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How can I tell if grub is installed on a disk, and if it is what settings it has (noteably, what it has for the root parameter)

I need to check a lot of disks in software RAID1 arrays to make sure both disks have grub installed, with the grub on each disk having the appropriate root value.

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2 Answers 2

up vote 16 down vote accepted

you can use file(1) to identify GRUB in an MBR. e.g.

# file -s /dev/sda
/dev/sda: x86 boot sector; GRand Unified Bootloader, stage1 version 0x3, stage2 address 0x2000, stage2 segment 0x200; partition 1: ID=0xfd, starthead 1, startsector 63, 1044162 sectors; partition 2: ID=0x82, starthead 0, startsector 1044225, 1028160 sectors; partition 3: ID=0xfd, starthead 0, startsector 2072385, 1951447680 sectors, code offset 0x48

The root= paramater is not stored in the MBR, that's stored in GRUB's menu.lst file which is stored on a file-system (typically in the /boot/grub directory of the root fs or the grub directory of the /boot filesystem - but not always, it could be anywhere).

you'll have to parse the output of file above, determine which disk/partition the menu.lst file is on, mount it, read it in and parse it. You'll also want to read in the grub/default file to figure out which grub menu entry is the default, because that's probably the one that has the root= parameter that you're most interested in.

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In that case I need a way to make sure grub is looking for menu.lst in the proper place... somewhere the boot loader must be told which disk & partition it has to go to so it can get the boot menu. –  DrStalker Sep 2 '09 at 5:51
    
there's a lot of useful info about grub at the grub home page gnu.org/software/grub , the grub wiki grub.enbug.org , and at wikipedia en.wikipedia.org/wiki/GRUB –  cas Sep 2 '09 at 21:31
1  
This is no longer correct with grub2. See @benrifkah's answer instead. –  Paul Tomblin Apr 8 '13 at 21:19

file -s didn't work for me in Ubuntu Lucid. Another way to do this is to examine the first 512 bytes of the device using the dd command like this:

user@host:~$ sudo dd bs=512 count=1 if=/dev/sda 2>/dev/null | strings
ZRr=
`|f
\|f1
GRUB
Geom
Hard Disk
Read
 Error

This sends the output of the dd command through the strings command thus stripping out unprintable characters (transfer statistics get discarded to /dev/null).

If you see any messages that GRUB would display when it encounters failure then you've got grub installed.

Hat tip to louib on ubuntuforums.org for answering in this password protected post: http://ubuntuforums.org/showthread.php?t=363372

For the curious this is what I got from file -s:

user@host:~$ sudo file -s /dev/sda
/dev/sda: x86 boot sector; partition 1: ID=0x83, active, starthead 32, startsector 20
48, 337211392 sectors; partition 2: ID=0x5, starthead 254, startsector 337215486, 1434214
6 sectors, code offset 0x63

There's nothing about GRUB in there specifically.

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3  
Spot on. The accepted answer didn't work for me on several machines, but yours did. –  Cerin Aug 5 '11 at 3:35
5  
The accepted answer was written in 2009 and (as should be obvious from the mention of menu.lst rather than grub.cfg) was for grub1 aka "grub-legacy". obviously things have changed since then, and grub2 doesn't put an identifier in the MBR. if it worked on some machines for you but not on others, that would imply you have grub1 on some and grub2 on others. –  cas Sep 11 '11 at 23:09
    
Thanks for the clarifications Craig. The system I originally tried the file -s command on uses grub-pc which is apparently GRUB2 so it sounds like you're partially correct. However, I ran file -s on some other systems running GRUB 0.97 and on some it worked while on others it didn't. The difference between these systems was in the numbers in the magic files that come with the file command. Swapping magic number files confirmed that the definitions from the non-working system caused the working system to fail. –  benrifkah Sep 12 '11 at 19:02

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