nixos/system/modules/boot.nix

{ config, pkgs, lib, ... }:

{
    boot = {
        loader = {
            systemd-boot.enable = true;
            efi.canTouchEfiVariables = true;
        };
        initrd.postDeviceCommands = lib.mkAfter ''
            mkdir /btrfs_tmp
            mount /dev/disk/by-label/NIXOS /btrfs_tmp
            if [[ -e /btrfs_tmp/root ]]; then
                mkdir -p /btrfs_tmp/old_roots
                timestamp=$(dat +%Y-%m-%d_%H-%M-%S)
                mv /btrfs_tmp/root "/btrfs_tmp/old_roots/$timestamp"
            fi

            delete_subvolume_recursively() {
                IFS=$'\n'
                for i in $(btrfs subvolume list -o "$1" | cut -f 9 -d ' '); do
                    delete_subvolume_recursively "/btrfs_tmp/$i"
                done
                btrfs subvolume delete "$1"
            }

            for i in $(find /btrfs_tmp/old_roots/ -maxdepth 1 -mtime +30); do
                delete_subvolume_recursively "$i"
            done

            btrfs subvolume create /btrfs_tmp/root
            umount /btrfs_tmp
        '';
    };
}