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:
This is the default interface shown when GRUB is configured by
the installation program, this is the interface shown by
default. A menu of operating systems or kernels preconfigured are
displayed as a list, ordered by name. Use the arrow keys to select
an option other than the default selection and press the
Refer to Section 2.7 GRUB Menu Configuration File for more information on configuring this interface.
To access the menu entry editor, press the
After all changes are made, the
For information about changing runlevels with GRUB using the menu entry editor, refer to Section 2.10 Changing Runlevels at Boot Time.
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
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, 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.