Howto: Re-establishing boot
process from MBR
==================================
Introduction:
--------------
In a multi-OS boot system, booting is done from MBR with Grub or such a
boot-loader. In case the MBR is corrupted or over-written because of any
reason, such as re-installing Windows, the booting through Grub can be restored
from MBR, without need to re-install Linux.
Steps:
------
1. Download an LFS live DVD image from www.linuxfromscratch.org, and burn the
iso image onto a DVD. (This is a very handy tool and, hence, should always be
available). If it is not with you presently, use your windows or other
installation to download and burn. Boot from the live LFS DVD.
2. Type the command 'net-setup' and setup your internet and download or read
online the LFS Book. Open with lynx for reference, if
needed.
3. Give the following commands in succession: (These commands are from the LFS
book - only selected here for the limited job to be done)
export LFS=/mnt/lfs
mount -v -t ext3 /dev/ $LFS (where is the
partition on which your linux exists that you wish to boot.)
/sbin/swapon -v /dev/ (where is the
swap partition on your hard disk.)
mount -v --bind /dev $LFS/dev (mounting and
populating /dev)
mount -vt devpts devpts $LFS/dev/pts (mounting virtual
kernel File System)
mount -vt tmpfs shm $LFS/dev/shm
mount -vt proc proc $LFS/proc
mount -vt sysfs sysfs $LFS/sys
chroot "$LFS" $LFS (entering
the chroot environment - this command will show root prompt on the
partition that was mounted above. Check and ensure that it is the right one.)
/bin/bash --login +h (shall
bring the same prompt as above but with its environment)
4. Installing Grub on to MBR: Give following commands:
grub
(Note that grub does not recognize scaci drives etc. For it everything is 'hd'.
So
if you have drive such as 'sda', for grub it shall remain as 'hd0' only)
root (hdx,z)
(Where 'x' is the number of the drive starting at zero and 'z' is the number of
linux partition on that drive, also starting with zero.)
setup hdx (If
you had a working system before the mishap and you already had the
'stage1' and 'stage2' in place, grub will show success.)
5. Unmount the various virtual kernel file systems mounted at Step 3 above as
also the $LFS partition and reboot the system.
6. The system should boot normally unless something else had also gone wrong.
The things can go wrong in that the splash screen may not be the same what you
had earlier. For example, I boot from openSUSE and its splash screen is
different but what I got after rebooting above was different and after booting
with that grub menu I got kernel panic at some stage of booting. But the 'grub'
shall be available and you could give it the commands by pressing 'c' on your
keyboard. Like earlier, give it the following commands, in succession:
root (hdx,z)
kernel (hdx,z)/boot/vmlinuz (or whatever is the kernel name in
your boot partition)
initrd (hdx,z)/boot/initrd (or the specific name of your
initrd file in boot partition)
boot
The system should boot. After booting the system you could carryout whatever
further correction you wish to do. For example in my case, the X refused to
start. So I had to setup grub again here through 'YAST'. After that the normal
booting came up with its normal splash etc.
Conclusion:
---------------
If any such mishap ocurs, there is no need to panick. The good old LFS is there
for your rescue. Needless to say that there are a number of other means
available for rescue. Personally, however, I find this simple. I have used
these steps a number of times, mainly because of the need to re-install windows
Vista, which goes off pretty often, in my case.
Thought this may be of some use to the community.
Regards
Randhir Phagura
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