anchor.diff:      ''
         #! ${pkgs.runtimeShell}
   
  +      set -e
  +
         NIX_DISK_IMAGE=$(readlink -f "''${NIX_DISK_IMAGE:-${config.virtualisation.diskImage}}")
   
         if ! test -e "$NIX_DISK_IMAGE"; then
             ${qemu}/bin/qemu-img create -f qcow2 "$NIX_DISK_IMAGE" \
  -            ${toString config.virtualisation.diskSize}M || exit 1
  +            ${toString config.virtualisation.diskSize}M
         fi
   
         # Create a directory for storing temporary data of the running VM.
  -      if [ -z "$TMPDIR" -o -z "$USE_TMPDIR" ]; then
  +      if [ -z "$TMPDIR" ] || [ -z "$USE_TMPDIR" ]; then
             TMPDIR=$(mktemp -d nix-vm.XXXXXXXXXX --tmpdir)
         fi
   
  
positive.code:         ''
            # VM needs writable EFI vars
            if ! test -e "$NIX_EFI_VARS"; then
              cp ${bootDisk}/efi-vars.fd "$NIX_EFI_VARS" || exit 1
              chmod 0644 "$NIX_EFI_VARS" || exit 1
            fi
          ''}
        ''}
  
        cd "$TMPDIR" || exit 1
  
        ${lib.optionalString (cfg.emptyDiskImages != []) "idx=0"}
        ${flip concatMapStrings cfg.emptyDiskImages (size: ''
  
negative.code:       ''
          # Create a writable copy/snapshot of the boot disk.
          # A writable boot disk can be booted from automatically.
          ${qemu}/bin/qemu-img create -f qcow2 -F qcow2 -b ${bootDisk}/disk.img "$TMPDIR/disk.img"
  
          NIX_EFI_VARS=$(readlink -f "''${NIX_EFI_VARS:-${cfg.efiVars}}")
  
  