NOTE: CentOS Enterprise Linux 5 is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux 5 is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux 5.
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:
The following GRUB interfaces can only be accessed by pressing any key within the three seconds of the GRUB menu bypass screen.
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 operating system or kernel version and press the Enter key to boot it. If you do nothing on this screen, then after the time out period expires GRUB will load 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.