Wednesday Why: Grub Boot Configuration

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,1)
# kernel /vmlinuz-version ro root=/dev/main/root
# initrd /initrd-version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,1)/grub/splash.xpm.gz
hiddenmenu
title Fedora (2.6.23.1-49.fc8)

root (hd0,1)
kernel /vmlinuz-2.6.23.1-49.fc8 ro root=/dev/main/root rhgb quiet
initrd /initrd-2.6.23.1-49.fc8.img

title Fedora (2.6.23.1-42.fc8)

root (hd0,1)
kernel /vmlinuz-2.6.23.1-42.fc8 ro root=/dev/main/root rhgb quiet
initrd /initrd-2.6.23.1-42.fc8.img

title WindowsXP

rootnoverify (hd0,0)
chainloader +1

This file contains general options (in green), boot stanzas for Fedora (blue), and a boot stanza for Windows (red).

The general options are:

• default - selects the boot stanza which will be selected by default unless the user intervenes. These are numbered starting at zero.
• timeout - the time in seconds before the default is loaded, if the user does not press a key. To make Grub wait indefinitely for user input, remove this entry. To immediately proceed with the default entry, set the timeout to 0.
• hiddenmenu - hides the boot menu unless the user presses a key. To make the menu appear every time, remove this entry.
• splashimage - selects the image to be displayed in the background of the boot display.

The Fedora stanzas (and those for other Linux distributions, if any) each consist of a title line followed by lines for:

• root - sets the partition for the boot files. Note: this is not the root filesystem! -- this is the partition from which the boot files are loaded.
• kernel - selects the kernel and specifies kernel boot options.
• initrd - specifies a file which contains a compressed filesystem image which is loaded into memory and used as a ramdisk during the boot process.

Grub will load the kernel and initrd from the specified root device, then execute the kernel. Other operating systems, such as Windows, are loaded differently -- so Grub hands the boot process over to whatever bootloader was installed with the operating system, using these lines:

• rootnoverify - like the root line in a Linux boot stanza, this selects the partition which contains the boot files.
• chainloader - instructs Grub to execute the bootloader code located at the second (+1) sector of the selected partion.

Throughout this file, partitions are referred to as (hdX,N) where X is the drive number in the order in which drives are discovered by the BIOS, numbered starting at zero, and N is the partition number, also numbered starting at zero. If your system only has one drive installed, the partition /dev/sda2 (/dev/hda2 under Fedora 6 and earlier) will therefore correspond to (hd0,1) in Grub notation.

If you're using Xen, your Fedora boot stanza will look something like this:

title Fedora (2.6.21-2950.fc8xen)

root (hd0,1)
kernel /xen.gz-2.6.21-2950.fc8
module /vmlinuz-2.6.21-2950.fc8xen ro root=/dev/main/root rhgb quiet
module /initrd-2.6.21-2950.fc8xen.img

In this case, the "kernel" is actually the Xen hypervisor, and the actual dom0 kernel and initrd are loaded as modules to the hypervisor. Any kernel arguments must therefore be added to the module line which references the kernel (/vmlinux-*).

Since gub.conf is a regular text file, you can edit it with the text editor of your choice, such as vi, gedit, or kate. However, if you introduce an error into this file, you may not be able to boot your system from the boot menu. To reduce risk, it's a good idea to keep a known-good stanza in the file -- and in the worst case, you can always edit a boot stanza using the options provided by Grub during the boot process.