GNU GRUB ("GRUB" for short) is a boot loader package from the GNU Project. GRUB is the reference implementation of the Multiboot Specification, which allows a user to have several different operating systems on their computer at once, and to choose which one to run when the computer starts. GRUB can be used to select from different kernel images available on a particular operating system's partitions, as well as to pass boot-time parameters to such kernels.

GNU GRUB developed from a previous package called the Grand Unified Bootloader (a play on grand unified theory). It is predominantly used on Unix-like systems; the GNU operating system uses GNU GRUB as its boot loader, as do most general-purpose Linux distributions. Solaris has used GRUB as its bootloader on x86 systems since the Solaris 10 1/06 release.

Features

GRUB is dynamically configurable; it loads its configuration at startup, allowing boot-time changes such as selecting different kernels or initial RAM disks. To this end, GRUB provides a simple, bash-like, command line interface which lets users write new boot sequences.

GRUB is highly portable. It supports multiple executable formats, and is geometry translation independent; while Multiboot compliant, it supports non-multiboot operating systems such as Microsoft Windows and OS/2 via a chain loading function. GRUB supports all commonly used Unix file systems as well as VFAT and NTFS as used by Windows, and supports Logical Block Address (LBA) mode. GRUB allows users to view the contents of files on any supported file system.

GRUB can be used with a variety of different user interfaces. Most Linux distributions take advantage of GRUB's support for a graphical interface to provide a customized boot menu with a background image, and occasionally mouse support.Fact: date=November 2007 GRUB's text interface can be set to use a serial link to provide a remote terminal boot loader access.

GRUB can download operating system images from a network, and can thus support disk-free systems. GRUB supports automatic decompression of OS images prior to booting from them.

GRUB differs from other boot loaders by being able to communicate with a user directly via a GRUB prompt. A GRUB prompt is the stage before GRUB loads an operating system and can be triggered at a text-mode GRUB booting screen (which is controlled by the configuration file "menu.lst" (or "grub.conf": see below)) by pressing the "c" key. A GRUB prompt (similar to bash) can also be obtained by booting GRUB as a stand alone system without an operating system attached or in any GRUB installation with an operating system when the "menu.lst" file is absent. From the GRUB prompt a user can manually select and control booting from any installed operating system by using bash-like commands. To boot an operating system automatically, the appropriate commands are placed in a configuration file called "menu.lst" in a designated subdirectory.