GRUB features three powerful interfaces that provide different levels of functionality. Each of these interfaces allows users to boot operating systems, and even move between interfaces within the GRUB environment.

Menu Interface

If GRUB was automatically configured by the Red Hat Linux installation program, this is the interface shown by default. A menu of operating systems or kernels preconfigured with their own boot commands exists in this interface as a list, by name, after the system first starts. Use the arrow keys to select an option other than the default selection and press the [Enter] key to boot it. Alternatively, a timeout period may have been set, so that GRUB will start loading the default option.

From the menu interface, press the [e] key to edit the commands in the highlighted menu entry or the [c] key to load a command line interface.

Menu Entry Editor Interface

To access the menu entry editor, press the [e] key from the boot loader menu. The GRUB commands for that entry are displayed here, and users may alter these command lines before booting the operating system by adding ([o] after the current line; [O] before it), editing ([e]), or deleting ([d]) them.

After all changes are made, hit the [b] key to execute the commands and boot the operating system. The [Esc] key discards any changes and reloads the standard menu interface. The [c] key will load the command line interface.


For information about changing runlevels with GRUB using the menu entry editor, refer to the Section called SysV Init in Chapter 3.

Command Line Interface

The command line is the most basic GRUB interface, but it is also the one that grants the most control. The command line makes it possible to type any relevant GRUB commands followed by the [Enter] key to execute them. This interface features some advanced shell-like features, including [Tab] key completion, based on context, and [Ctrl] key combinations when typing commands, such as [Ctrl]-[a] to move to the beginning of a line, and [Ctrl]-[e] to move to the end of a line. In addition, the arrow, [Home], [End], and [Delete] keys work as they do in the bash shell.

Order of Interface Use

When the GRUB environment starts loading the second stage boot loader, it looks for its configuration file. When found, it uses the configuration file to build the menu list of operating systems to load and then displays the boot menu interface.

If the configuration file cannot be found, or if the configuration file is unreadable, GRUB will load the command line interface to allow users to manually type the commands necessary to boot an operating system.

If the configuration file is not valid, GRUB will print out the error and ask for input. This can be very helpful, because users will then be able to see precisely where the problem occurred and fix it in the file. Pressing any key will reload the menu interface, where it is then possible to edit the menu option and correct the problem based on the error reported by GRUB. If the correction fails, the error is reported and GRUB will begin again.