Learning Red Hat Linux (3rd Edition) [Electronic resources] نسخه متنی

C.3 The GRUB Loader

Most PCs can be booted from
a floppy drive or hard drive; most recently manufactured computers
can be booted from a CD-ROM drive. The first sector of a disk,
diskette, or partition is known as the
boot sector. The boot
sector associated with a disk or diskette (the first sector of the
disk or diskette) is known as the
Master Boot
Record (MBR). In order for a diskette or disk to
be bootable, it must contain a boot loader, which can reside in:

The boot sector of the floppy diskette

The MBR of the first hard disk or the first CD-ROM drive, if the PC
supports booting from a CD-ROM

The boot sector of a Linux filesystem partition on the first hard disk

The boot sector of an extended partition on the first hard disk

GRUB is a sophisticated boot loader that can load Linux, Microsoft
Windows 3.x and 9x, NT,
2000, XP, and other popular operating systems. Most users install
GRUB on the MBR of their system's first hard disk.
That way, when the system is started, it boots GRUB, which can be
used to load Linux, Microsoft Windows, or another operating system.

Unless you direct otherwise, the Red Hat Linux installation procedure
automatically installs GRUB. So you don't need to
install GRUB; you just need to configure it.

Similarly, when you boot by using GRUB, you can also boot parameters
to control the boot process; you can specify GRUB's
boot parameters by selecting an operating system from
GRUB's menu and pressing e. In response, GRUB displays an editor screen
that shows the commands associated with the selected operating
system, as shown in Figure C-1.

Figure C-1. The GRUB editor screen

Commands that can be used in the editor screen are listed in Table C-1.

The principal GRUB commands are:

chainloader

Used to load a Microsoft operating system, including DOS, Windows
3.x, 9x, NT, or 2000.

initrd

Specifies the file containing an initial RAM disk used in loading
Linux. This command is necessary, for instance, when booting Linux
from a SCSI drive.

kernel

Specifies the file containing the Linux kernel to be booted.

root

rootnoverify

Specifies the partition to be mounted as the root partition. The
root command causes the filesystem
to be verified before the partition is mounted.

GRUB refers to hard disks using the syntax (hdn), where
n specifies the device number assigned by the system BIOS. For
example, (hd0) refers to the first hard drive. Partitions are
designated by the related syntax
(hdn,m), where
m is the number of the partition. For example,
(hd1,0) refers to the first partition of the second hard drive.

GRUB can refer to the disk blocks that comprise a file by using a
special syntax known as a
blocklist.
A blocklist consists of a comma-separated list of block ranges, each
of which consists of a starting block number, followed by a plus sign
(+), followed by the number of blocks in the range. For example, the
blocklist 0+1,10+20 refers to a block range beginning at block 0 and
including 1 block and a block range beginning at block 10 and
including 20 blocks.

If the starting block number of a block range is omitted, the block
range is implied to begin with block 0. For example, the block range
+1 begins at block 0 and includes 1 block.

A blocklist can appear after a reference to a partition; if no
partition is identified, the partition identified by the preceding
root command is implied. For
example, in:

root (hd0,0)
chainlist +1

the blocklist +1 is implicitly associated with (hd0,0) and is
equivalent to the blocklist (hd0,0)+1.

A typical GRUB entry for booting Linux resembles the following:

root (hd0,0)
kernel /boot/vmlinuz-2.4.18-14 root=LABEL=/
initrd /initrd-2.4.18-14.img

This entry boots the specified kernel residing on partition 0 of the
first hard drive, mounting the partition labelled
"/" as the root partition. Linux
boot parameters, such as those described in the upcoming
Section C.4, can be specified by
using the kernel command.

If the kernel requires access to special drivers residing on a
RAM disk, a RAM disk can be identified as
in this entry:

root (hd0,0)
kernel /boot/vmlinuz-2.4.18-14 ro root=LABEL=/
initrd /initrd-2.4.18-14.img

Notice that the name of the RAM disk file is specified as though the
file resides in the root directory, whereas it actually resides in
the /boot directory. This is necessary because
GRUB initially mounts the /boot filesystem, as
specified in the preceding root
command. The mounted filesystem is treated by GRUB as its root
filesystem.

A typical GRUB entry for booting a
Microsoft operating system
has this form:

rootnoverify (hd0,0)
chainloader +1

This entry boots the Microsoft operating system residing on partition
of the first hard drive. The blocklist +1 points to the first block
of the root partition. Microsoft operating systems place their boot
file at this location.

To boot your system, GRUB uses your system's BIOS,
which may not be able to load a Linux kernel (or other program)
stored beyond cylinder 1023 of your hard drive. Booting a kernel
stored beyond cylinder 1023 requires a motherboard that supports
logical byte
addressing (LBA). Most motherboards manufactured
in the last several years are supposed to support LBA. Unfortunately,
some motherboards that claim to support LBA do not. If
you're installing Linux on a preexisting hard drive,
you may not be able to place your Linux kernel in an appropriate
location. In that case, you won't be able to use
GRUB to boot your system.

To learn more about GRUB, see the following resources:

The manpage for GRUB

The GRUB home page, http://www.gnu.org/software/grub

The Multiboot-with-GRUB mini HOWTO, http://www.redhat.com/mirrors/LDP/HOWTO/mini/Multiboot-with-GRUBl