Configuring GRUB Legacy

When you use the GRUB Legacy interactive menu, you need to tell it just what options to show. You do that using special GRUB commands.

The GRUB Legacy system stores the menu commands in a standard text file. The configuration file used by GRUB Legacy is called menu.lst and is stored in the /boot/grub folder (while not a requirement, some Linux distributions create a separate /boot partition on the hard drive). Red Hat–derived Linux distributions (such as CentOS and Fedora) use grub.conf instead of menu.lst for the configuration file.

The GRUB Legacy configuration file consists of two sections:

• Global definitions

• Operating system boot definitions

The global definitions section defines commands that control the overall operation of the GRUB Legacy boot menu. The global definitions must appear first in the configuration file. There are only a handful of global settings that you can make; TABLE 4-1 shows these settings.

TABLE 4-1 GRUB Legacy global commands.

For GRUB Legacy, to define a value for a command you just list the value as a command line parameter:

The color command defines the color scheme for the menu. The first pair define the foreground/background pair for normal menu entries, while the second pair define the foreground/background pair for the selected menu entry.

After the global definitions, you place definitions for the individual operating systems that are installed on the system. Each operating system should have its own definition section. There are a lot of boot definition settings that you can use to customize how the boot loader finds the operating system kernel file. Fortunately, just a few commands are required to define the operating system. The ones to remember are the following:

• title—the first line for each boot definition section, this is what appears in the boot menu

• root—defines the disk and partition where the GRUB /boot folder partition is on the system

• kernel—defines the kernel image file stored in the /boot folder to load

• initrd—defines the initial RAM Disk file, which contains hardware drivers necessary for the kernel to interact with the system hardware

• rootnoverify—defines non-Linux boot partitions, such as Windows

The root command defines the hard drive and partition that contains the /boot folder for GRUB Legacy. Unfortunately, GRUB Legacy uses a somewhat odd way of referencing those values:

GRUB Legacy doesn’t refer to hard drives the way Linux does; it uses a number system to reference both disks and partitions, starting at 0 instead of 1. For example, to reference the first partition on the first hard drive on the system, you’d use (hd0,0). To reference the second partition on the first hard drive, you’d use (hd0,1).

Here’s a sample GRUB configuration file that defines both a Windows and a Linux partition for booting:

This example shows two boot options—one for an Ubuntu Linux system and one for a Windows system. The Ubuntu system is installed on the first partition of the second hard drive, while the Windows system is installed on the first partition of the first hard drive.

Installing GRUB Legacy

Once you build the GRUB Legacy configuration file (as either menu.lst or grub.conf, depending on your Linux distribution), you must install GRUB in the MBR. The command to do this is grub-install.

The grub-install command uses a single parameter—the partition to install GRUB on. You can specify the partition using either Linux or GRUB Legacy format. For example, to use Linux format you’d use

to install GRUB on the MBR of the first hard drive. To use GRUB Legacy format, you must enclose the hard drive format in quotes:

If you’re using the chainloading method and prefer to install a copy of GRUB Legacy on the boot sector of a partition instead of to the MBR of a hard drive, you must specify the partition, again using either Linux or GRUB format:

You don’t need to reinstall GRUB Legacy in the MBR after making changes to the configuration file — GRUB Legacy reads the configuration file each time it runs.

Interacting with GRUB Legacy

When you boot a system that uses the GRUB Legacy boot loader you’ll see a menu that shows the boot options you defined in the configuration file. If you wait for the timeout to expire, the default boot option will process. Alternatively, you can use the arrow keys to select one of the boot options, then press the ENTER key to select it.

You can also edit boot options on the fly from the GRUB menu. First, arrow to the boot option you want to modify, then press the E key. A new screen will appear, showing the currently configured options for that entry. Use the arrow key to move the cursor to the line you need to modify, then press the E key to edit it. Press the B key to boot the system using the new values. You can also press the C key at any time to enter an interactive shell mode, allowing you to submit commands on the fly.

Configuring GRUB2

There are also a few changes to the commands used in GRUB2. For example, instead of the title command, GRUB uses the menuentry command, and you must also enclose each individual boot section with braces immediately following the menuentry command. Here’s an example of a sample GRUB2 configuration file:

Description

Notice that GRUB2 uses the set command to assign values to the root keyword as well as uses an equal sign to assign the device. GRUB2 utilizes environment variables to configure settings instead of commands.

To make things more confusing, GRUB2 changes the numbering system for partitions. While it still uses 0 for the first hard drive, the first partition is set to 1. So to define the /boot folder on the first partition of the first hard drive, you now need to use:

Also, notice that the rootnoverify and kernel commands are not used in GRUB2. Non-Linux boot options are now defined the same as Linux boot options using the root environment variable, and you define the kernel location using the linux command.

The configuration process for GRUB2 is also somewhat different. While the /boot/grub/grub.cfg file is the configuration file that GRUB2 uses, you should never modify that file. Instead, separate configuration files are stored in the /etc/grub.d folder. This allows you (or the system) to create individual configuration files for each boot option installed on your system (for example, one configuration file for booting Linux, and another for booting Windows).

For global commands, use the /etc/default/grub configuration file. You’ll notice that the format for some of the global commands has changed from the GRUB Legacy commands, such as GRUB_TIMEOUT instead of just timeout.

Most Linux distributions generate the new grub.cfg configuration file automatically after certain events, such as upgrading the kernel.

Installing GRUB2

Unlike GRUB Legacy, you don’t need to install GRUB2; all you need to do is rebuild the main installation file. This is done by running the grub-mkconfig program.

The grub-mkconfig program reads configuration files stored in the /etc/grub.d folder and assembles the commands into the single grub.cfg configuration file.

You can update the configuration file manually by running the grub-mkconfig command:

Notice that you must manually redirect the output of the grub-mkconfig program to the grub.cfg configuration file. By default the grub-mkconfig program outputs the new configuration file commands to standard output.

Interacting with GRUB2

The GRUB2 boot loader produces a boot menu similar to the GRUB Legacy method. You can use arrow keys to switch between boot options, the E key to edit a boot entry, or the C key to bring up the GRUB2 command line to submit interactive boot commands.