让Ubuntu Grub2 引导上一次正确启动的内核，需要设置两个变量，如下:
/etc/default/grub
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true
然后运行update-grub命令。
参考
Saved
Saving an OS can be achieved by running sudo grub-set-default ifGRUB_DEFAULT=saved is set in /etc/default/grub. It may also be saved if GRUB_SAVEDEFAULT=true is also set in /etc/default/grub. In this case, the default OS remains until a new OS is manually selected from the GRUB 2 menu or thegrub-set-default command is executed.