[ -z "$KERNEL" ] && KERNEL="/vmlinuz"
[ -z "$MKFS" ] && MKFS="mke2fs -F"
[ -z "$MOUNT" ] && MOUNT="$SUDO mount -o loop"
[ -z "$TUNE2FS" ] && TUNE2FS="tune2fs"
[ -z "$UMOUNT" ] && UMOUNT="$SUDO umount"