How do I tell if grub is installed on a device?

Question

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.

Answer 1

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.

Answer 2

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.