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Re-use a NixOS Derivation outside of Nix

The scenario

You're running NixOS. You want to do some development work on a project that uses a build system other than Nix. If you only use that build system for this one project, you may not want to install all of the tools it requires in NixOS. Instead, it would be nice to set up a nix shell that has those tools.

If you're not familiar with the build system, writing a Nix derivation may seem a daunting task. However, there is a package in NixOS for this project, so someone must have written a derivation. Perhaps you can use that?

Finding the derivation

First, we need to find that derivation. Go to https://github.com/NixOS/nixpkgs and click on the "Go to File" button near the top of the page. This activates the file finder. Type the name of the package. You're looking for something like:

pkgs/blah/blah/package-name/default.nix

For example, the derivation for the curl package is

pkgs/tools/networking/curl/default.nix

That directory may contain other files such as patches and scripts. If you only want a Nix shell (once you're in it you'll use the tools for that project's build system), the default.nix file is probably all you need. Copy that to the root directory of your project.

Some packages, such emacs or python3 have more complicated set-ups. These instructions assume you're dealing with a simpler package.

A slight wrinkle

If you try to enter a nix shell using the default.nix you got from NixOS/nixplkgs, you'll get an error message similar to this:

$ nix-shell
error: cannot evaluate a function that has an argument without a value ('lib')

       Nix attempted to evaluate a function as a top level expression; in
       this case it must have its arguments supplied either by default
       values, or passed explicitly with '--arg' or '--argstr'. See
       https://nixos.org/manual/nix/stable/#ss-functions.

       at /home/amy/waybar-amy/default.nix:1:3:

            1| { lib
             |   ^
            2| , stdenv

The problem is that this derivation was written to be used as part of a NixOS build, and it expects certain arguments to be supplied.

The solution

We can easily create a shell.nix that will inject all of the dependencies that default.nix expects. Here's what to put in it:

let
  pkgs = import <nixpkgs> {};
in
pkgs.callPackage ./default.nix {}

Now the nix-shell command should work as expected.

Want to know more?

The code above is a simplified version of how pkgs/top-level/all-packages.nix invokes the default.nixes for each package in NixOS. Youll find it here. This is a good file to become acquainted with if you want to contribute to NixOS.

Acknowledgements

Thank you to jtojnar on the Nix Discourse forum for this elegant solution!