Fix broken Linux vm without restoring / re-installing

Hi folks
If you use Linux VM's (or Hosts) and you need to fix bootloader etc after say copying partitions -- you want to move / increase say '/' (the root) partition then you can often do it easily by simple command chroot which uses a "temporary" root system on a device or file.

Easy enough to do

say I'm running on /dev/sdb but I want to reinstall the boot loader on /dev/sda

1) as root mount /dev/sda2 /mnt (where /dev/sda2 is your root partition you want to use

2) run this next :
sudo mount --bind /dev /mnt/dev &&
sudo mount --bind /dev/pts /mnt/dev/pts &&
sudo mount --bind /proc /mnt/proc &&
sudo mount --bind /sys /mnt/sys


You need those to access the programs etc


now simply chroot /mnt /bin/bash

Note if you need network access inside chroot do this BEFORE the chroot command. Also if you have a spearate /boot and /efi partition that you want to use mount them on /mnt before the chroot too.

cp -L /etc/resolv.conf /mnt/etc/resolv.conf

Then you can re-install grub/grub2 , reinstall kernel etc etc
after -- simply reboot although better first to "unbind" everything

so exit root by typing exit

sudo umount /mnt/sys &&
sudo umount /mnt/proc &&
sudo umount /mnt/dev/pts &&
sudo umount /mnt/dev &&
sudo umount /mnt

reboot your repaired device.


cheers
jimbo