Use grub to fix boot failure of virtualbox Ubuntu guest

instruction.md

1. Refer to this tutorial

2. Commands:

   grub> set pager=1
   grub> ls
   (hd0) (hd0,msdos2) (hd0,msdos1)
   grub> ls (hd0,1)/
   lost+found/ bin/ boot/ cdrom/ dev/ etc/ home/  lib/
   lib64/ media/ mnt/ opt/ proc/ root/ run/ sbin/ 
   srv/ sys/ tmp/ usr/ var/ vmlinuz vmlinuz.old 
   initrd.img initrd.img.old
   grub> cat (hd0,1)/etc/issue
   Ubuntu 14.04 LTS \n \l
   grub> set root=(hd0,1)
   grub> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
   grub> initrd /boot/initrd.img-3.13.0-29-generic
   grub> boot
   

3. After reboot, fix the grub permenantly:

   # update-grub
   Generating grub configuration file ...
   Found background: /usr/share/images/grub/Apollo_17_The_Last_Moon_Shot_Edit1.tga
   Found background image: /usr/share/images/grub/Apollo_17_The_Last_Moon_Shot_Edit1.tga
   Found linux image: /boot/vmlinuz-3.13.0-29-generic
   Found initrd image: /boot/initrd.img-3.13.0-29-generic
   Found linux image: /boot/vmlinuz-3.13.0-27-generic
   Found initrd image: /boot/initrd.img-3.13.0-27-generic
   Found linux image: /boot/vmlinuz-3.13.0-24-generic
   Found initrd image: /boot/initrd.img-3.13.0-24-generic
   Found memtest86+ image: /boot/memtest86+.elf
   Found memtest86+ image: /boot/memtest86+.bin
   done
   # grub-install /dev/sda
   Installing for i386-pc platform.
   Installation finished. No error reported.
   

4. If you're in the GRUB rescue shell the commands are different, and you have to load the normal.mod andlinux.mod modules:

   grub rescue> set prefix=(hd0,1)/boot/grub
   grub rescue> set root=(hd0,1)
   grub rescue> insmod normal
   grub rescue> normal
   grub rescue> insmod linux
   grub rescue> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
   grub rescue> initrd /boot/initrd.img-3.13.0-29-generic
   grub rescue> boot