How to determine which Linux partition w/ Grub controls the MBR?

CJS · 06-22-2008, 08:24 PM

If I have multiple linux distros installed in separate partitions on my HD, what is a good way to determine exactly which distro (partition) is controlling the MBR? I know I can figure it out by examining the menu.lst files for all the partitions, but is there some Grub command in the grub CLI that will just tell me what partition controls the MBR?

Thanks for any help.

syg00 · 06-22-2008, 08:47 PM

Not that I'm aware of - the code in the MBR only has relative sector number in it, not partition-id. You could eye-ball it and do the math yourself, but ...

tungvs · 06-22-2008, 08:49 PM

In my opinion, the MBR is independent of OSs. The last-installed OS write its code into the MBR region and the code runs itself, even when you format or delete the OS's partition - if the partition does not contain the MBR, of courses

. (reinstalling the OS or installing new OS will overwrite the MBR). So I think there's no OS "controls" the MBR.

CJS · 06-23-2008, 08:26 AM

Quote:

Originally Posted by syg00

Not that I'm aware of - the code in the MBR only has relative sector number in it, not partition-id. You could eye-ball it and do the math yourself, but ...

Well, I found out from another forum a solution to my question, and it is a true gem of pure linux voodoo:

Code:

sudo dd if=/dev/sda  bs=1 skip=1049 count=2 | hexdump

This command reads the necessary info from your boot drive's MBR. Of course replace /dev/sda with your boot drive if it is different. The output will look something like:

Code:

2+0 records in
2+0 records out
2 bytes (2 B) copied, 0.0177551 s, 0.1 kB/s
0000000 ff04                                   
0000002

The critical piece of info is the "FF04". "FF04" that means (hd0,4) because FF means the controlling linux partition is on the boot drive, and the 04 is the fifth partition. "FF02" is (hd0,2) or the third partition on the boot drive, "FF03" is fourth partition, etc.

If the controlling linux partition is on a HD other than the boot drive, the first two numbers XY in the four-digit hex sequence will correspond to (hdY,X), instead of being "FF". Thus if get "3103" that means (hd1,3) or 2nd HD, 4th partition. Or "5105" means (hd1,5) or 2nd HD, 6th partition.