nixos build virtualbox image for dummies

default.nix

# This file describes how to build custom virtualbox instances. This examples ignores things like cross compiling etc..

# this defines the function signature of our expression, pkgs is defaulted to the systems packages,
# you could also overrides that if you need a specific release / channel.
{ pkgs ? import <nixpkgs> {}}:
# "let" expressions are just there to introduce scoped variables and functions,
# in this case it is declaring just a helper function
let
  # this creates a helper function that abstracts a bit of the boilerplate away
  # `mkVM` takes one argument, a list of `modules` to include in the image.
  mkVM = mods: (import <nixpkgs/nixos/lib/eval-config.nix> {
    modules = [
      # include the standard virtualbox demo install that probably includes things like KDE or whatever
      <nixpkgs/nixos/modules/installer/virtualbox-demo.nix>

    ] ++ mods;
  }).config.system.build.virtualBoxOVA;
in
# here starts the actual declaration of "target" included in this file.
{
  # delcares the target of an example image with vim and tmux
  # build with: nix-build . -A withTmuxAndVIM
  withTmuxAndVIM = mkVM [
      # add some custom pkgs like vim and tmux, could be our internal packages
      ({pkgs, ...}: {
        environment.systemPackages = with pkgs; [ vim tmux ];
      })
  ];
}