[SOLVED] Grub basics

[charlie205]

I have a 20GB hard drive. I installed Ubuntu 8.04 on a 12 GB partition (actually 11 for / and 1 for swap). I installed CentOS on the remaining 8 GB partition (no swap). The CentOS installer gave me the option to specify another OS to add support for in Grub. I selected /dev/hda1 (the ubuntu install) and named it Ubuntu. The system boots Centos fine, but Ubuntu won't boot when selected. I think the /boot/grub/grub.conf needs to be adjusted. Here are the contents:

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You do not have a /boot partition. This means that
# all kernel and initrd paths are relative to /, eg.
# root (hd0,2)
# kernel /boot/vmlinuz-version ro root=/dev/hda3
# initrd /boot/initrd-version.img
#boot=/dev/hda
default=1
timeout=10
splashimage=(hd0,2)/boot/grub/splash.xpm.gz
title CentOS-3 (2.4.21-50.EL)
root (hd0,2)
kernel /boot/vmlinuz-2.4.21-50.EL ro root=LABEL=/
initrd /boot/initrd-2.4.21-50.EL.img
title Ubuntu
rootnoverify (hd0,0)
chainloader +1




What reference should I input for "title Ubuntu" so Grub can find and boot Ubuntu?

Also, once Ubuntu is accessible, I assume it will have it's own /boot/grub/grub.conf. Which one takes precedence?

Any help is appreciated! Thanks!!

[devils casper]

RedHat based distros doesn't detect other installed Linux distros and one has to add an entry in GRUB Manually.
Mount root partition of Ubuntu in CentOS and open /boot/grub/menu.lst file. Copy first title block from it and paste in /boot/grub/grub.conf file of CentOS.

su -
mkdir /media/ubuntu
mount -t ext3 /dev/sda1 /media/ubuntu
cd /media/ubuntu/boot/grub
less menu.lst

Copy first title block and paste in /boot/grub/grub.conf file of CentOS.

Also, once Ubuntu is accessible, I assume it will have it's own /boot/grub/grub.conf. Which one takes precedence?

GRUB will be handled by CentOS's grub.conf file unless you install Ubuntu's GRUB in boot sector of Ubuntu's root partition and add chainloading code in grub.conf file of CentOS.

[charlie205]

is any kind of "write grub" command required to write the changes to the disk's boot sector?

[devils casper]

I wont suggest you to install Ubuntu's GRUB in Boot Sector now. Just copy/paste first title block. You will have an option to boot up Ubuntu in GRUB Menu and Ubuntu will boot up fine.

[charlie205]

this worked perfectly, thanks!

can you recommend a decent overview of how grub actually works? i'm a bit confused about it now.

I thought each physical hard drive had one boot sector (also called MBR) and this is where grub is installed. A conf file would exists in the file system which would write changes to that boot sector (i now know in Lilo you had to write changes, but grub searches for the conf file to detect changes on boot automagically).

It sounds like your saying there is an installation of grub (or there can be) in every boot partition on a physical disk, so for example, if i have one 20GB disk with a pair of 10GB partitions I would have one MBR and two boot sectors (one for each partition).

I need to figure out how it actually works. If you can point me to a good resource so I can RTFM i would appreciate it.

thanks a bunch

[khafa]

something like this?
grub

[devils casper]

It sounds like your saying there is an installation of grub (or there can be) in every boot partition on a physical disk, so for example, if i have one 20GB disk with a pair of 10GB partitions I would have one MBR and two boot sectors (one for each partition).

Thats correct. Each partition has its own Boot Sector. You can install GRUB in MBR as well as in boot sector. GRUB can chainload other Boot Loaders easily.
Let say, I have Windows XP + 2 Linux distros ( A and B ) installed in my HD and I am using GRUB of A for dual booting.

* A : GRUB of A in MBR of HD
* Windows XP : Windows Boot Loader in boot sector of Windows OS Partition ( thats default and GRUB chainloads Windows Boot Loader )
* B : I can boot up B distro in three ways :

1. Install GRUB in Boot Sector of B's root partition and chainload it like Windows OS. ( Its recommended )
2. Do not install GRUB and add title block ( title, kernel, initrd lines ) in A's grub.conf ( or menu.lst ) file as I suggested you earlier.
3. Install GRUB of B in MBR ( Its default in all distros ). Installer will overwrite A's GRUB, detect A and Windows XP and setup multiboot itself. But there are a few drawbacks in this method. Installer of RedHat based distros do not detect other Linux distros and one has to add entries manually.

I hope it helps. Feel free to ask if you have any confusion.

[charlie205]

khafa - thanks, that looks like it

casper - thanks a bunch...this is a good start. Thanks for your help getting the grub thing working before also!