Restoring grub on MBR

Wolfgang Messingschlager w.mess at freenet.de
Sat Sep 20 09:00:30 PDT 2008


randhir phagura wrote:
>                                                   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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> http://www.live.com/?scope=video&form=MICOAL


Hi Randir,


very interesting info. But if you install often or sometimes Windows I 
prefer to create the partitions on Linux and then save the MBR in a file 
with the dd command e.g.

TS=`date +%Y%m%d`
dd if=/dev/hda of=/boot/mbr_$TS bs=512 count=1

This can be done on a regular basis e.g. as a cron job.

Then you can easily restore your MBR with dd as well e.g.

dd if=/boot/mbr_... of=/dev/hda bs=512 count=1


Wolfgang



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