0-readme.md

Boot into temporary NixOS system from any systemd system with nix installed

This builds a NixOS system, copies it into a tmpfs, then instructs systemd to shut down the main system and switch into the new one (without replacing the kernel).

nixos-tmpfs.sh

#!/usr/bin/env bash
set -exuo pipefail

mkdir -p /tmp/new-root-tmpfs
findmnt /tmp/new-root-tmpfs || mount -t tmpfs tmpfs /tmp/new-root-tmpfs
system=$(nix-build '<nixpkgs/nixos>' --arg configuration '{lib, ...}:
let ifExists = p: lib.optional (lib.pathExists p) p; in {
  boot.supportedFilesystems = ["zfs"];
  services.openssh.enable = true;
  networking.hostId = "23234242";
  networking.hostName = "nixos-tmpfs";
  networking.wireless.enable = true;
  networking.wireless.networks.ssid.psk = "password";
  fileSystems."/" = {
      device = "none";
      fsType = "tmpfs";
  };
  boot.loader.grub.enable = false;
  users.users.root.openssh.authorizedKeys.keyFiles =
    ifExists /etc/ssh/authorized_keys.d/root
    ++ ifExists /root/.ssh/authorized_keys;
}' -A config.system.build.toplevel)

nix copy $system --to /tmp/new-root-tmpfs --no-require-sigs
ln -s $system /tmp/new-root-tmpfs/system
mkdir -p /tmp/new-root-tmpfs/etc
rsync /etc/ssh/*host* /tmp/new-root-tmpfs/etc/ssh/

[[ $(read -p "Switch into the tmpfs system? ") = y ]] && systemctl switch-root /tmp/new-root-tmpfs /system/init