How to Recover Linux Grub Boot Loader

Normally, the order of installation should be first windows then linux so that the grub boot loader is installed and recognizes the windows partition too.

At my job i had to do it the other way around: we had a system with Centos 7 linux and we needed to install Windows to make this machine we useful either to rigging either to production staff.

I inserted the windows dvd and made a new partition in the unallocated space then install.

When i restarted the machine the linux boot loader had disappeared so i was googling how to recover it. Truth is it's pretty simple and after a really few minor mistakes i managed to make it work:

We need first to enter in RESCUE mode with a linux installation dvd/cd.

The system will be most probably mounted in /mnt/sysimage so we make the

chroot /mnt/sysimage

after that we do:

/sbin/grub2-install /dev/sda

to make grub recognize our windows entry just type:

sudo grub2-mkconfig > /dev/null

If everything went well the windows loader entry has been detected, all we need now is to save it to the grub.cfg:

grub2-mkconfig -o /boot/grub2/grub.cfg

Restart and voilà, we have our boot loader back with our windows O.S. entry!!!