GNU GRUB (short for GNU GRand Unified Bootloader, commonly referred to as GRUB) is a

boot loader A bootloader, also spelled as boot loader or called boot manager and bootstrap loader, is a computer program that is responsible for booting a computer. When a computer is turned off, its softwareincluding operating systems, application code, a ...

package from the

GNU Project The GNU Project () is a free software, mass collaboration project announced by Richard Stallman on September 27, 1983. Its goal is to give computer users freedom and control in their use of their computers and computing devices by collaborati ...

. GRUB is the

reference implementation In the software development process, a reference implementation (or, less frequently, sample implementation or model implementation) is a program that implements all requirements from a corresponding specification. The reference implementation o ...

of the

Free Software Foundation The Free Software Foundation (FSF) is a 501(c)#501(c)(3), 501(c)(3) non-profit organization founded by Richard Stallman on October 4, 1985, to support the free software movement, with the organization's preference for software being distributed ...

's Multiboot Specification, which provides a user the choice to boot one of multiple

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...

s installed on a computer or select a specific

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learnin ...

configuration available on a particular operating system's partitions. GNU GRUB was developed from a package called the ''Grand Unified Bootloader'' (a play on

Grand Unified Theory A Grand Unified Theory (GUT) is a model in particle physics in which, at high energies, the three gauge interactions of the Standard Model comprising the electromagnetic, weak, and strong forces are merged into a single force. Although this ...

). It is predominantly used for

Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...

systems. The

GNU operating system GNU () is an extensive collection of free software (383 packages as of January 2022), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operat ...

uses GNU GRUB as its boot loader, as do most

Linux distribution A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...

s and the

Solaris operating system Solaris is a proprietary Unix operating system originally developed by Sun Microsystems. After the Sun acquisition by Oracle in 2010, it was renamed Oracle Solaris. Solaris superseded the company's earlier SunOS in 1993, and became known for it ...

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introd ...

systems, starting with the Solaris 10 1/06 release.

Operation

GNU GRUB on MBR partitioned hard disk drives

Booting

When a computer is turned on, its

BIOS In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...

finds the primary bootable device (usually the computer's hard disk) and runs the initial bootstrap program from the

master boot record A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBR ...

(MBR). The MBR is the first

sector Sector may refer to: Places * Sector, West Virginia, U.S. Geometry * Circular sector, the portion of a disc enclosed by two radii and a circular arc * Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc * Spherical sector, a po ...

of the hard disk. This bootstrap program must be small because it has to fit in a single sector. For a long time, the size of a sector has been 512 bytes. Since 2009 there are hard disks available with a sector size of 4096 bytes, called

Advanced Format Advanced Format (AF) is any disk sector format used to store data on magnetic disks in hard disk drives (HDDs) that exceeds 512, 520, or 528 bytes per sector, such as the 4096, 4112, 4160, and 4224-byte (4 KB) sectors of an Advanced Format ...

disks, but , such hard disks are still accessed in 512-byte sectors, using the 512e emulation. The legacy

MBR partition table A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MB ...

supports a maximum of four partitions and occupies 64 bytes, combined. Together with the optional disk signature (four bytes) and disk timestamp (six bytes), this leaves between 434 and 446 bytes available for the

machine code In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...

of a boot loader. Although such a small space can be sufficient for very simple boot loaders, it is not big enough to contain a boot loader supporting complex and multiple

file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...

s, menu-driven selection of boot choices, etc. Boot loaders with bigger footprints are therefore split into pieces, where the smallest piece fits in the MBR, while one or more larger pieces are stored in other locations such as empty sectors between the MBR and the first partition. The code in the MBR then does little more than starting the second part. The purpose of the remaining part(s) of the boot loader is to actually boot an operating system by configuring it and starting the

. Kernels are in most cases stored as files residing on appropriate file systems, but the concept of a file system is unknown to the BIOS. Thus, in BIOS-based systems, the duty of a boot loader is to access the content of those files, so it can be loaded into the

RAM Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * Ra ...

and executed. One possible approach for boot loaders is to load kernel images by directly accessing hard disk sectors without understanding the underlying file system. Usually, an additional level of

indirection In computer programming, indirection (also called dereferencing) is the ability to reference something using a name, reference, or container instead of the value itself. The most common form of indirection is the act of manipulating a value throug ...

is required, in form of ''maps'' or ''map files'' auxiliary files that contain a list of physical sectors occupied by kernel images. Such maps need to be updated each time a kernel image changes its physical location on disk, due to installing new kernel images, file system defragmentation, etc. Also, in case of the maps changing their physical location, their locations need to be updated within the boot loader's MBR code, so the sectors indirection mechanism continues to work. This is not only cumbersome, but it also leaves the system in need of manual repairs in case something goes wrong during system updates. Another approach is to make a boot loader aware of the underlying file systems, so kernel images are configured and accessed using their actual

file path A path is a string of characters used to uniquely identify a location in a directory structure. It is composed by following the directory tree hierarchy in which components, separated by a delimiting character, represent each directory. The del ...

s. That requires a boot loader to contain a driver for each of the supported file systems, so they can be understood and accessed by the boot loader itself. This approach eliminates the need for hardcoded locations of hard disk sectors and existence of map files, and does not require MBR updates after kernel images are added or moved around. The configuration of a boot loader is stored in a regular file, which is also accessed in a file system-aware way to obtain boot configurations before the actual booting of any kernel images. Thus, fewer things can go wrong during system updates. As a downside, such boot loaders are larger and more complex. GNU GRUB uses the second approach, by understanding the underlying file systems. The boot loader itself is split into multiple ''stages'' so that it fits in the MBR boot scheme. Two major versions of GRUB are in common use: GRUB version 1, called ''GRUB legacy,'' is only prevalent in older releases of Linux distributions. ''GRUB 2'' was written from scratch and intended to replace its predecessor, and is now used by a majority of Linux distributions.

Version 0 (GRUB Legacy)

GRUB 0.x follows a two-stage approach. The master boot record (MBR) usually contains GRUB ''stage 1,'' or can contain a standard MBR implementation which chainloads GRUB ''stage 1'' from the active partition's boot sector. Given the small size of a boot sector (512 bytes), ''stage 1'' can do little more than load the next stage of GRUB by loading a few disk sectors from a fixed location near the start of the disk (within its first 1024 cylinders). ''Stage 1'' can load ''stage 2'' directly, but it is normally set up to load the ''stage 1.5.'', located in the first 30

KiB The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...

of hard disk immediately following the MBR and before the first partition. In case this space is not available (unusual partition table, special disk drivers, GPT or LVM disk) the install of ''stage 1.5'' will fail. The ''stage 1.5'' image contains file system drivers, enabling it to directly load ''stage 2'' from any known location in the filesystem, for example from /boot/grub. ''Stage 2'' will then load the default configuration file and any other modules needed.

Version 2 (GRUB 2)

Startup on systems using
BIOS In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
firmware

* See illustration in last image on the right. * boot.img (stage 1) is written to the first 440 bytes of the

Master Boot Record A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBR ...

(MBR boot code in sector 0), or optionally in a partition boot sector (PBR). It addresses diskboot.img by a 64-bit LBA address. The actual sector number is written by grub-install. diskboot.img is the first sector of core.img with the sole purpose to load the rest of core.img identified by LBA sector numbers also written by grub-install. ** On MBR partitioned disks, core.img (stage 1.5) is stored in the empty sectors (if available) between the MBR and the first partition. Recent operating systems suggest a 1 MiB gap here for alignment (2047*512 byte or 255*4KiB sectors). This gap used to be 62 sectors (31 KiB) as a reminder of the sector number limit of

Cylinder-Head-Sector Cylinder-head-sector (CHS) is an early method for giving addresses to each physical block of data on a hard disk drive. It is a 3D-coordinate system made out of a vertical coordinate ''head'', a horizontal (or radial) coordinate ''cylinder'', an ...

(C/H/S) addressing used by

before 1996, therefore core.img is designed to be smaller than 32 KiB. **On GPT partitioned disks core.img is written to its own partition. It must be flagged "BIOS_grub", must not be formatted and can be as tiny as 1 MiB. * stage 2: core.img loads /boot/grub/i386-pc/normal.mod from the partition configured by grub-install. If the partition index has changed, GRUB will be unable to find the normal.mod, and presents the user with the GRUB Rescue prompt. * Depending on how GRUB2 was installed, the /boot/grub/ is either in the ''root'' partition of the Linux distribution, or in the separate ''/boot'' partition. * after normal.mod loaded: normal.mod parses /boot/grub/grub.cfg, optionally loads modules (eg. for graphical UI and file system support) and shows the menu.

Startup on systems using
UEFI UEFI (Unified Extensible Firmware Interface) is a set of specifications written by the UEFI Forum. They define the architecture of the platform firmware used for booting and its interface for interaction with the operating system. Examples of f ...
firmware

* /efi//grubx64.efi (for

x64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mo ...

UEFI systems) is installed as a file in the EFI System Partition, and booted by the firmware directly, without a boot.img in MBR sector 0. This file is like stage1 and stage1.5. * /boot/grub/ can be installed on the EFI System Partition or the separate ''/boot'' partition. * For x64 UEFI systems, stage2 are the /boot/grub/x86_64-efi/normal.mod file and other /boot/grub/ files.

After startup

GRUB presents a menu where the user can choose from operating systems (OS) found by grub-install. GRUB can be configured to automatically load a specified OS after a user-defined timeout. If the timeout is set to zero seconds, pressing and holding while the computer is booting makes it possible to access the boot menu. In the operating system selection menu GRUB accepts a couple of commands: * By pressing , it is possible to edit kernel parameters of the selected menu item ''before the operating system is started''. The reason for doing this in GRUB (i.e. not editing the parameters in an already booted system) can be an emergency case: the system has failed to boot. Using the kernel parameters line it is possible, among other things, to specify a module to be disabled (blacklisted) for the kernel. This could be required if the specific kernel module is broken and thus prevents boot-up. For example, to blacklist the kernel module nvidia-current, one could append modprobe.blacklist=nvidia-current at the end of the kernel parameters. * By pressing , the user enters the GRUB command line. The GRUB command line is not a regular Linux shell, like e.g.

bash Bash or BASH may refer to: Arts and entertainment * ''Bash!'' (Rockapella album), 1992 * ''Bash!'' (Dave Bailey album), 1961 * '' Bash: Latter-Day Plays'', a dramatic triptych * ''BASH!'' (role-playing game), a 2005 superhero game * "Bash" ('' ...

, and accepts only certain GRUB-specific commands, documented by various Linux distributions. Once boot options have been selected, GRUB loads the selected kernel into memory and passes control to the kernel. Alternatively, GRUB can pass control of the boot process to another boot loader, using ''

chain loading Chain loading is a method used by computer programs to replace the currently executing program with a new program, using a common data area to pass information from the current program to the new program. It occurs in several areas of computing. ...

''. This is the method used to load operating systems that do not support the Multiboot Specification or are not supported directly by GRUB.

Identifying partitions (UUID workaround)

A computer can have multiple hard disks connected to it. These could be identified via their SATA port. Each time the computer POSTs, the hard disk connected to port 1 (of the motherboard), ''could'' be assigned the same identifier, e.g. . But what if such consistency cannot be guaranteed? What if the constellation of connected hard disks changed from one start up to another? What if a hard disk shall be connected to another computer? By entering into either the in ''GRUB rescue console'' (available after loading core.img ) or the ''GRUB console'' (available after loading normal.mod) a list of all available hard disks and partitions can be obtained. Either by guessing or by further investigation (e.g. can these numbers be assigned to actual hard disks and partitions. As it cannot be guaranteed that the numbering of hard disks via dumb numbers is consistent, GNU GRUB can use the UUID to identify partitions (actually file system instances) – that have this feature – instead. The file systems ext2, ext3, ext4 and xfs use an

Universally Unique Identifier A universally unique identifier (UUID) is a 128-bit nominal number, label used for information in computer systems. The term globally unique identifier (GUID) is also used. When generated according to the standard methods, UUIDs are, for practic ...

(UUID) to uniquely identify an instance. The UUID is created when a partition is formatted. The UUID is part of the file system and written to the superblock. All operations but a formatting should leave the UUID unaltered, but it is possible to change it or duplicate it (by using dd to clone an entire partition). The file is used to configure GRUB. It is the file where the commands which shall be executed during each start up are stored. Without an existing and valid , GRUB will present a prompt. An absolute minimal might contain only the following two commands (cf.

initial ramdisk In Linux systems, initrd (''initial ramdisk'') is a scheme for loading a temporary root file system into memory, to be used as part of the Linux startup process. initrd and initramfs refer to two different methods of achieving this. Both are comm ...

linux (hd0,1)/kernel/vmlinuz-3.20.1-4 ro  # use the file name "vmlinuz-…" located in the directory /kernel on the first partition of the first hard disk as linux kernel image
initrd (hd0,1)/boot/initrd.img-3.20.1-4   # use the file named "initrd.img–…" located in the directory /boot on the first partition of the first hard disk as initial ramdisk

More fancy will describe a menu to be presented, will use multiple colors, and maybe a background picture.

History

GRUB was initially developed by Erich Boleyn as part of work on booting the

GNU GNU () is an extensive collection of free software (383 packages as of January 2022), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operat ...

Hurd GNU Hurd is a collection of microkernel servers written as part of GNU, for the GNU Mach microkernel. It has been under development since 1990 by the GNU Project of the Free Software Foundation, designed as a replacement for the Unix kernel, and ...

, developed by the

.GRUB Manual – 1.2 Grub History
Gnu.org (2012-06-23). Retrieved on 2012-12-01. In 1999, Gordon Matzigkeit and Yoshinori K. Okuji made GRUB an official software package of the

and opened the development process to the public. , the majority of Linux distributions have adopted GNU GRUB 2, as well as other systems such as Sony's

PlayStation 4 The PlayStation 4 (PS4) is a home video game console developed by Sony Interactive Entertainment. Announced as the successor to the PlayStation 3 in February 2013, it was launched on November 15, 2013, in North America, November 29, 2013 in E ...

Development

GRUB version 1 (also known as "GRUB Legacy") is no longer under development and is being phased out. The GNU GRUB developers have switched their focus to GRUB 2, a complete rewrite with goals including making GNU GRUB cleaner, more robust, more portable and more powerful. GRUB 2 started under the name PUPA. PUPA was supported by the Information-technology Promotion Agency (IPA) in Japan. PUPA was integrated into GRUB 2 development around 2002, when GRUB version 0.9x was renamed GRUB Legacy. Some of the goals of the GRUB 2 project include support for non-x86 platforms,

internationalization and localization In computing, internationalization and localization (American) or internationalisation and localisation (British English), often abbreviated i18n and L10n, are means of adapting computer software to different languages, regional peculiarities and ...

, non-ASCII characters, dynamic modules,

memory management Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when ...

, a scripting

mini-language A domain-specific language (DSL) is a computer language specialized to a particular application domain. This is in contrast to a general-purpose language (GPL), which is broadly applicable across domains. There are a wide variety of DSLs, ranging f ...

, migrating platform specific (x86) code to platform specific modules, and an object-oriented framework. GNU GRUB version 2.00 was officially released on June 26, 2012. Three of the most widely used

Linux distributions A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...

use GRUB 2 as their mainstream boot loader.

Ubuntu Ubuntu ( ) is a Linux distribution based on Debian and composed mostly of free and open-source software. Ubuntu is officially released in three editions: ''Desktop'', ''Server'', and ''Core'' for Internet of things devices and robots. All the ...

adopted it as the default boot loader in its 9.10 version of October 2009.

Fedora A fedora () is a hat with a soft brim and indented crown.Kilgour, Ruth Edwards (1958). ''A Pageant of Hats Ancient and Modern''. R. M. McBride Company. It is typically creased lengthwise down the crown and "pinched" near the front on both sides ...

followed suit with Fedora 16 released in November 2011.

OpenSUSE openSUSE () is a free and open-source software, free and open source RPM Package Manager, RPM-based Linux distribution developed by the openSUSE project. The initial release of the community project was a beta version of SUSE Linux 10.0. Addi ...

adopted GRUB 2 as the default boot loader with its 12.2 release of September 2012.

Solaris Solaris may refer to: Arts and entertainment Literature, television and film * ''Solaris'' (novel), a 1961 science fiction novel by Stanisław Lem ** ''Solaris'' (1968 film), directed by Boris Nirenburg ** ''Solaris'' (1972 film), directed by ...

also adopted GRUB 2 on the x86 platform in the Solaris 11.1 release. In late 2015, the exploit of pressing backspace 28 times to bypass the login password was found and quickly fixed.

Variants

GNU GRUB is

free and open-source software Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software and open-source software where anyone is freely licensed to use, copy, study, and change the software in any way, and the source ...

, so several variants have been created. Some notable ones, which have not been merged into GRUB mainline: *

OpenSolaris OpenSolaris () is a discontinued open-source computer operating system based on Solaris and created by Sun Microsystems. It was also, perhaps confusingly, the name of a project initiated by Sun to build a developer and user community around th ...

includes a modified GRUB Legacy that supports Solaris VTOC slices, automatic 64-bit kernel selection, and booting from

ZFS ZFS (previously: Zettabyte File System) is a file system with volume management capabilities. It began as part of the Sun Microsystems Solaris operating system in 2001. Large parts of Solaris – including ZFS – were published under an open ...

(with

compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression *Gas compression *Compression ratio, of a c ...

and multiple boot environments). *

Google Summer of Code The Google Summer of Code, often abbreviated to GSoC, is an international annual program in which Google awards stipends to contributors who successfully complete a free and open-source software coding project during the summer. , the program is ...

2008 had a project to support GRUB legacy to boot from

ext4 ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3. ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for ...

formatted partitions. * The

Syllable A syllable is a unit of organization for a sequence of speech sounds typically made up of a syllable nucleus (most often a vowel) with optional initial and final margins (typically, consonants). Syllables are often considered the phonological "bu ...

project made a modified version of GRUB to load the system from its

AtheOS File System The AtheOS file system (''AFS'') was originally used in the AtheOS operating system under Master boot record, MBR partition ID , and is now a part of the Syllable Desktop, Syllable and the Syllable-based Wave OS operating system. AFS started with e ...

. * ''TrustedGRUB'' extends GRUB by implementing verification of the system integrity and boot process security, using the

Trusted Platform Module Trusted Platform Module (TPM, also known as ISO/IEC 11889) is an international standard for a secure cryptoprocessor, a dedicated microcontroller designed to secure hardware through integrated cryptographic keys. The term can also refer to a ch ...

(TPM). * The Intel BIOS Implementation Test Suite (BITS) provides a GRUB environment for testing BIOSes and in particular their initialization of Intel processors, hardware, and technologies. BITS supports scripting via Python, and includes Python APIs to access various low-level functionality of the hardware platform, including ACPI, CPU and chipset registers, PCI, and PCI Express. * GRUB4DOS is a GRUB legacy fork that improves the installation experience on DOS and

Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...

by putting everything besides the GRLDR config in one image file. It can be loaded directly from DOS, or by

NTLDR NTLDR (abbreviation of ''NT loader'') is the boot loader for all releases of Windows NT operating system from 1993 with the release of Windows NT 3.1 up until Windows XP and Windows Server 2003. From Windows Vista onwards it was replaced by the BOO ...

Windows Boot Manager The booting process of Windows Vista and later versions differ from the startup process part of previous versions of Windows. In this article, unless otherwise specified, what is said about Windows Vista also applies to all later NT operatin ...

. GRUB4DOS is under active development and as of 2021 supports UEFI.

Utilities

GRUB configuration tools

The setup tools in use by various distributions often include modules to set up GRUB. For example,

YaST2 YaST (Yet another Setup Tool) is a Linux operating system setup and configuration tool. YaST is featured in the openSUSE Linux distribution, as well as in SUSE's derived commercial distributions. It is also part of the defunct United Linux. YaS ...

on SUSE Linux and

openSUSE openSUSE () is a free and open-source software, free and open source RPM Package Manager, RPM-based Linux distribution developed by the openSUSE project. The initial release of the community project was a beta version of SUSE Linux 10.0. Addi ...

distributions and

Anaconda Anacondas or water boas are a group of large snakes of the genus ''Eunectes''. They are found in tropical South America. Four species are currently recognized. Description Although the name applies to a group of snakes, it is often used to re ...

RHEL Red Hat Enterprise Linux (RHEL) is a commercial open-source Linux distribution developed by Red Hat for the commercial market. Red Hat Enterprise Linux is released in server versions for x86-64, Power ISA, ARM64, and IBM Z and a desktop version ...

distributions. StartUp-Manager and GRUB Customizer are graphical configuration editors for Debian-based distributions. The development of StartUp-Manager stopped on 6 May 2011 after the lead developer cited personal reasons for not actively developing the program. GRUB Customizer is also available for Arch-based distributions. For GRUB 2 there are KDE Control Modules. GRLDR ICE is a tiny tool for modifying the default configuration of grldr file for GRUB4DOS.

Boot repair utilities

Boot-Repair is a simple graphical tool for recovering from frequent boot-related problems with GRUB and

bootloader. This application is available under

GNU GPL license The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the four freedoms to run, study, share, and modify the software. The license was the first copyleft for general us ...

. Boot-Repair can repair GRUB on multiple Linux distributions including, but not limited to, Debian, Ubuntu,

Mint MiNT is Now TOS (MiNT) is a free software alternative operating system kernel for the Atari ST system and its successors. It is a multi-tasking alternative to TOS and MagiC. Together with the free system components fVDI device drivers, XaAES g ...

, Fedora, openSUSE, and

Arch Linux Arch Linux () is an independently developed, x86-64 general-purpose Linux distribution that strives to provide the latest stable versions of most software by following a Rolling release, rolling-release model. The default installation is a minim ...

Installer for Windows

Grub2Win is a Windows open-source software package. It allows GNU GRUB to boot from a Windows directory. The setup program installs GNU GRUB version 2.06 to an NTFS partition. A Windows GUI application is then used to customize the GRUB boot menu, themes, UEFI boot order, scripts etc. All GNU GRUB scripts and commands are supported for both UEFI and legacy systems. Grub2Win can configure GRUB for multiboot of Windows, Ubuntu, openSuse, Fedora and many other Linux distributions. It is freely available under

GNU GPL License The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the four freedoms to run, study, share, and modify the software. The license was the first copyleft for general us ...

SourceForge SourceForge is a web service that offers software consumers a centralized online location to control and manage open-source software projects and research business software. It provides source code repository hosting, bug tracking, mirrorin ...

Alternative boot managers

The strength of GRUB is the wide range of supported platforms, file systems, and operating systems, making it the default choice for distributions and embedded systems. However, there are boot managers targeted at the end user that give more friendly user experience, graphical OS selector and simpler configuration: *

rEFInd rEFInd is a boot manager for Unified Extensible Firmware Interface, UEFI and EFI-based machines. It can be used to boot multiple operating systems that are installed on a single non-volatile device. It also provides a way to launch UEFI applicat ...

– Macintosh-style graphical boot manager, only for UEFI-based computers (BIOS not supported).
CloverEFI
– Macintosh-style graphical boot manager for BIOS and UEFI-based computers. It emulates UEFI with a heavily modifie
DUET
from the TianoCore project and requires a FAT formatted partition even on BIOS systems. As a benefit, it has a basic filesystem driver in the partition boot sector, avoiding the brittleness of a second or third stage and the infamous GRUB Rescue prompt. The user interface looks similar to rEFInd: both inherit from the abandoned boot manage
rEFIt
Non-graphical alternatives: *

systemd-boot systemd-boot is a free and open-source boot manager created by obsoleting the gummiboot project and merging it into systemd in May 2015. gummiboot was developed by the Red Hat employees Kay Sievers and Harald Hoyer and designed as a minimal ...

– Light, UEFI-only boot manager with text-based OS selector menu.

External links

How-Tos and troubleshooting

Distribution wikis have many solutions for common issues and custom setups that might help you:
Arch Linux /GRUB

Ubuntu /Grub2
(also see Links at the bottom) * Fedora /GRUB_2 * Gentoo /GRUB2
Grub2 theme tutorial

Documentation

GRUB manual
– most detailed documentation, including all commands *
GRUB wiki archived in 2010

Introductory articles

Boot with GRUB
an April 2001 article in

Linux Journal ''Linux Journal'' (''LJ'') is an American monthly technology magazine originally published by Specialized System Consultants, Inc. (SSC) in Seattle, Washington since 1994. In December 2006 the publisher changed to Belltown Media, Inc. in Houston ...

Technicalities

Booting Linux on x86 using Grub2
– in-depth article *

Unified Extensible Firmware Interface UEFI (Unified Extensible Firmware Interface) is a set of specifications written by the UEFI Forum. They define the architecture of the platform firmware used for booting and its interface for interaction with the operating system. Examples of ...

(UEFI firmware, common since 2012) *

GUID Partition Table The GUID Partition Table (GPT) is a standard for the layout of partition tables of a physical computer storage device, such as a hard disk drive or solid-state drive, using universally unique identifiers, which are also known as globally unique i ...

(GPT) – handles hard drives bigger than 2 TiB and more than 4 partitions *

Master boot record A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBR ...

used with BIOS firmware (motherboards roughly before 2012)
BIOS Boot Specification Version 1.01 (January 11, 1996)
– hard to find

Notes

References

{{GNU, state=collapsed Free boot loaders Free software primarily written in assembly language Free software programmed in C

GRUB Grub can refer to Grub (larva), of the beetle superfamily Scarabaeoidea, or as a slang term for food. It can also refer to: Places * Grub, Appenzell Ausserrhoden, Switzerland * Grub, St. Gallen, Switzerland * Grub (Amerang), a hamlet in Bavaria, ...

Research projects 1995 software