How to restore the Ubuntu boot item after Windows is reinstalled

Ubuntu + Windows multi-system boot is completed by grub of ubuntu by default. If we reinstall Windows, Windows will change the Master Boot Record to its own, at this time, the startup Item of Ubuntu cannot be found, and we need to manually restore the grub boot. Next I will introduce how to restore the Ubuntu boot item after Windows is reinstalled:

Use ubuntu live USB or livd cd to enter the ubuntu live environment, and enter sudo fdisk-l in the terminal to view your ubuntu Partition Number, it is usually/dev/sdax (X is a number, whichever is your own)

Run sudo mkdir/media/ubuntu.
Mount partition: sudo/dev/sdaX mount/media/ubuntu
Restore grub: sudo grub-install-root-directory =/media/ubuntu // dev/sda (note that there is no X)

If the execution is complete and there is no error, grub will be installed again. After the restart, go to ubuntu through grub and run sudo update-grub to update the startup list. Then, go to Win.