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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.
GRUB features three interfaces which provide different levels of
functionality. Each of these interfaces allows users to boot the
Linux kernel or another operating system.
The interfaces are as follows:
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The following GRUB interfaces can only be accessed by pressing
any key within the three seconds of the GRUB menu bypass
screen.
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- Menu Interface
-
This is the default interface shown when GRUB is configured by
the installation program. A menu of operating systems or
preconfigured kernels 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, after which GRUB loads the
default option.
Press the [e] key to enter the entry
editor interface or the [c] key to load
a command line interface.
Refer to 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.
- Command Line Interface
-
The command line interface 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.
Refer to Section 2.6 GRUB
Commands for a list of common commands.
When GRUB loads its second stage boot loader, it first searches
for its configuration file. Once found, the menu interface bypass
screen is displayed. If a key is pressed within three seconds, GRUB
builds a menu list and displays the menu interface. If no key is
pressed, the default kernel entry in the GRUB menu is used.
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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