Nix shell to apptainer

Preface

Previously I have written about setting up nix to run on the University of Iowa’s compute cluster. At the time I noted some issues with this approach. These made the approach unusable long term and I needed another solution. The new solution I ended up adopting was building apptainer¹ images from nix-shell definitions.

The Method

This is carried out in two steps. First we use dockerTools.buildNixShellImage to construct a docker image then convert this to an apptainer image.

The following example is from my own projects. I use npins and standalone shell.nix files. The same idea works with a flake, you just have to juggle the stuff around a bit. Here we create a new file container.nix:

{
  system ? builtins.currentSystem,
  pins ? import ./npins,
  pkgs ? import pins.nixpkgs { inherit system; },
}:
pkgs.dockerTools.buildNixShellImage {
  tag = "latest";
  drv = import ./shell.nix { };
  compressor = "none";
}

We then build the container with nix-build container.nix. Our docker image now lives in the nix store somewhere with the link result pointing to it. We can now convert this to apptainer:

apptainer \
  pull \
  container.sif \
  docker-archive://$(readlink result)

This approach does not spark joy, but it works and that’s what I needed right now.

Why didn’t you just —

I tried other stuff, it didn’t work.