nixpkgs=/etc/channels/nixpkgs" "nixos-config=/etc/nixos/config…
	environment.etc."channels/nixpkgs".source = inputs.nixpkgs.outPath;
	```

	This requires your user to logout from your current session to be
	effective. You can then check nix-shell and nix shell use the same
	nixpkgs source with this snippet. This asks the full path of the test
	program named "hello" and compares both results, they should match if
	they use the same nixpkgs.

	```shell
	[ "$(nix-shell -p hello --run "which hello")" = "$(nix shell nixpkgs#hello -c w…
	```

	# Conclusion

	Flakes are awesome, and are in the way of becoming the future of Nix. I
	hope this article shed some light about nix commands, and saved you
	some bandwidth.

	# Credits

	I found this information on a blog post of the company Tweag (which is
	my current employer) in a series of articles about Nix flakes. That's
	a bit sad I didn't find this information in the official NixOS
	documentation, but as flakes are still experimental, they are not
	really covered.

	Tweag blog: Nix Flakes, Part 3: Managing NixOS systems

	As I found this information on their blog post, and I'm fine giving
	credits to people, so I have to link their blog post license here.

	Creative Commons Attribution 4.0 International license