Nixos-shell

Nixos-shell is a small helper script for spawning lightweight NixOS virtual machines in a shell.

Installation

Add following line to your system configuration to install the program

environment.systemPackages = [ pkgs.nixos-shell ];

Usage

Simple port forward

Create a single example file containing the system configuration for the virtual machine

myvm.nix

{ pkgs, ... }: {
  services.dokuwiki.sites."localhost" = {
    enable = true;
    settings.title = "My Wiki";
  };
};

In this example, we'll have a virtual guest machine running an instance of DokuWiki on port 80. Start the VM while forwarding port 8080 on the host to port 80 on the guest

QEMU_NET_OPTS="hostfwd=tcp::8080-:80" nixos-shell myvm.nix

After the VM is successfully booted, DokuWiki will be available on http://localhost:8080

Mounting host directories

This snippet mounts the directory calendar which resides in the working directory where you run nixos-shell on the host. It gets mounted to /var/lib/nextcloud/store-apps/calendar on the guest. The target directory must exist before mounting gets executed.

myvm.nix

{ pkgs, ... }: {
  nixos-shell.mounts.extraMounts = {
    "/var/lib/nextcloud/store-apps/calendar" = {
       target = ./calendar;
       cache = "none";
    };
  };
};

Mounting is done through the network filesystem protocol 9p. Currently it's not possible to mount the target directory with a specific UID/GID, so you'll have to change the permissions on the host directory according to your needs.

Inside Nix Flake

Using following Flakes example, you can start a virtual machine using nixos-shell by just typing nix run

flake.nix

{
  description = "Spawns lightweight nixos vm in a shell";

  inputs.nixpkgs.url = "nixpkgs/nixos-23.11";

  outputs = { self, nixpkgs }: let
    pkgs = nixpkgs.legacyPackages.x86_64-linux;
    start =
      pkgs.writeShellScriptBin "start" ''
        set -e
        export QEMU_NET_OPTS="hostfwd=tcp::8080-:80"
        ${pkgs.nixos-shell}/bin/nixos-shell --flake .
      '';
  in {

    nixosConfigurations.vm = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        (import ./myvm.nix)
        self.nixosModules.nixos-shell
      ];
    };

    nixosModules.nixos-shell = import ./share/modules/nixos-shell.nix;

    packages = { inherit start; };
    defaultPackage.x86_64-linux = start;

  };
}

The configuration of the virtual machine is inside the file myvm.nix in the same directory. The virtual machine will use the nixpkgs source defined in the flake inputs.