GRUB: The Grand Unified Bootloader (Linux in a Nutshell, 4th Edition)

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:

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:

4.3.1. Installing GRUB

Installing GRUB involves two stages. First, you install the GRUB files on your system, either by compiling and installing the source tarball or from a package. That puts the GRUB files in the correct locations on your system. The second step is to install the GRUB software as your boot manager. This is the step we describe in this section.

If you installed GRUB as part of your Linux installation, the distribution's installation program took care of both stages of installing GRUB, and you'll most likely see the GRUB menu when you boot Linux. If you didn't install GRUB as part of your Linux installation, you have two choices. The easiest way to install GRUB is with the grub-install shell script that comes with GRUB. If grub-install doesn't work, or if you want to do the installation manually, you can run the grub command and issue the installation commands yourself.

The following sections describe how to create a GRUB boot floppy and how to install GRUB.

4.3.1.1. Creating a GRUB boot floppy

You can create a GRUB boot floppy for everyday use or to have for an emergency. The following instructions make a floppy that boots to the GRUB command line:

From the directory where GRUB was installed (e.g., /usr/share/grub/i386-pc), use the dd command to write the file stage1 to the floppy:
```
% dd if=stage1 of=/dev/fd0 bs=512 count=1
```
This command writes one block, with a blocksize of 512, from the input file stage1 to the floppy device /dev/fd0.
Now write the file stage2 to the floppy, skipping over the first block (seek=1) so you don't overwrite stage1:
```
% dd if=stage2 of=/dev/fd0 bs=512 seek=1
```

Put together, the process looks like this:

% dd if=stage1 of=/dev/fd0 bs=512 count=1
1+0 records in
1+0 records out
% dd if=stage2 of=/dev/fd0 bs=512 seek=1
254+1 records in
254+1 records out

The boot floppy is now ready to boot to the GRUB command line.

You can also make a boot floppy that boots to the GRUB menu:

Create a GRUB configuration file (/boot/grub/grub.conf) if you don't already have one. The configuration file is described later in Section 4.3.2.
Create a filesystem on your floppy disk. For example:
```
$ mke2fs /dev/fd0
```

Mount the floppy drive and create the directory /boot/grub:

% mount /mnt
% mkdir /mnt/boot
% mkdir /mnt/boot/grub

Copy the stage1, stage2, and grub.conf GRUB images from /boot/grub on your Linux partition to /mnt/boot/grub.
Run the grub command. This example assumes the command is in /sbin/grub, but it might be in /usr/sbin/grub on your system:
```
$ /sbin/grub --batch <<EOT
root (fd0)
setup (fd0)
quit
EOT
```

You should now be able to boot to the GRUB menu from the floppy disk you just created.

4.3.1.2. Using grub-install

GRUB comes with a shell script, grub-install, which uses the GRUB shell to automate the installation. The command syntax is:

grub-install options install-device

where install-device is the name of the device on which you want to install GRUB, specified as either the GRUB device name (e.g., (hd0)) or the system device (e.g., /dev/hda). For example, you might issue following the command (as root):

# grub-install /dev/hda

This command installs GRUB into the MBR of the first hard drive. The grub-install options are:

--force-lba

Force GRUB to use LBA mode, to allow booting from partitions beyond cylinder 1024.

--grub-shell=file

Specify that file is to be used as the GRUB shell. You might want to use this option to append options to grub. For example:

% grub-install --grub-shell="grub --read-only" /dev/fd0

-h, --help

Print a help message on standard output and exit.

--recheck

Force probing of a device map. You should run grub-install with this option if you add or remove a disk from your system. The device map is found at /boot/grub/device.map.

--root-directory=dir

Install GRUB images in the directory dir instead of the GRUB root directory.

-v, --version

Print the GRUB version number to standard output and exit.

4.3.1.3. Installing from the GRUB command line

To install GRUB from the native command environment, make a GRUB boot floppy as described previously. You will use that floppy to boot to the GRUB command line to do the installation. If you know which partition holds the GRUB files, you're all set. Otherwise, you can find the partition with the find command:

grub> find /boot/grub/stage1
(hd0,0)

Here, the files are on (hd0,0). Use that information to set the GRUB root device:

grub> root (hd0,0)

Run the setup command to install GRUB. To install GRUB on the MBR, run setup as follows:

grub> setup (hd0)

If you are going to chainload Linux and want to install GRUB on the boot sector of the Linux partition, run setup like this:

grub> setup (hd0,0)

4.3.2. The GRUB Configuration File

GRUB uses a configuration file that sets up the menu interface. The configuration file is called grub.conf and is found with the other GRUB files in the /boot/grub directory. grub.conf is also known as menu.lst, and at least on some distributions (e.g., Red Hat 8), menu.lst is a symbolic link to grub.conf.

The configuration file begins with a section containing global commands that apply to all boot entries, followed by an entry for each Linux image or other operating system that you want to be able to boot. Here is an example of a global section (a hash sign, #, begins a comment):

default=0                                  # default to the first entry
timeout=20                                 # set the timeout to 20 seconds
splashimage=(hd0,0)/grub/splash.xpm.gz     # the splash image displayed with
                                           # the menu

Certain GRUB commands are available only in the global section of the configuration file, for use with the GRUB menu. These commands are described in the following list. All other commands can be used either in the configuration file or on the command line and are described later in Section 4.4.

default num: Set the default menu entry to num. The default entry is started if the user does not make a selection before the timeout time. Menu entries are numbered from 0. If no default is specified, the first entry (0) is used as the default.
fallback num: Specify the entry to be used if for any reason the default entry has errors. If this command is specified and the default doesn't work, GRUB boots the fallback entry automatically instead of waiting for user input.
hiddenmenu: Specify that the menu is not to be displayed. The user can press Esc before the end of the timeout period to have the menu displayed; otherwise the default entry is booted at the end of the timeout.
timeout time: Specify the timeout period, in seconds. The timeout is the amount of time GRUB waits for user input before booting the default entry.
title name: Start a new boot entry with specified name.

Following the global section, the configuration file includes an entry for each boot image. An entry begins with a title command that specifies the text that will appear on the menu for that entry when the system boots. A typical boot entry might look like this one from a Red Hat 8.0 system:

title Red Hat Linux (2.4.18-14)
root (hd0,1)
kernel /vmlinuz-2.4.18-14 ro root=LABEL=/ hdc=ide-scsi
initrd /initrd-2.4.18-14.img

This entry provides the information GRUB needs to boot to Linux. When the menu is displayed, it will include an entry that says:

Red Hat Linux (2.4.18-14)

The GRUB root is on the second partition of the first hard drive (hd0,1). The kernel command specifies which Linux kernel to run and passes some parameters to the kernel, and the initrd command sets up an initial RAM disk.

The configuration file also provides some security features, such as the ability to set passwords and to lock certain entries so only the root user can boot them. The configuration file can be set up so that a password is required to run interactively (i.e., for editing menu entries or using the command interface) or simply to protect certain menu entries while leaving other entries available to all users. See the explanation of the password and lock commands in Section 4.4.

In addition to providing a password feature, GRUB provides the command md5crypt to encrypt passwords in MD5 format, and a corresponding Linux command, grub-md5-crypt. grub-md5-crypt is a shell script that acts as a frontend to the grub shell, calling md5crypt. Passwords encrypted either directly with md5crypt or with grub-md5-crypt can be used with the password command to set up a GRUB password. grub-md5-crypt has three possible options:

--help: Print help message and exit.
--grub-shell=file: Specify that file is to be used as the GRUB shell.
--version: Print version information and exit.

GRUB version 0.92 (640K lower / 3072K upper memory) [ Minimal BASH-like line editing is supported. For the first word, TAB lists possible command completions. Anywhere else TAB lists the possible completions of a device/filename. ] grub>

grub> Possible commands are: blocklist boot cat chainloader cmp color configfile debug device displayapm displaymem dump embed find fstest geometry halt help hide impsprobe initrd install ioprobe kernel lock makeactive map md5crypt module modulenounzip pager partnew parttype password pause quit read reboot root rootnoverify savedefault serial setkey setup terminal testload testvbe unhide uppermem vbeprobe

grub> help blocklist FILE boot cat FILE chainloader [--force] FILE color NORMAL [HIGHLIGHT] configfile FILE device DRIVE DEVICE displayapm displaymem find FILENAME geometry DRIVE [CYLINDER HEAD SECTOR [ halt [--no-apm] help [--all] [PATTERN ...] hide PARTITION initrd FILE [ARG ...] kernel [--no-mem-option] [--type=TYPE] makeactive map TO_DRIVE FROM_DRIVE md5crypt module FILE [ARG ...] modulenounzip FILE [ARG ...] pager [FLAG] partnew PART TYPE START LEN parttype PART TYPE quit reboot root [DEVICE [HDBIAS]] rootnoverify [DEVICE [HDBIAS]] serial [--unit=UNIT] [--port=PORT] [-- setkey [TO_KEY FROM_KEY] setup [--prefix=DIR] [--stage2=STAGE2_ terminal [--dumb] [--timeout=SECS] [-- testvbe MODE unhide PARTITION uppermem KBYTES vbeprobe [MODE]

4.3. GRUB: The Grand Unified Bootloader

4.3.1. Installing GRUB

4.3.1.1. Creating a GRUB boot floppy

4.3.1.2. Using grub-install

4.3.1.3. Installing from the GRUB command line

4.3.2. The GRUB Configuration File

4.3.3. Using the Menu Interface

4.3.4. The GRUB Shell