As soon as we finish installing Nix on Darwin, we’re greeted with a call to action:

Alright! We're done!
Try it! Open a new terminal, and type:

  $ nix-shell -p nix-info --run "nix-info -m"

Thank you for using this installer. If you have any feedback or need
help, don't hesitate:

You can open an issue at

Hello world (bloated)

All right then, let’s do it!

$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"aarch64-darwin"`
 - host os: `Darwin 21.3.0, macOS 12.2`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.6.0`
 - channels(root): `"nixpkgs"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixpkgs`

Cool, it works. Let’s break it down a bit.

Hello world (classic)

Nix shell creates an ephemeral shell environment with the customizations you want. The most basic customization is to make a given set of packages available. There’s a hello package:

$ nix-shell -p hello
$ hello
Hello, world!

In case you’re curious, this is a GNU binary:

$ hello --version
hello (GNU Hello) 2.10

Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

I have no idea why they are in version 2.10 and what their changelog is. It’s such a simple binary…

If you exit the shell, hello seemingly vanishes:

$ exit
$ hello
zsh: command not found: hello

An easy way to think of nix-shell is like an ephemeral sandbox where all your desired packages are made available when you enter it. It’s possible to provide more than one package, naturally. It’s also possible to provide a shell.nix file with the package declarations, so that when you can nix-shell without any arguments.

$ cat shell.nix
{ pkgs ? import <nixpkgs> {} }:
  pkgs.mkShell {
    # nativeBuildInputs is usually what you want -- tools you need to run
    nativeBuildInputs = [ pkgs.buildPackages.hello ];
$ nix-shell
$ hello
Hello, world!

Hello world (oneshot)

$ nix-shell -p hello --run hello
Hello, world!

This oneshot style doesn’t enter the shell, it just runs the given --run command and then exits.

This post just scratched the surface of what nix-shell can do. See the references below for more in-depth guides about it.