GRUB features three powerful interfaces that provide different levels of
functionality. Each of these interfaces allow you to boot operating
systems, and you can even move between interfaces within the GRUB
environment.
If GRUB was automatically configured for you by the Red Hat Linux installation program,
this is the interface you already know. 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. You can
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, you can also press the [e] key
to edit the highlighted menu entry's commands or the
[c] key to move to a command line interface.
To get to this interface, you must press the [e] key
from the menu interface. The GRUB commands for that entry are
displayed here, and you can 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 you have made your changes and the lines look like you want, you
can hit the [b] key to execute them and boot the
operating system. The [Esc] key forgets your changes
and takes you to the standard menu interface. The [c]
key will take you to the command line interface.
This is the most basic GRUB interface, but it is also the one that
gives you the most control. You can type any relevant GRUB commands
here 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.
When the GRUB environment starts upon 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 moves you into the menu interface.
If the configuration file cannot be found, or if the configuration
file is unreadable, GRUB will take you to the command line interface
to allow you 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 you will be able
to see precisely where the problem occurred and fix it in the
file. Pressing any key will take you back to the menu interface, where
you can edit that menu option and correct the problem based on the
error reported by GRUB. If your correction fails, the error is
reported and you are allowed to start again.
