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Linux in a Nutshell, 6th Edition by Robert Love, Stephen Figgins, Ellen Siever, Arnold Robbins

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GRUB: The Grand Unified Bootloader

Like LILO, the GRUB boot loader can load other operating systems in addition to Linux. GRUB has become the default bootloader for most Linux variants. It was written by Erich Boleyn to boot operating systems on PC-based hardware and is now developed and maintained by the GNU project. GRUB was intended to boot operating systems that conform to the Multiboot Specification, which was designed to create one booting method that would work on any conforming PC-based operating system. In addition to multiboot-conforming systems, GRUB can boot directly into Linux, FreeBSD, OpenBSD, and NetBSD. It can also boot other operating systems such as Microsoft Windows indirectly, through the use of a chainloader. The chainloader loads an intermediate file, and that file loads the operating system’s boot loader.

GRUB provides a graphical menu interface. It also provides a command interface that is accessible both while the system is booting (the native command environment) and from the command line once Linux is running.

While LILO works perfectly well, especially if you usually boot the default image, GRUB has some advantages. The graphical menu interface shows you exactly what your choices are for booting, so you don’t have to remember them. It also lets you easily edit an entry on the fly, or drop down into the command interface. In addition, if you are using the menu interface and something goes wrong, GRUB automatically puts you into the command interface so you can attempt to recover and boot manually. Another advantage of GRUB is that if you install a new kernel or update the configuration file, that’s all you have to do; with LILO, you also have to remember to rerun the lilo command to reinstall the boot loader. On the other hand, if you are used to LILO, don’t need to see the prompts often, and have a stable system, LILO is quick and convenient.

A GRUB installation consists of at least two and sometimes three executables, known as stages. The stages are:

Stage 1

Stage 1 is the piece of GRUB that resides in the MBR or the boot sector of another partition or drive. Since the main portion of GRUB is too large to fit into the 512 bytes of a boot sector, Stage 1 is used to transfer control to the next stage, either Stage 1.5 or Stage 2.

Stage 1.5

Stage 1.5 is loaded by Stage 1 only if the hardware requires it. Stage 1.5 is filesystem-specific; that is, there is a different version for each filesystem that GRUB can load. The name of the filesystem is part of the filename (e2fs_stage1_5, fat_stage1_5, etc.). Stage 1.5 loads Stage 2.

Stage 2

Stage 2 runs the main body of the GRUB code. It displays the menu, lets you select the operating system to be run, and starts the system you’ve chosen.

If it was compiled with netboot support, GRUB can also be used to boot over a network. We don’t describe that process here; see the file netboot/README.netboot in the GRUB source directory for detailed information.

One of the first things to understand about GRUB is that it uses its own naming conventions. Drives are numbered starting from 0; thus, the first hard drive is hd0, the second hard drive is hd1, the first floppy drive is fd0, and so on. Partitions are also numbered from 0, and the entire name is put in parentheses. For example, the first partition of the first drive, /dev/hda1, is known as (hd0,0) to GRUB, and the third partition of the second drive is (hd1,2). GRUB makes no distinction between drive types; thus the first drive is hd0 regardless of whether it is IDE, SCSI, or SATA.

Files are specified either by the filename or by blocklist, which is used to specify files such as chainloaders that aren’t part of a filesystem. A filename looks like a standard Unix path specification with the GRUB device name prepended; for example:

(hd0,0)/grub/grub.conf

If the device name is omitted, the GRUB root device is assumed. The GRUB root device is the disk or partition where the kernel image is stored, set with the root command. See GRUB Commands in GRUB Commands for the command descriptions.

When you use blocklist notation, you tell GRUB which blocks on the disk contain the file you want. Each section of a file is specified as the offset on the partition where the block begins plus the number of blocks in the section. The offset starts at 0 for the first block on the partition. The syntax for blocklist notation is:

[device][offset+length[,offset]+length...

In this case, too, the device name is optional for a file on the root device. With blocklist notation, you can also omit the offset if it is 0. A typical use of blocklist notation is when using a chainloader to boot Windows. If GRUB is installed in the MBR, you can chainload Windows by setting the root device to the partition that has the Windows boot loader, making it the active partition, and then using the chainloader command to read the Windows boot sector:

     rootnoverify (hd0,0)
     makeactive
     chainloader +1

In this example, the blocklist notation (+1) does not include either the device name or the offset because we set the root device to the Windows partition, and the Windows loader begins at offset 0 of that partition.

GRUB also includes a device map. The device map is an ASCII file, usually /boot/grub/device.map. Since the operating system isn’t loaded yet when you use GRUB to boot Linux (or any other operating system), GRUB knows only the BIOS drive names. The purpose of the device map is to map the BIOS drives to Linux devices. For example:

     (fd0)   /dev/fd0
     (hd0)   /dev/hda

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