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.
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.
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.
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.
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.
