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:
Note
The following GRUB interfaces can only be accessed by pressing
any key within the three seconds of the GRUB menu bypass screen.
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.
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.
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.
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.
