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2.5. GRUB Interfaces

GRUB features three interfaces, which provide different levels of functionality. Each of these interfaces allows users to boot the linux kernel or other operating systems.

The interfaces are as follows:

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 are displayed as a list, ordered by name. 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 is set, so that GRUB will start loading the default option.

Press the [e] key to enter the entry editor interface or the [c] key to load a command line interface.

See Section 2.7 GRUB Menu Configuration File for more information on configuring this 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 a command line ([o] inserts a new line after the current line and [O] inserts a new line before it), editing one ([e]), or deleting one ([d]).

After all changes are made, the [b] key executes the commands and boots the operating system. The [Esc] key discards any changes and reloads the standard menu interface. The [c] key loads the command line interface.

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For information about changing runlevels with GRUB using the menu entry editor, refer to Section 2.10 Changing Runlevels at Boot Time.

Command Line Interface

The command line interface is the most basic of the GRUB interfaces, 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.

See Section 2.6 GRUB Commands, for a list of common commands.

2.5.1. Order of the Interfaces

When GRUB loads its second stage boot loader, it first searches for its configuration file. Once found, it builds a menu list and displays the menu interface.

If the configuration file cannot be found, or if the configuration file is unreadable, GRUB loads the command line interface, allowing the user to type commands to complete the boot process.

If the configuration file is not valid, GRUB prints out the error and asks for input. This helps the user see precisely where the problem occurred. Pressing any key reloads 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, GRUB reports an error and reloads the menu interface.

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