GNU GRUB manual

This is the documentation of GNU GRUB, the GRand Unified Bootloader, a flexible and powerful boot loader program for a wide range of architectures.

This edition documents version 2.14.

This manual is for GNU GRUB (version 2.14, 8 January 2026).

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections.

1 Introduction to GRUB

1.1 Overview

Briefly, a boot loader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to an operating system kernel software (such as Linux or GNU Mach). The kernel, in turn, initializes the rest of the operating system (e.g. a GNU system).

GNU GRUB is a very powerful boot loader, which can load a wide variety of free operating systems, as well as proprietary operating systems with chain-loading¹. GRUB is designed to address the complexity of booting a personal computer; both the program and this manual are tightly bound to that computer platform, although porting to other platforms may be addressed in the future.

One of the important features in GRUB is flexibility; GRUB understands filesystems and kernel executable formats, so you can load an arbitrary operating system the way you like, without recording the physical position of your kernel on the disk. Thus you can load the kernel just by specifying its file name and the drive and partition where the kernel resides.

When booting with GRUB, you can use either a command-line interface (see The flexible command-line interface), or a menu interface (see The simple menu interface). Using the command-line interface, you type the drive specification and file name of the kernel manually. In the menu interface, you just select an OS using the arrow keys. The menu is based on a configuration file which you prepare beforehand (see Writing your own configuration file). While in the menu, you can switch to the command-line mode, and vice-versa. You can even edit menu entries before using them.

In the following chapters, you will learn how to specify a drive, a partition, and a file name (see Naming convention) to GRUB, how to install GRUB on your drive (see Installation), and how to boot your OSes (see Booting), step by step.

1.2 History of GRUB

GRUB originated in 1995 when Erich Boleyn was trying to boot the GNU Hurd with the University of Utah’s Mach 4 microkernel (now known as GNU Mach). Erich and Brian Ford designed the Multiboot Specification (see Motivation in The Multiboot Specification), because they were determined not to add to the large number of mutually-incompatible PC boot methods.

Erich then began modifying the FreeBSD boot loader so that it would understand Multiboot. He soon realized that it would be a lot easier to write his own boot loader from scratch than to keep working on the FreeBSD boot loader, and so GRUB was born.

Erich added many features to GRUB, but other priorities prevented him from keeping up with the demands of its quickly-expanding user base. In 1999, Gordon Matzigkeit and Yoshinori K. Okuji adopted GRUB as an official GNU package, and opened its development by making the latest sources available via anonymous CVS. See How to obtain and build GRUB, for more information.

Over the next few years, GRUB was extended to meet many needs, but it quickly became clear that its design was not keeping up with the extensions being made to it, and we reached the point where it was very difficult to make any further changes without breaking existing features. Around 2002, Yoshinori K. Okuji started work on PUPA (Preliminary Universal Programming Architecture for GNU GRUB), aiming to rewrite the core of GRUB to make it cleaner, safer, more robust, and more powerful. PUPA was eventually renamed to GRUB 2, and the original version of GRUB was renamed to GRUB Legacy. Small amounts of maintenance continued to be done on GRUB Legacy, but the last release (0.97) was made in 2005 and at the time of writing it seems unlikely that there will be another.

By around 2007, GNU/Linux distributions started to use GRUB 2 to limited extents, and by the end of 2009 multiple major distributions were installing it by default.

1.3 Differences from previous versions

GRUB 2 is a rewrite of GRUB (see History of GRUB), although it shares many characteristics with the previous version, now known as GRUB Legacy. Users of GRUB Legacy may need some guidance to find their way around this new version.

The configuration file has a new name (grub.cfg rather than menu.lst or grub.conf), new syntax (see Writing your own configuration file) and many new commands (see Available commands). Configuration cannot be copied over directly, although most GRUB Legacy users should not find the syntax too surprising.
grub.cfg is typically automatically generated by grub-mkconfig (see Simple configuration handling). This makes it easier to handle versioned kernel upgrades.
Partition numbers in GRUB device names now start at 1, not 0 (see Naming convention).
The configuration file is now written in something closer to a full scripting language: variables, conditionals, and loops are available.
A small amount of persistent storage is available across reboots, using the save_env and load_env commands in GRUB and the grub-editenv utility. This is not available in all configurations (see The GRUB environment block).
GRUB 2 has more reliable ways to find its own files and those of target kernels on multiple-disk systems, and has commands (see search) to find devices using file system labels or Universally Unique Identifiers (UUIDs).
GRUB 2 is available for several other types of system in addition to the PC BIOS systems supported by GRUB Legacy: PC EFI, PC coreboot, PowerPC, SPARC, and MIPS Lemote Yeeloong are all supported.
Many more file systems are supported, including but not limited to ext4, HFS+, and NTFS.
GRUB 2 can read files directly from LVM and RAID devices.
A graphical terminal and a graphical menu system are available.
GRUB 2’s interface can be translated, including menu entry names.
The image files (see GRUB image files) that make up GRUB have been reorganised; Stage 1, Stage 1.5, and Stage 2 are no more.
GRUB 2 puts many facilities in dynamically loaded modules, allowing the core image to be smaller, and allowing the core image to be built in more flexible ways.

1.4 GRUB features

The primary requirement for GRUB is that it be compliant with the Multiboot Specification, which is described in Motivation in The Multiboot Specification.

The other goals, listed in approximate order of importance, are:

Basic functions must be straightforward for end-users.
Rich functionality to support kernel experts and designers.
Backward compatibility for booting FreeBSD, NetBSD, OpenBSD, and Linux. Proprietary kernels (such as DOS, Windows NT, and OS/2) are supported via a chain-loading function.

Except for specific compatibility modes (chain-loading and the Linux piggyback format), all kernels will be started in much the same state as in the Multiboot Specification. Only kernels loaded at 1 megabyte or above are presently supported. Any attempt to load below that boundary will simply result in immediate failure and an error message reporting the problem.

In addition to the requirements above, GRUB has the following features (note that the Multiboot Specification doesn’t require all the features that GRUB supports):

Recognize multiple executable formats

Support many of the a.out variants plus ELF. Symbol tables are also loaded.

Support non-Multiboot kernels

Support many of the various free 32-bit kernels that lack Multiboot compliance (primarily FreeBSD, NetBSD², OpenBSD, and Linux). Chain-loading of other boot loaders is also supported.

Load multiples modules

Fully support the Multiboot feature of loading multiple modules.

Load a configuration file

Support a human-readable text configuration file with preset boot commands. You can also load another configuration file dynamically and embed a preset configuration file in a GRUB image file. The list of commands (see Available commands) are a superset of those supported on the command-line. An example configuration file is provided in Writing your own configuration file.

Provide a menu interface

A menu interface listing preset boot commands, with a programmable timeout, is available. There is no fixed limit on the number of boot entries, and the current implementation has space for several hundred.

Have a flexible command-line interface

A fairly flexible command-line interface, accessible from the menu, is available to edit any preset commands, or write a new boot command set from scratch. If no configuration file is present, GRUB drops to the command-line.

The list of commands (see Available commands) are a subset of those supported for configuration files. Editing commands closely resembles the Bash command-line (see Command Line Editing in Bash Features), with TAB -completion of commands, devices, partitions, and files in a directory depending on context.

Support multiple filesystem types

Support multiple filesystem types transparently, plus a useful explicit blocklist notation. The currently supported filesystem types are Amiga Fast FileSystem (AFFS), AtheOS fs, BeFS, BtrFS (including raid0, raid1, raid10, gzip and lzo), cpio (little- and big-endian bin, odc and newc variants), EROFS (only uncompressed support for now), Linux ext2/ext3/ext4, DOS FAT12/FAT16/FAT32, exFAT, F2FS, HFS, HFS+, ISO9660 (including Joliet, Rock-ridge and multi-chunk files), JFS, Minix fs (versions 1, 2 and 3), nilfs2, NTFS (including compression), ReiserFS, ROMFS, Amiga Smart FileSystem (SFS), Squash4, tar, UDF, BSD UFS/UFS2, XFS, and ZFS (including lzjb, gzip, zle, mirror, stripe, raidz1/2/3 and encryption in AES-CCM and AES-GCM). See Filesystem syntax and semantics, for more information. Note: Only a subset of filesystems are supported in lockdown mode (such as when secure boot is enabled, see Lockdown when booting on a secure setup for more information).

Support automatic decompression

Can decompress files which were compressed by gzip or xz³. This function is both automatic and transparent to the user (i.e. all functions operate upon the uncompressed contents of the specified files). This greatly reduces a file size and loading time, a particularly great benefit for floppies.⁴

It is conceivable that some kernel modules should be loaded in a compressed state, so a different module-loading command can be specified to avoid uncompressing the modules.

Access data on any installed device

Support reading data from any or all floppies or hard disk(s) recognized by the BIOS, independent of the setting of the root device.

Be independent of drive geometry translations

Unlike many other boot loaders, GRUB makes the particular drive translation irrelevant. A drive installed and running with one translation may be converted to another translation without any adverse effects or changes in GRUB’s configuration.

Detect all installed RAM

GRUB can generally find all the installed RAM on a PC-compatible machine. It uses an advanced BIOS query technique for finding all memory regions. As described on the Multiboot Specification (see Motivation in The Multiboot Specification), not all kernels make use of this information, but GRUB provides it for those who do.

Support Logical Block Address mode

In traditional disk calls (called CHS mode), there is a geometry translation problem, that is, the BIOS cannot access over 1024 cylinders, so the accessible space is limited to at least 508 MB and to at most 8GB. GRUB can’t universally solve this problem, as there is no standard interface used in all machines. However, several newer machines have the new interface, Logical Block Address (LBA) mode. GRUB automatically detects if LBA mode is available and uses it if available. In LBA mode, GRUB can access the entire disk.

Support network booting

GRUB is basically a disk-based boot loader but also has network support. You can load OS images from a network by using the TFTP protocol.

Support remote terminals

To support computers with no console, GRUB provides remote terminal support, so that you can control GRUB from a remote host. Only serial terminal support is implemented at the moment.

1.5 The role of a boot loader

The following is a quotation from Gordon Matzigkeit, a GRUB fanatic:

Some people like to acknowledge both the operating system and kernel when they talk about their computers, so they might say they use “GNU/Linux” or “GNU/Hurd”. Other people seem to think that the kernel is the most important part of the system, so they like to call their GNU operating systems “Linux systems.”

I, personally, believe that this is a grave injustice, because the boot loader is the most important software of all. I used to refer to the above systems as either “LILO”⁵ or “GRUB” systems.

Unfortunately, nobody ever understood what I was talking about; now I just use the word “GNU” as a pseudonym for GRUB.

So, if you ever hear people talking about their alleged “GNU” systems, remember that they are actually paying homage to the best boot loader around… GRUB!

We, the GRUB maintainers, do not (usually) encourage Gordon’s level of fanaticism, but it helps to remember that boot loaders deserve recognition. We hope that you enjoy using GNU GRUB as much as we did writing it.

2 Naming convention

The device syntax used in GRUB is a wee bit different from what you may have seen before in your operating system(s), and you need to know it so that you can specify a drive/partition.

Look at the following examples and explanations:

(fd0)

First of all, GRUB requires that the device name be enclosed with ‘(’ and ‘)’. The ‘fd’ part means that it is a floppy disk. The number ‘0’ is the drive number, which is counted from zero. This expression means that GRUB will use the whole floppy disk.

(hd0,msdos2)

Here, ‘hd’ means it is a hard disk drive. The first integer ‘0’ indicates the drive number, that is, the first hard disk, the string ‘msdos’ indicates the partition scheme, while the second integer, ‘2’, indicates the partition number (or the PC slice number in the BSD terminology). The partition numbers are counted from one, not from zero (as was the case in previous versions of GRUB). This expression means the second partition of the first hard disk drive. In this case, GRUB uses one partition of the disk, instead of the whole disk.

(hd0,msdos5)

This specifies the first extended partition of the first hard disk drive. Note that the partition numbers for extended partitions are counted from ‘5’, regardless of the actual number of primary partitions on your hard disk.

(hd1,msdos1,bsd1)

This means the BSD ‘a’ partition on first PC slice number of the second hard disk.

Of course, to actually access the disks or partitions with GRUB, you need to use the device specification in a command, like ‘set root=(fd0)’ or ‘parttool (hd0,msdos3) hidden-’. To help you find out which number specifies a partition you want, the GRUB command-line (see The flexible command-line interface) options have argument completion. This means that, for example, you only need to type

set root=(

followed by a TAB , and GRUB will display the list of drives, partitions, or file names. So it should be quite easy to determine the name of your target partition, even with minimal knowledge of the syntax.

Note that GRUB does not distinguish IDE from SCSI - it simply counts the drive numbers from zero, regardless of their type. Normally, any IDE drive number is less than any SCSI drive number, although that is not true if you change the boot sequence by swapping IDE and SCSI drives in your BIOS.

Now the question is, how to specify a file? Again, consider an example:

(hd0,msdos1)/vmlinuz

This specifies the file named ‘vmlinuz’, found on the first partition of the first hard disk drive. Note that the argument completion works with file names, too.

That was easy, admit it. Now read the next chapter, to find out how to actually install GRUB on your drive.

3 OS-specific notes about grub tools

On OS which have device nodes similar to Unix-like OS GRUB tools use the OS name. E.g. for GNU/Linux:

# grub-install /dev/sda

On AROS we use another syntax. For volumes:

//:<volume name>

E.g.

//:DH0

For disks we use syntax:

//:<driver name>/unit/flags

E.g.

# grub-install //:ata.device/0/0

On Windows we use UNC path. For volumes it’s typically

\\?\Volume{<GUID>}
\\?\<drive letter>:

E.g.

\\?\Volume{17f34d50-cf64-4b02-800e-51d79c3aa2ff}
\\?\C:

For disks it’s

\\?\PhysicalDrive<number>

E.g.

# grub-install \\?\PhysicalDrive0

Beware that you may need to further escape the backslashes depending on your shell.

When compiled with cygwin support then cygwin drive names are automatically when needed. E.g.

# grub-install /dev/sda

4 Installation

In order to install GRUB as your boot loader, you need to first install the GRUB system and utilities under your UNIX-like operating system (see How to obtain and build GRUB). You can do this either from the source tarball, or as a package for your OS.

After you have done that, you need to install the boot loader on a drive (floppy or hard disk) by using the utility grub-install (see Invoking grub-install) on a UNIX-like OS.

GRUB comes with boot images, which are normally put in the directory /usr/lib/grub/<cpu>-<platform> (for BIOS-based machines /usr/lib/grub/i386-pc). Hereafter, the directory where GRUB images are initially placed (normally /usr/lib/grub/<cpu>-<platform>) will be called the image directory, and the directory where the boot loader needs to find them (usually /boot) will be called the boot directory.

4.1 Installing GRUB using grub-install

For information on where GRUB should be installed on PC BIOS platforms, see BIOS installation.

In order to install GRUB under a UNIX-like OS (such as GNU), invoke the program grub-install (see Invoking grub-install) as the superuser (root).

The usage is basically very simple. You only need to specify one argument to the program, namely, where to install the boot loader. The argument has to be either a device file (like ‘/dev/hda’). For example, under Linux the following will install GRUB into the MBR of the first IDE disk:

# grub-install /dev/sda

Likewise, under GNU/Hurd, this has the same effect:

# grub-install /dev/hd0

But all the above examples assume that GRUB should put images under the /boot directory. If you want GRUB to put images under a directory other than /boot, you need to specify the option --boot-directory. The typical usage is that you create a GRUB boot floppy with a filesystem. Here is an example:

# mke2fs /dev/fd0
# mount -t ext2 /dev/fd0 /mnt
# mkdir /mnt/boot
# grub-install --boot-directory=/mnt/boot /dev/fd0
# umount /mnt

Some BIOSes have a bug of exposing the first partition of a USB drive as a floppy instead of exposing the USB drive as a hard disk (they call it “USB-FDD” boot). In such cases, you need to install like this:

# losetup /dev/loop0 /dev/sdb1
# mount /dev/loop0 /mnt/usb
# grub-install --boot-directory=/mnt/usb/bugbios --force --allow-floppy /dev/loop0

This install doesn’t conflict with standard install as long as they are in separate directories.

On EFI systems for fixed disk install you have to mount EFI System Partition. If you mount it at /boot/efi then you don’t need any special arguments:

# grub-install

Otherwise you need to specify where your EFI System partition is mounted:

# grub-install --efi-directory=/mnt/efi

For removable installs you have to use --removable and specify both --boot-directory and --efi-directory:

# grub-install --efi-directory=/mnt/usb --boot-directory=/mnt/usb/boot --removable

4.2 Making a GRUB bootable CD-ROM

GRUB supports the no emulation mode in the El Torito specification⁶. This means that you can use the whole CD-ROM from GRUB and you don’t have to make a floppy or hard disk image file, which can cause compatibility problems.

For booting from a CD-ROM, GRUB uses a special image called cdboot.img, which is concatenated with core.img. The core.img used for this should be built with at least the ‘iso9660’ and ‘biosdisk’ modules. Your bootable CD-ROM will usually also need to include a configuration file grub.cfg and some other GRUB modules.

To make a simple generic GRUB rescue CD, you can use the grub-mkrescue program (see Invoking grub-mkrescue):

$ grub-mkrescue -o grub.iso

You will often need to include other files in your image. To do this, first make a top directory for the bootable image, say, ‘iso’:

$ mkdir iso

Make a directory for GRUB:

$ mkdir -p iso/boot/grub

If desired, make the config file grub.cfg under iso/boot/grub (see Writing your own configuration file), and copy any files and directories for the disc to the directory iso/.

Finally, make the image:

$ grub-mkrescue -o grub.iso iso

This produces a file named grub.iso, which then can be burned into a CD (or a DVD), or written to a USB mass storage device.

The root device will be set up appropriately on entering your grub.cfg configuration file, so you can refer to file names on the CD without needing to use an explicit device name. This makes it easier to produce rescue images that will work on both optical drives and USB mass storage devices.

4.3 The map between BIOS drives and OS devices

If the device map file exists, the GRUB utilities (grub-probe, etc.) read it to map BIOS drives to OS devices. This file consists of lines like this:

(device) file

device is a drive specified in the GRUB syntax (see How to specify devices), and file is an OS file, which is normally a device file.

Historically, the device map file was used because GRUB device names had to be used in the configuration file, and they were derived from BIOS drive numbers. The map between BIOS drives and OS devices cannot always be guessed correctly: for example, GRUB will get the order wrong if you exchange the boot sequence between IDE and SCSI in your BIOS.

Unfortunately, even OS device names are not always stable. Modern versions of the Linux kernel may probe drives in a different order from boot to boot, and the prefix (/dev/hd* versus /dev/sd*) may change depending on the driver subsystem in use. As a result, the device map file required frequent editing on some systems.

GRUB avoids this problem nowadays by using UUIDs or file system labels when generating grub.cfg, and we advise that you do the same for any custom menu entries you write. If the device map file does not exist, then the GRUB utilities will assume a temporary device map on the fly. This is often good enough, particularly in the common case of single-disk systems.

However, the device map file is not entirely obsolete yet, and it is used for overriding when current environment is different from the one on boot. Most common case is if you use a partition or logical volume as a disk for virtual machine. You can put any comments in the file if needed, as the GRUB utilities assume that a line is just a comment if the first character is ‘#’.

4.4 BIOS installation

MBR

The partition table format traditionally used on PC BIOS platforms is called the Master Boot Record (MBR) format; this is the format that allows up to four primary partitions and additional logical partitions. With this partition table format, there are two ways to install GRUB: it can be embedded in the area between the MBR and the first partition (called by various names, such as the "boot track", "MBR gap", or "embedding area", and which is usually at least 1000 KiB), or the core image can be installed in a file system and a list of the blocks that make it up can be stored in the first sector of that partition.

Modern tools usually leave MBR gap of at least 1023 KiB. This amount is sufficient to cover most configurations. Hence this value is recommended by the GRUB team.

Historically many tools left only 31 KiB of space. This is not enough to parse reliably difficult structures like Btrfs, ZFS, RAID or LVM, or to use difficult disk access methods like ahci. Hence GRUB will warn if attempted to install into small MBR gap except in a small number of configurations that were grandfathered. The grandfathered config must:

use biosdisk as disk access module for /boot
not use any additional partition maps to access /boot
/boot must be on one of following filesystems: AFFS, AFS, BFS, cpio, newc, odc, ext2/3/4, FAT, exFAT, F2FS, HFS, uncompressed HFS+, ISO9660, JFS, Minix, Minix2, Minix3, NILFS2, NTFS, ReiserFS, ROMFS, SFS, tar, UDF, UFS1, UFS2, XFS

Note: Only a subset of filesystems are supported in lockdown mode (such as when secure boot is enabled, see Lockdown when booting on a secure setup for more information).

MBR gap has few technical problems. There is no way to reserve space in the embedding area with complete safety, and some proprietary software is known to use it to make it difficult for users to work around licensing restrictions. GRUB works around it by detecting sectors by other software and avoiding them and protecting its own sectors using Reed-Solomon encoding.

GRUB team recommends having MBR gap of at least 1000 KiB.

Should it not be possible, GRUB has support for a fallback solution which is heavily recommended against. Installing to a filesystem means that GRUB is vulnerable to its blocks being moved around by filesystem features such as tail packing, or even by aggressive fsck implementations, so this approach is quite fragile; and this approach can only be used if the /boot filesystem is on the same disk that the BIOS boots from, so that GRUB does not have to rely on guessing BIOS drive numbers.

The GRUB development team generally recommends embedding GRUB before the first partition, unless you have special requirements. You must ensure that the first partition starts at least 1000 KiB (2000 sectors) from the start of the disk; on modern disks, it is often a performance advantage to align partitions on larger boundaries anyway, so the first partition might start 1 MiB from the start of the disk.

GPT

Some newer systems use the GUID Partition Table (GPT) format. This was specified as part of the Extensible Firmware Interface (EFI), but it can also be used on BIOS platforms if system software supports it; for example, GRUB and GNU/Linux can be used in this configuration. With this format, it is possible to reserve a whole partition for GRUB, called the BIOS Boot Partition. GRUB can then be embedded into that partition without the risk of being overwritten by other software and without being contained in a filesystem which might move its blocks around.

When creating a BIOS Boot Partition on a GPT system, you should make sure that it is at least 31 KiB in size. (GPT-formatted disks are not usually particularly small, so we recommend that you make it larger than the bare minimum, such as 1 MiB, to allow plenty of room for growth.) You must also make sure that it has the proper partition type. Using GNU Parted, you can set this using a command such as the following:

# parted /dev/disk set partition-number bios_grub on

If you are using gdisk, set the partition type to ‘0xEF02’. With partitioning programs that require setting the GUID directly, it should be ‘21686148-6449-6e6f-744e656564454649’.

Caution: Be very careful which partition you select! When GRUB finds a BIOS Boot Partition during installation, it will automatically overwrite part of it. Make sure that the partition does not contain any other data.

5 Booting

GRUB can load Multiboot-compliant kernels in a consistent way, but for some free operating systems you need to use some OS-specific magic.

5.1 How to boot operating systems

GRUB has three distinct boot methods: loading an operating system directly, using kexec from userspace, and chainloading another bootloader. Generally speaking, the first two are more desirable because you don’t need to install or maintain other boot loaders and GRUB is flexible enough to load an operating system from an arbitrary disk/partition. However, chainloading is sometimes required, as GRUB doesn’t support all existing operating systems natively.

5.1.1 How to boot an OS directly with GRUB

Multiboot (see Motivation in The Multiboot Specification) is the native format supported by GRUB. For the sake of convenience, there is also support for Linux, FreeBSD, NetBSD and OpenBSD. If you want to boot other operating systems, you will have to chain-load them (see Chain-loading an OS).

FIXME: this section is incomplete.

Run the command boot (see boot).

However, DOS and Windows have some deficiencies, so you might have to use more complicated instructions. See DOS/Windows, for more information.

5.1.2 Kexec with grub2-emu

GRUB can be run in userspace by invoking the grub2-emu tool. It will read all configuration scripts as if booting directly (see How to boot an OS directly with GRUB). With the --kexec flag, and kexec(8) support from the operating system, the linux command will directly boot the target image. For systems that lack working systemctl(1) support for kexec, passing the --kexec flag twice will fallback to invoking kexec(8) directly; note however that this fallback may be unsafe outside read-only environments, as it does not invoke shutdown machinery.

5.1.3 Chain-loading an OS

Operating systems that do not support Multiboot and do not have specific support in GRUB (specific support is available for Linux, FreeBSD, NetBSD and OpenBSD) must be chain-loaded, which involves loading another boot loader and jumping to it in real mode or via the firmware.

The chainloader command (see chainloader) is used to set this up. It is normally also necessary to load some GRUB modules and set the appropriate root device. Putting this together, we get something like this, for a Windows system on the first partition of the first hard disk:

menuentry "Windows" {
	insmod chain
	insmod ntfs
	set root=(hd0,1)
	chainloader +1
}

On systems with multiple hard disks, an additional workaround may be required. See DOS/Windows.

Chain-loading is only supported on PC BIOS and EFI platforms.

5.2 Loopback booting

GRUB is able to read from an image (be it one of CD or HDD) stored on any of its accessible storages (refer to see loopback command). However the OS itself should be able to find its root. This usually involves running a userspace program running before the real root is discovered. This is achieved by GRUB loading a specially made small image and passing it as ramdisk to the kernel. This is achieved by commands kfreebsd_module, knetbsd_module_elf, kopenbsd_ramdisk, initrd (see initrd), initrd16 (see initrd16), multiboot_module, multiboot2_module or xnu_ramdisk depending on the loader. Note that for knetbsd the image must be put inside miniroot.kmod and the whole miniroot.kmod has to be loaded. In kopenbsd payload this is disabled by default. Additionally, behaviour of initial ramdisk depends on command line options. Several distributors provide the image for this purpose or it’s integrated in their standard ramdisk and activated by special option. Consult your kernel and distribution manual for more details. Other loaders like appleloader, chainloader (BIOS, EFI, coreboot), freedos, ntldr, plan9 and truecrypt provide no possibility of loading initial ramdisk and as far as author is aware the payloads in question don’t support either initial ramdisk or discovering loopback boot in other way and as such not bootable this way. Please consider alternative boot methods like copying all files from the image to actual partition. Consult your OS documentation for more details.

5.3 Booting from LVM cache logical volume

The LVM cache logical volume is the logical volume consisting of the original and the cache pool logical volume. The original is usually on a larger and slower storage device while the cache pool is on a smaller and faster one. The performance of the original volume can be improved by storing the frequently used data on the cache pool to utilize the greater performance of faster device.

GRUB boots from LVM cache logical volume merely by reading it’s original logical volume so that dirty data in cache pool volume is disregarded. This is not a problem for "writethrough" cache mode as it ensures that any data written will be stored both on the cache and the origin LV. For the other cache mode "writeback", which delays writing from the cache pool back to the origin LV to boost performance, GRUB may fail to boot in the wake of accidental power outage due to it’s inability to assemble the cache device for reading the required dirty data left behind. The situation will be improved after adding full support to the LVM cache logical volume in the future.

5.4 Some caveats on OS-specific issues

Here, we describe some caveats on several operating systems.

5.4.1 GNU/Hurd

Since GNU/Hurd is Multiboot-compliant, it is easy to boot it; there is nothing special about it. But do not forget that you have to specify a root partition to the kernel.

Set GRUB’s root device to the same drive as GNU/Hurd’s. The command search --set=root --file /boot/gnumach.gz or similar may help you (see search).

Load the kernel and the modules, like this:

grub> multiboot /boot/gnumach.gz root=device:hd0s1
grub> module  /hurd/ext2fs.static ext2fs --readonly \
                   --multiboot-command-line='${kernel-command-line}' \
                   --host-priv-port='${host-port}' \
                   --device-master-port='${device-port}' \
                   --exec-server-task='${exec-task}' -T typed '${root}' \
                   '$(task-create)' '$(task-resume)'
grub> module /lib/ld.so.1 exec /hurd/exec '$(exec-task=task-create)'

Finally, run the command boot (see boot).

5.4.2 GNU/Linux

It is relatively easy to boot GNU/Linux from GRUB, because it somewhat resembles to boot a Multiboot-compliant OS.

Set GRUB’s root device to the same drive as GNU/Linux’s. The command search --set=root --file /vmlinuz or similar may help you (see search).
Load the kernel using the command linux (see linux):
```
grub> linux /vmlinuz root=/dev/sda1
```
If you need to specify some kernel parameters, just append them to the command. For example, to set acpi to ‘off’, do this:
```
grub> linux /vmlinuz root=/dev/sda1 acpi=off
```
See the documentation in the Linux source tree for complete information on the available options.

With linux GRUB uses 32-bit protocol. Some BIOS services like APM or EDD aren’t available with this protocol. In this case you need to use linux16
```
grub> linux16 /vmlinuz root=/dev/sda1 acpi=off
```
If you use an initrd, execute the command initrd (see initrd) after linux:
```
grub> initrd /initrd
```
If you used linux16 you need to use initrd16:
```
grub> initrd16 /initrd
```
Finally, run the command boot (see boot).

5.4.3 NetBSD

Booting a NetBSD kernel from GRUB is also relatively easy: first set GRUB’s root device, then load the kernel and the modules, and finally run boot.

Set GRUB’s root device to the partition holding the NetBSD root file system. For a disk with a NetBSD disk label, this is usually the first partition (a:). In that case, and assuming that the partition is on the first hard disk, set GRUB’s root device as follows:
```
grub> insmod part_bsd
grub> set root=(hd0,netbsd1)
```
For a disk with a GUID Partition Table (GPT), and assuming that the NetBSD root partition is the third GPT partition, do this:
```
grub> insmod part_gpt
grub> set root=(hd0,gpt3)
```
Load the kernel using the command knetbsd:
```
grub> knetbsd /netbsd
```
Various options may be given to knetbsd. These options are, for the most part, the same as in the NetBSD boot loader. For instance, to boot the system in single-user mode and with verbose messages, do this:
```
grub> knetbsd /netbsd -s -v
```
If needed, load kernel modules with the command knetbsd_module_elf. A typical example is the module for the root file system:
```
grub> knetbsd_module_elf /stand/amd64/6.0/modules/ffs/ffs.kmod
```
Finally, run the command boot (see boot).

5.4.4 DOS/Windows

GRUB cannot boot DOS or Windows directly, so you must chain-load them (see Chain-loading an OS). However, their boot loaders have some critical deficiencies, so it may not work to just chain-load them. To overcome the problems, GRUB provides you with two helper functions.

If you have installed DOS (or Windows) on a non-first hard disk, you have to use the disk swapping technique, because that OS cannot boot from any disks but the first one. The workaround used in GRUB is the command drivemap (see drivemap), like this:

drivemap -s (hd0) (hd1)

This performs a virtual swap between your first and second hard drive.

Caution: This is effective only if DOS (or Windows) uses BIOS to access the swapped disks. If that OS uses a special driver for the disks, this probably won’t work.

Another problem arises if you installed more than one set of DOS/Windows onto one disk, because they could be confused if there are more than one primary partitions for DOS/Windows. Certainly you should avoid doing this, but there is a solution if you do want to do so. Use the partition hiding/unhiding technique.

If GRUB hides a DOS (or Windows) partition (see parttool), DOS (or Windows) will ignore the partition. If GRUB unhides a DOS (or Windows) partition, DOS (or Windows) will detect the partition. Thus, if you have installed DOS (or Windows) on the first and the second partition of the first hard disk, and you want to boot the copy on the first partition, do the following:

parttool (hd0,1) hidden-
parttool (hd0,2) hidden+
set root=(hd0,1)
chainloader +1
parttool  ${root}  boot+
boot

6 Writing your own configuration file

GRUB is configured using grub.cfg, usually located under /boot/grub. This file is quite flexible, but most users will not need to write the whole thing by hand.

6.1 Simple configuration handling

The program grub-mkconfig (see Invoking grub-mkconfig) generates grub.cfg files suitable for most cases. It is suitable for use when upgrading a distribution, and will discover available kernels and attempt to generate menu entries for them.

grub-mkconfig does have some limitations. While adding extra custom menu entries to the end of the list can be done by editing /etc/grub.d/40_custom or creating /boot/grub/custom.cfg, changing the order of menu entries or changing their titles may require making complex changes to shell scripts stored in /etc/grub.d/. This may be improved in the future. In the meantime, those who feel that it would be easier to write grub.cfg directly are encouraged to do so (see Booting, and Writing full configuration files directly), and to disable any system provided by their distribution to automatically run grub-mkconfig.

The file /etc/default/grub controls the operation of grub-mkconfig. It is sourced by a shell script, and so must be valid POSIX shell input; normally, it will just be a sequence of ‘KEY=value’ lines, but if the value contains spaces or other special characters then it must be quoted. For example:

GRUB_TERMINAL_INPUT="console serial"

Valid keys in /etc/default/grub are as follows:

‘GRUB_DEFAULT’

The default menu entry. This may be a number, in which case it identifies the Nth entry in the generated menu counted from zero, or the title of a menu entry, or the special string ‘saved’. Using the id may be useful if you want to set a menu entry as the default even though there may be a variable number of entries before it.

For example, if you have:

menuentry 'Example GNU/Linux distribution' --class gnu-linux --id example-gnu-linux {
	...
}

then you can make this the default using:

GRUB_DEFAULT=example-gnu-linux

Previously it was documented the way to use entry title. While this still works it’s not recommended since titles often contain unstable device names and may be translated

If you set this to ‘saved’, then the default menu entry will be that saved by ‘GRUB_SAVEDEFAULT’ or grub-set-default. This relies on the environment block, which may not be available in all situations (see The GRUB environment block).

The default is ‘0’.

‘GRUB_SAVEDEFAULT’

If this option is set to ‘true’, then, when an entry is selected, save it as a new default entry for use by future runs of GRUB. This is only useful if ‘GRUB_DEFAULT=saved’; it is a separate option because ‘GRUB_DEFAULT=saved’ is useful without this option, in conjunction with grub-set-default. Unset by default. This option relies on the environment block, which may not be available in all situations (see The GRUB environment block).

‘GRUB_TIMEOUT’

Boot the default entry this many seconds after the menu is displayed, unless a key is pressed. The default is ‘5’. Set to ‘0’ to boot immediately without displaying the menu, or to ‘-1’ to wait indefinitely.

If ‘GRUB_TIMEOUT_STYLE’ is set to ‘countdown’ or ‘hidden’, the timeout is instead counted before the menu is displayed.

‘GRUB_TIMEOUT_STYLE’

If this option is unset or set to ‘menu’, then GRUB will display the menu and then wait for the timeout set by ‘GRUB_TIMEOUT’ to expire before booting the default entry. Pressing a key interrupts the timeout.

If this option is set to ‘countdown’ or ‘hidden’, then, before displaying the menu, GRUB will wait for the timeout set by ‘GRUB_TIMEOUT’ to expire. If ESC or F4 are pressed, or SHIFT is held down during that time, it will display the menu and wait for input. If a hotkey associated with a menu entry is pressed, it will boot the associated menu entry immediately. If the timeout expires before either of these happens, it will boot the default entry. In the ‘countdown’ case, it will show a one-line indication of the remaining time.

‘GRUB_DEFAULT_BUTTON’

‘GRUB_TIMEOUT_BUTTON’

‘GRUB_TIMEOUT_STYLE_BUTTON’

‘GRUB_BUTTON_CMOS_ADDRESS’

Variants of the corresponding variables without the ‘_BUTTON’ suffix, used to support vendor-specific power buttons. See Using GRUB with vendor power-on keys.

‘GRUB_DISTRIBUTOR’

Set by distributors of GRUB to their identifying name. This is used to generate more informative menu entry titles.

‘GRUB_TERMINAL_INPUT’

Select the terminal input device. You may select multiple devices here, separated by spaces.

Valid terminal input names depend on the platform, but may include ‘console’ (native platform console), ‘serial’ (serial terminal), ‘serial_<port>’ (serial terminal with explicit port selection), ‘at_keyboard’ (PC AT keyboard), or ‘usb_keyboard’ (USB keyboard using the HID Boot Protocol, for cases where the firmware does not handle this).

The default is to use the platform’s native terminal input.

‘GRUB_TERMINAL_OUTPUT’

Select the terminal output device. You may select multiple devices here, separated by spaces.

Valid terminal output names depend on the platform, but may include ‘console’ (native platform console), ‘serial’ (serial terminal), ‘serial_<port>’ (serial terminal with explicit port selection), ‘gfxterm’ (graphics-mode output), ‘vga_text’ (VGA text output), ‘mda_text’ (MDA text output), ‘morse’ (Morse-coding using system beeper) or ‘spkmodem’ (simple data protocol using system speaker).

‘spkmodem’ is useful when no serial port is available. Connect the output of sending system (where GRUB is running) to line-in of receiving system (usually developer machine). On receiving system compile ‘spkmodem-recv’ from ‘util/spkmodem-recv.c’ and run:

parecord --channels=1 --rate=48000 --format=s16le | ./spkmodem-recv

The default is to use the platform’s native terminal output.

‘GRUB_TERMINAL’

If this option is set, it overrides both ‘GRUB_TERMINAL_INPUT’ and ‘GRUB_TERMINAL_OUTPUT’ to the same value.

‘GRUB_SERIAL_COMMAND’

A command to configure the serial port when using the serial console. See serial. Defaults to ‘serial’.

‘GRUB_CMDLINE_LINUX’

Command-line arguments to add to menu entries for the Linux kernel.

‘GRUB_CMDLINE_LINUX_DEFAULT’

Unless ‘GRUB_DISABLE_RECOVERY’ is set to ‘true’, two menu entries will be generated for each Linux kernel: one default entry and one entry for recovery mode. This option lists command-line arguments to add only to the default menu entry, after those listed in ‘GRUB_CMDLINE_LINUX’.

‘GRUB_CMDLINE_LINUX_RECOVERY’

Unless ‘GRUB_DISABLE_RECOVERY’ is set to ‘true’, two menu entries will be generated for each Linux kernel: one default entry and one entry for recovery mode. This option lists command-line arguments to add only to the recovery menu entry, before those listed in ‘GRUB_CMDLINE_LINUX’. The default is ‘single’.

‘GRUB_CMDLINE_NETBSD’

‘GRUB_CMDLINE_NETBSD_DEFAULT’

As ‘GRUB_CMDLINE_LINUX’ and ‘GRUB_CMDLINE_LINUX_DEFAULT’, but for NetBSD.

‘GRUB_CMDLINE_GNUMACH’

As ‘GRUB_CMDLINE_LINUX’, but for GNU Mach.

‘GRUB_CMDLINE_XEN’

‘GRUB_CMDLINE_XEN_DEFAULT’

The values of these options are passed to Xen hypervisor Xen menu entries, for all respectively normal entries.

‘GRUB_CMDLINE_LINUX_XEN_REPLACE’

‘GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT’

The values of these options replace the values of ‘GRUB_CMDLINE_LINUX’ and ‘GRUB_CMDLINE_LINUX_DEFAULT’ for Linux and Xen menu entries.

‘GRUB_TOP_LEVEL’

‘GRUB_TOP_LEVEL_XEN’

This option should be an absolute path to a kernel image. If provided, the image specified will be made the top-level entry if it is found in the scan.

‘GRUB_TOP_LEVEL_OS_PROBER’

This option should be a line of output from os-prober. As ‘GRUB_TOP_LEVEL’, if provided, the image specified will be made the top-level entry if it is found in the scan.

‘GRUB_EARLY_INITRD_LINUX_CUSTOM’

‘GRUB_EARLY_INITRD_LINUX_STOCK’

List of space-separated early initrd images to be loaded from ‘/boot’. This is for loading things like CPU microcode, firmware, ACPI tables, crypto keys, and so on. These early images will be loaded in the order declared, and all will be loaded before the actual functional initrd image.

‘GRUB_EARLY_INITRD_LINUX_STOCK’ is for your distribution to declare images that are provided by the distribution. It should not be modified without understanding the consequences. They will be loaded first.

‘GRUB_EARLY_INITRD_LINUX_CUSTOM’ is for your custom created images.

The default stock images are as follows, though they may be overridden by your distribution:

intel-uc.img intel-ucode.img amd-uc.img amd-ucode.img early_ucode.cpio microcode.cpio

‘GRUB_DISABLE_LINUX_UUID’

Normally, grub-mkconfig will generate menu entries that use universally-unique identifiers (UUIDs) to identify the root filesystem to the Linux kernel, using a ‘root=UUID=...’ kernel parameter. This is usually more reliable, but in some cases it may not be appropriate. To disable the use of UUIDs, set this option to ‘true’.

‘GRUB_DISABLE_LINUX_PARTUUID’

If grub-mkconfig cannot identify the root filesystem via its universally-unique indentifier (UUID), grub-mkconfig can use the UUID of the partition containing the filesystem to identify the root filesystem to the Linux kernel via a ‘root=PARTUUID=...’ kernel parameter. This is not as reliable as using the filesystem UUID, but is more reliable than using the Linux device names. When ‘GRUB_DISABLE_LINUX_PARTUUID’ is set to ‘false’, the Linux kernel version must be 2.6.37 (3.10 for systems using the MSDOS partition scheme) or newer. This option defaults to ‘true’. To enable the use of partition UUIDs, set this option to ‘false’.

‘GRUB_DISABLE_RECOVERY’

If this option is set to ‘true’, disable the generation of recovery mode menu entries.

‘GRUB_DISABLE_UUID’

Normally, grub-mkconfig will generate menu entries that use universally-unique identifiers (UUIDs) to identify various filesystems to search for files. This is usually more reliable, but in some cases it may not be appropriate. To disable this use of UUIDs, set this option to ‘true’. Setting this option to ‘true’, will also set the options ‘GRUB_DISABLE_LINUX_UUID’ and ‘GRUB_DISABLE_LINUX_PARTUUID’ to ‘true’, unless they have been explicitly set to ‘false’.

‘GRUB_VIDEO_BACKEND’

If graphical video support is required, either because the ‘gfxterm’ graphical terminal is in use or because ‘GRUB_GFXPAYLOAD_LINUX’ is set, then grub-mkconfig will normally load all available GRUB video drivers and use the one most appropriate for your hardware. If you need to override this for some reason, then you can set this option.

After grub-install has been run, the available video drivers are listed in /boot/grub/video.lst.

‘GRUB_GFXMODE’

Set the resolution used on the ‘gfxterm’ graphical terminal. Note that you can only use modes which your graphics card supports via VESA BIOS Extensions (VBE), so for example native LCD panel resolutions may not be available. The default is ‘auto’, which tries to select a preferred resolution. See gfxmode.

‘GRUB_BACKGROUND’

Set a background image for use with the ‘gfxterm’ graphical terminal. The value of this option must be a file readable by GRUB at boot time, and it must end with .png, .tga, .jpg, or .jpeg. The image will be scaled if necessary to fit the screen. Image height and width will be restricted by an artificial limit of 16384.

‘GRUB_THEME’

Set a theme for use with the ‘gfxterm’ graphical terminal.

‘GRUB_GFXPAYLOAD_LINUX’

Set to ‘text’ to force the Linux kernel to boot in normal text mode, ‘keep’ to preserve the graphics mode set using ‘GRUB_GFXMODE’, ‘widthxheight’[‘xdepth’] to set a particular graphics mode, or a sequence of these separated by commas or semicolons to try several modes in sequence. See gfxpayload.

Depending on your kernel, your distribution, your graphics card, and the phase of the moon, note that using this option may cause GNU/Linux to suffer from various display problems, particularly during the early part of the boot sequence. If you have problems, set this option to ‘text’ and GRUB will tell Linux to boot in normal text mode.

‘GRUB_DISABLE_OS_PROBER’

The grub-mkconfig has a feature to use the external os-prober program to discover other operating systems installed on the same machine and generate appropriate menu entries for them. It is disabled by default since automatic and silent execution of os-prober, and creating boot entries based on that data, is a potential attack vector. Set this option to ‘false’ to enable this feature in the grub-mkconfig command.

‘GRUB_OS_PROBER_SKIP_LIST’

List of space-separated case insensitive UUIDs of filesystems to be ignored from os-prober output. For EFI chainloaders it’s <UUID>@<EFI FILE>. For backward compatibility with previous behaviour, <UUID>@/dev/* is also accepted for non-EFI chainloaders even if the device does not match, and comma and semicolon are also accepted as separator.

‘GRUB_DISABLE_SUBMENU’

Normally, grub-mkconfig will generate top level menu entry for the kernel with highest version number and put all other found kernels or alternative menu entries for recovery mode in submenu. For entries returned by os-prober first entry will be put on top level and all others in submenu. If this option is set to ‘true’, flat menu with all entries on top level will be generated instead. Changing this option will require changing existing values of ‘GRUB_DEFAULT’, ‘fallback’ (see fallback) and ‘default’ (see default) environment variables as well as saved default entry using grub-set-default and value used with grub-reboot.

‘GRUB_ENABLE_CRYPTODISK’

If set to ‘y’, grub-mkconfig and grub-install will check for encrypted disks and generate additional commands needed to access them during boot. Note that in this case unattended boot is not possible because GRUB will wait for passphrase to unlock encrypted container.

‘GRUB_INIT_TUNE’

Play a tune on the speaker when GRUB starts. This is particularly useful for users unable to see the screen. The value of this option is passed directly to play.

‘GRUB_BADRAM’

If this option is set, GRUB will issue a badram command to filter out specified regions of RAM.

‘GRUB_PRELOAD_MODULES’

This option may be set to a list of GRUB module names separated by spaces. Each module will be loaded as early as possible, at the start of grub.cfg.

The following options are still accepted for compatibility with existing configurations, but have better replacements:

‘GRUB_HIDDEN_TIMEOUT’

Wait this many seconds before displaying the menu. If ESC or F4 are pressed, or SHIFT is held down during that time, display the menu and wait for input according to ‘GRUB_TIMEOUT’. If a hotkey associated with a menu entry is pressed, boot the associated menu entry immediately. If the timeout expires before either of these happens, display the menu for the number of seconds specified in ‘GRUB_TIMEOUT’ before booting the default entry.

If you set ‘GRUB_HIDDEN_TIMEOUT’, you should also set ‘GRUB_TIMEOUT=0’ so that the menu is not displayed at all unless ESC or F4 are pressed, or SHIFT is held down.

This option is unset by default, and is deprecated in favour of the less confusing ‘GRUB_TIMEOUT_STYLE=countdown’ or ‘GRUB_TIMEOUT_STYLE=hidden’.

‘GRUB_HIDDEN_TIMEOUT_QUIET’

In conjunction with ‘GRUB_HIDDEN_TIMEOUT’, set this to ‘true’ to suppress the verbose countdown while waiting for a key to be pressed before displaying the menu.

This option is unset by default, and is deprecated in favour of the less confusing ‘GRUB_TIMEOUT_STYLE=countdown’.

‘GRUB_HIDDEN_TIMEOUT_BUTTON’

Variant of ‘GRUB_HIDDEN_TIMEOUT’, used to support vendor-specific power buttons. See Using GRUB with vendor power-on keys.

This option is unset by default, and is deprecated in favour of the less confusing ‘GRUB_TIMEOUT_STYLE=countdown’ or ‘GRUB_TIMEOUT_STYLE=hidden’.

‘GRUB_FORCE_EFI_ALL_VIDEO’

When set to true, this will allow grub-mkconfig to generate a GRUB config that supports loading the all_video module on the EFI platform instead of just the efi_gop and efi_uga modules.

This option is unset by default.

For more detailed customisation of grub-mkconfig’s output, you may edit the scripts in /etc/grub.d directly. /etc/grub.d/40_custom is particularly useful for adding entire custom menu entries; simply type the menu entries you want to add at the end of that file, making sure to leave at least the first two lines intact.

6.2 Root Identification Heuristics

If the target operating system uses the Linux kernel, grub-mkconfig attempts to identify the root file system via a heuristic algoirthm. This algorithm selects the identification method of the root file system by considering three factors. The first is if an initrd for the target operating system is also present. The second is ‘GRUB_DISABLE_LINUX_UUID’ and if set to ‘true’, prevents grub-mkconfig from identifying the root file system by its UUID. The third is ‘GRUB_DISABLE_LINUX_PARTUUID’ and if set to ‘true’, prevents grub-mkconfig from identifying the root file system via the UUID of its enclosing partition. If the variables are assigned any other value, that value is considered equivalent to ‘false’. The variables are also considered to be set to ‘false’ if they are not set.

When booting, the Linux kernel will delegate the task of mounting the root filesystem to the initrd. Most initrd images determine the root file system by checking the Linux kernel’s command-line for the ‘root’ key and use its value as the identification method of the root file system. To improve the reliability of booting, most initrd images also allow the root file system to be identified by its UUID. Because of this behavior, the grub-mkconfig command will set ‘root’ to ‘root=UUID=...’ to provide the initrd with the filesystem UUID of the root file system.

If no initrd is detected or ‘GRUB_DISABLE_LINUX_UUID’ is set to ‘true’ then grub-command will identify the root filesystem by setting the kernel command-line variable ‘root’ to ‘root=PARTUUID=...’ unless ‘GRUB_DISABLE_LINUX_PARTUUID’ is also set to ‘true’. If ‘GRUB_DISABLE_LINUX_PARTUUID’ is also set to ‘true’, grub-command will identify by its Linux device name.

The following table summarizes the behavior of the grub-mkconfig command.

Initrd detected	GRUB_DISABLE_LINUX_PARTUUID Set To	GRUB_DISABLE_LINUX_UUID Set To	Linux Root ID Method
false	false	false	part UUID
false	false	true	part UUID
false	true	false	dev name
false	true	true	dev name
true	false	false	fs UUID
true	false	true	part UUID
true	true	false	fs UUID
true	true	true	dev name

Remember, ‘GRUB_DISABLE_LINUX_PARTUUID’ and ‘GRUB_DISABLE_LINUX_UUID’ are also considered to be set to ‘true’ and ‘false’, respectively, when they are unset.

6.3 Writing full configuration files directly

grub.cfg is written in GRUB’s built-in scripting language, which has a syntax quite similar to that of GNU Bash and other Bourne shell derivatives.

Words

A word is a sequence of characters considered as a single unit by GRUB. Words are separated by metacharacters, which are the following plus space, tab, and newline:

{ } | & $ ; < >

Quoting may be used to include metacharacters in words; see below.

Reserved words

Reserved words have a special meaning to GRUB. The following words are recognised as reserved when unquoted and either the first word of a simple command or the third word of a for command:

! [[ ]] { }
case do done elif else esac fi for function
if in menuentry select then time until while

Not all of these reserved words have a useful purpose yet; some are reserved for future expansion.

Quoting

Quoting is used to remove the special meaning of certain characters or words. It can be used to treat metacharacters as part of a word, to prevent reserved words from being recognised as such, and to prevent variable expansion.

There are three quoting mechanisms: the escape character, single quotes, and double quotes.

A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows, with the exception of newline.

Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of ‘$’ and ‘\’. The ‘$’ character retains its special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: ‘$’, ‘"’, ‘\’, or newline. A backslash-newline pair is treated as a line continuation (that is, it is removed from the input stream and effectively ignored⁷). A double quote may be quoted within double quotes by preceding it with a backslash.

Variable expansion

The ‘$’ character introduces variable expansion. The variable name to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.

Normal variable names begin with an alphabetic character, followed by zero or more alphanumeric characters. These names refer to entries in the GRUB environment (see GRUB environment variables).

Positional variable names consist of one or more digits. They represent parameters passed to function calls, with ‘$1’ representing the first parameter, and so on.

The special variable name ‘?’ expands to the exit status of the most recently executed command. When positional variable names are active, other special variable names ‘@’, ‘*’ and ‘#’ are defined and they expand to all positional parameters with necessary quoting, positional parameters without any quoting, and positional parameter count respectively.

Comments

A word beginning with ‘#’ causes that word and all remaining characters on that line to be ignored.

Simple commands

A simple command is a sequence of words separated by spaces or tabs and terminated by a semicolon or a newline. The first word specifies the command to be executed. The remaining words are passed as arguments to the invoked command.

The return value of a simple command is its exit status. If the reserved word ! precedes the command, then the return value is instead the logical negation of the command’s exit status.

Compound commands

A compound command is one of the following:

for name in word …; do list; done: The list of words following in is expanded, generating a list of items. The variable name is set to each element of this list in turn, and list is executed each time. The return value is the exit status of the last command that executes. If the expansion of the items following in results in an empty list, no commands are executed, and the return status is 0.
if list; then list; [elif list; then list;] … [else list;] fi: The if list is executed, where list is a series of simple commands separated by a ";". If its exit status of the last command is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its last command’s exit status is zero, the corresponding then list is executed and the command completes. Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed, or zero if no condition tested true.
while cond; do list; done
until cond; do list; done: The while command continuously executes the do list as long as the last command in cond returns an exit status of zero, where cond is a list of simple commands separated by a ";". The until command is identical to the while command, except that the test is negated; the do list is executed as long as the last command in cond returns a non-zero exit status. The exit status of the while and until commands is the exit status of the last do list command executed, or zero if none was executed.
function name { command; … }: This defines a function named name. The body of the function is the list of commands within braces, each of which must be terminated with a semicolon or a newline. This list of commands will be executed whenever name is specified as the name of a simple command. Function definitions do not affect the exit status in $?. When executed, the exit status of a function is the exit status of the last command executed in the body.
menuentry title [--class=class …] [--users=users] [--unrestricted] [--hotkey=key] [--id=id] { command; … }: See menuentry.

Built-in Commands

Some built-in commands are also provided by GRUB script to help script writers perform actions that are otherwise not possible. For example, these include commands to jump out of a loop without fully completing it, etc.

break [n]: Exit from within a for, while, or until loop. If n is specified, break n levels. n must be greater than or equal to 1. If n is greater than the number of enclosing loops, all enclosing loops are exited. The return value is 0 unless n is not greater than or equal to 1.
continue [n]: Resume the next iteration of the enclosing for, while or until loop. If n is specified, resume at the nth enclosing loop. n must be greater than or equal to 1. If n is greater than the number of enclosing loops, the last enclosing loop (the top-level loop) is resumed. The return value is 0 unless n is not greater than or equal to 1.
return [n]: Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed in the function body. If used outside a function the return status is false.
setparams [arg] …: Replace positional parameters starting with $1 with arguments to setparams.
shift [n]: The positional parameters from n+1 … are renamed to $1…. Parameters represented by the numbers $# down to $#-n+1 are unset. n must be a non-negative number less than or equal to $#. If n is 0, no parameters are changed. If n is not given, it is assumed to be 1. If n is greater than $#, the positional parameters are not changed. The return status is greater than zero if n is greater than $# or less than zero; otherwise 0.

6.4 Multi-boot manual config

Currently autogenerating config files for multi-boot environments depends on os-prober and has several shortcomings. Due to that it is disabled by default. It is advised to use the power of GRUB syntax and do it yourself. A possible configuration is detailed here, feel free to adjust to your needs.

First create a separate GRUB partition, big enough to hold GRUB. Some of the following entries show how to load OS installer images from this same partition, for that you obviously need to make the partition large enough to hold those images as well. Mount this partition on/mnt/boot and disable GRUB in all OSes and manually install self-compiled latest GRUB with:

grub-install --boot-directory=/mnt/boot /dev/sda

In all the OSes install GRUB tools but disable installing GRUB in bootsector, so you’ll have menu.lst and grub.cfg available for use. Also disable os-prober use by setting:

GRUB_DISABLE_OS_PROBER=true

in /etc/default/grub

Then write a grub.cfg (/mnt/boot/grub/grub.cfg):

menuentry "OS using grub2" {
   insmod xfs
   search --set=root --label OS1 --hint hd0,msdos8
   configfile /boot/grub/grub.cfg
}
menuentry "OS using grub2-legacy" {
   insmod ext2
   search --set=root --label OS2 --hint hd0,msdos6
   legacy_configfile /boot/grub/menu.lst
}
menuentry "Windows XP" {
   insmod ntfs
   search --set=root --label WINDOWS_XP --hint hd0,msdos1
   ntldr /ntldr
}
menuentry "Windows 7" {
   insmod ntfs
   search --set=root --label WINDOWS_7 --hint hd0,msdos2
   ntldr /bootmgr
}
menuentry "FreeBSD" {
          insmod zfs
          search --set=root --label freepool --hint hd0,msdos7
          kfreebsd /freebsd@/boot/kernel/kernel
          kfreebsd_module_elf /freebsd@/boot/kernel/opensolaris.ko
          kfreebsd_module_elf /freebsd@/boot/kernel/zfs.ko
          kfreebsd_module /freebsd@/boot/zfs/zpool.cache type=/boot/zfs/zpool.cache
          set kFreeBSD.vfs.root.mountfrom=zfs:freepool/freebsd
          set kFreeBSD.hw.psm.synaptics_support=1
}
menuentry "experimental GRUB" {
          search --set=root --label GRUB --hint hd0,msdos5
          multiboot /experimental/grub/i386-pc/core.img
}
menuentry "Fedora 16 installer" {
          search --set=root --label GRUB --hint hd0,msdos5
          linux /fedora/vmlinuz lang=en_US keymap=sg resolution=1280x800
          initrd /fedora/initrd.img
}
menuentry "Fedora rawhide installer" {
          search --set=root --label GRUB --hint hd0,msdos5
          linux /fedora/vmlinuz repo=ftp://mirror.switch.ch/mirror/fedora/linux/development/rawhide/x86_64 lang=en_US keymap=sg resolution=1280x800
          initrd /fedora/initrd.img
}
menuentry "Debian sid installer" {
          search --set=root --label GRUB --hint hd0,msdos5
          linux /debian/dists/sid/main/installer-amd64/current/images/hd-media/vmlinuz
          initrd /debian/dists/sid/main/installer-amd64/current/images/hd-media/initrd.gz
}

Notes:

Argument to search after –label is FS LABEL. You can also use UUIDs with –fs-uuid UUID instead of –label LABEL. You could also use direct root=hd0,msdosX but this is not recommended due to device name instability.

6.5 Embedding a configuration file into GRUB

GRUB supports embedding a configuration file directly into the core image, so that it is loaded before entering normal mode. This is useful, for example, when it is not straightforward to find the real configuration file, or when you need to debug problems with loading that file. grub-install uses this feature when it is not using BIOS disk functions or when installing to a different disk from the one containing /boot/grub, in which case it needs to use the search command (see search) to find /boot/grub.

To embed a configuration file, use the -c option to grub-mkimage. The file is copied into the core image, so it may reside anywhere on the file system, and may be removed after running grub-mkimage.

After the embedded configuration file (if any) is executed, GRUB will load the ‘normal’ module (see normal), which will then read the real configuration file from $prefix/grub.cfg. By this point, the root variable will also have been set to the root device name. For example, prefix might be set to ‘(hd0,1)/boot/grub’, and root might be set to ‘hd0,1’. Thus, in most cases, the embedded configuration file only needs to set the prefix and root variables, and then drop through to GRUB’s normal processing. A typical example of this might look like this:

search.fs_uuid 01234567-89ab-cdef-0123-456789abcdef root
set prefix=($root)/boot/grub

(The ‘search_fs_uuid’ module must be included in the core image for this example to work.)

In more complex cases, it may be useful to read other configuration files directly from the embedded configuration file. This allows such things as reading files not called grub.cfg, or reading files from a directory other than that where GRUB’s loadable modules are installed. To do this, include the ‘configfile’ and ‘normal’ modules in the core image, and embed a configuration file that uses the configfile command to load another file. The following example of this also requires the echo, search_label, and test modules to be included in the core image:

search.fs_label grub root
if [ -e /boot/grub/example/test1.cfg ]; then
    set prefix=($root)/boot/grub
    configfile /boot/grub/example/test1.cfg
else
    if [ -e /boot/grub/example/test2.cfg ]; then
        set prefix=($root)/boot/grub
        configfile /boot/grub/example/test2.cfg
    else
        echo "Could not find an example configuration file!"
    fi
fi

The embedded configuration file may not contain menu entries directly, but may only read them from elsewhere using configfile.

7 Theme file format

7.1 Introduction

The GRUB graphical menu supports themes that can customize the layout and appearance of the GRUB boot menu. The theme is configured through a plain text file that specifies the layout of the various GUI components (including the boot menu, timeout progress bar, and text messages) as well as the appearance using colors, fonts, and images. Example is available in docs/example_theme.txt

7.2 Theme Elements

7.2.1 Colors

Colors can be specified in several ways:

HTML-style “#RRGGBB” or “#RGB” format, where *R*, *G*, and *B* are hexadecimal digits (e.g., “#8899FF”)
as comma-separated decimal RGB values (e.g., “128, 128, 255”)
with “SVG 1.0 color names” (e.g., “cornflowerblue”) which must be specified in lowercase.

7.2.2 Fonts

The fonts GRUB uses “PFF2 font format” bitmap fonts. Fonts are specified with full font names. Currently there is no provision for a preference list of fonts, or deriving one font from another. Fonts are loaded with the “loadfont” command in GRUB (loadfont). To see the list of loaded fonts, execute the “lsfonts” command (lsfonts). If there are too many fonts to fit on screen, do “set pager=1” before executing “lsfonts”.

7.2.3 Progress Bar

Figure 7.1

Figure 7.2

Progress bars are used to display the remaining time before GRUB boots the default menu entry. To create a progress bar that will display the remaining time before automatic boot, simply create a “progress_bar” component with the id “__timeout__”. This indicates to GRUB that the progress bar should be updated as time passes, and it should be made invisible if the countdown to automatic boot is interrupted by the user.

Progress bars may optionally have text displayed on them. This text is controlled by variable “text” which contains a printf template with the only argument %d is the number of seconds remaining. Additionally special values “@TIMEOUT_NOTIFICATION_SHORT@”, “@TIMEOUT_NOTIFICATION_MIDDLE@”, “@TIMEOUT_NOTIFICATION_LONG@” are replaced with standard and translated templates.

7.2.4 Circular Progress Indicator

The circular progress indicator functions similarly to the progress bar. When given an id of “__timeout__”, GRUB updates the circular progress indicator’s value to indicate the time remaining. For the circular progress indicator, there are two images used to render it: the *center* image, and the *tick* image. The center image is rendered in the center of the component, while the tick image is used to render each mark along the circumference of the indicator.

7.2.5 Labels

Text labels can be placed on the boot screen. The font, color, and horizontal alignment can be specified for labels. If a label is given the id “__timeout__”, then the “text” property for that label is also updated with a message informing the user of the number of seconds remaining until automatic boot. This is useful in case you want the text displayed somewhere else instead of directly on the progress bar.

7.2.7 Styled Boxes

One of the most important features for customizing the layout is the use of *styled boxes*. A styled box is composed of 9 rectangular (and potentially empty) regions, which are used to seamlessly draw the styled box on screen:

Northwest (nw)	North (n)	Northeast (ne)
West (w)	Center (c)	East (e)
Southwest (sw)	South (s)	Southeast (se)

To support any size of box on screen, the center slice and the slices for the top, bottom, and sides are all scaled to the correct size for the component on screen, using the following rules:

The edge slices (north, south, east, and west) are scaled in the direction of the edge they are adjacent to. For instance, the west slice is scaled vertically.
The corner slices (northwest, northeast, southeast, and southwest) are not scaled.
The center slice is scaled to fill the remaining space in the middle.

As an example of how an image might be sliced up, consider the styled box used for a terminal view.

Figure 7.3

7.2.8 Creating Styled Box Images

The Inkscape_ scalable vector graphics editor is a very useful tool for creating styled box images. One process that works well for slicing a drawing into the necessary image slices is:

Create or open the drawing you’d like use.
Create a new layer on the top of the layer stack. Make it visible. Select this layer as the current layer.
Draw 9 rectangles on your drawing where you’d like the slices to be. Clear the fill option, and set the stroke to 1 pixel wide solid stroke. The corners of the slices must meet precisely; if it is off by a single pixel, it will probably be evident when the styled box is rendered in the GRUB menu. You should probably go to File | Document Properties | Grids and enable a grid or create a guide (click on one of the rulers next to the drawing and drag over the drawing; release the mouse button to place the guide) to help place the rectangles precisely.
Right click on the center slice rectangle and choose Object Properties. Change the "Id" to “slice_c“ and click Set. Repeat this for the remaining 8 rectangles, giving them Id values of “slice_n“, “slice_ne“, “slice_e“, and so on according to the location.
Save the drawing.
Select all the slice rectangles. With the slice layer selected, you can simply press Ctrl+A to select all rectangles. The status bar should indicate that 9 rectangles are selected.
Click the layer hide icon for the slice layer in the layer palette. The rectangles will remain selected, even though they are hidden.
Choose File | Export Bitmap and check the *Batch export 9 selected objects* box. Make sure that *Hide all except selected* is unchecked. click *Export*. This will create PNG files in the same directory as the drawing, named after the slices. These can now be used for a styled box in a GRUB theme.

7.3 Theme File Manual

The theme file is a plain text file. Lines that begin with “#“ are ignored and considered comments. (Note: This may not be the case if the previous line ended where a value was expected.)

The theme file contains two types of statements:

Global properties.
Component construction.

7.3.1 Global Properties

7.3.2 Format

Global properties are specified with the simple format:

name1: value1
name2: "value which may contain spaces"
name3: #88F

In this example, name3 is assigned a color value.

7.3.3 Global Property List

title-text	Specifies the text to display at the top center of the screen as a title.
title-font	Defines the font used for the title message at the top of the screen.
title-color	Defines the color of the title message.
message-font	Currently unused. Left for backward compatibility.
message-color	Currently unused. Left for backward compatibility.
message-bg-color	Currently unused. Left for backward compatibility.
desktop-image	Specifies the image to use as the background. It will be scaled to fit the screen size or proportionally scaled depending on the scale method.
desktop-image-scale-method	Specifies the scaling method for the desktop-image. Options are “stretch“, “crop“, “padding“, “fitwidth“, “fitheight“. “stretch“ for fitting the screen size. Otherwise it is proportional scaling of a part of desktop-image to the part of the screen. “crop“ part of the desktop-image will be proportionally scaled to fit the screen sizes. “padding“ the entire desktop-image will be contained on the screen. “fitwidth“ for fitting the desktop-image’s width with screen width. “fitheight“ for fitting the desktop-image’s height with the screen height. Default is “stretch“.
desktop-image-h-align	Specifies the horizontal alignment of the desktop-image if desktop-image-scale-method isn’t equeal to “stretch“. Options are “left“, “center“, “right“. Default is “center“.
desktop-image-v-align	Specifies the vertical alignment of the desktop-image if desktop-image-scale-method isn’t equeal to “stretch“. Options are “top“, “center“, “bottom“. Default is “center“.
desktop-color	Specifies the color for the background if desktop-image is not specified.
terminal-box	Specifies the file name pattern for the styled box slices used for the command line terminal window. For example, “terminal-box: terminal_*.png“ will use the images “terminal_c.png“ as the center area, “terminal_n.png“ as the north (top) edge, “terminal_nw.png“ as the northwest (upper left) corner, and so on. If the image for any slice is not found, it will simply be left empty.
terminal-border	Specifies the border width of the terminal window.
terminal-left	Specifies the left coordinate of the terminal window.
terminal-top	Specifies the top coordinate of the terminal window.
terminal-width	Specifies the width of the terminal window.
terminal-height	Specifies the height of the terminal window.

7.3.4 Component Construction

Greater customizability comes is provided by components. A tree of components forms the user interface. *Containers* are components that can contain other components, and there is always a single root component which is an instance of a *canvas* container.

Components are created in the theme file by prefixing the type of component with a ’+’ sign:

+ label { text="GRUB" font="aqui 11" color="#8FF" }

properties of a component are specified as "name = value" (whitespace surrounding tokens is optional and is ignored) where *value* may be:

a single word (e.g., “align = center“, “color = #FF8080“),
a quoted string (e.g., “text = "Hello, World!"“), or
a tuple (e.g., “preferred_size = (120, 80)“).

7.3.5 Component List

The following is a list of the components and the properties they support.

label A label displays a line of text.

Properties:

id	Set to “__timeout__“ to display the time elapsed to an automatical boot of the default entry.
text	The text to display. If “id“ is set to “__timeout__“ and no “text“ property is set then the amount of seconds will be shown. If set to “@KEYMAP_SHORT@“, “@KEYMAP_MIDDLE@“ or “@KEYMAP_LONG@“ then predefined hotkey information will be shown.
font	The font to use for text display.
color	The color of the text.
align	The horizontal alignment of the text within the component. Options are “left“, “center“ and “right“.
visible	Set to “false“ to hide the label.

image A component that displays an image. The image is scaled to fit the component.
Properties:

file The full path to the image file to load.

progress_bar Displays a horizontally oriented progress bar. It can be rendered using simple solid filled rectangles, or using a pair of pixmap styled boxes.

Properties:

id	Set to “__timeout__“ to display the time elapsed to an automatical boot of the default entry.
fg_color	The foreground color for plain solid color rendering.
bg_color	The background color for plain solid color rendering.
border_color	The border color for plain solid color rendering.
text_color	The text color.
bar_style	The styled box specification for the frame of the progress bar. Example: “progress_frame_*.png“ If the value is equal to “highlight_style“ then no styled boxes will be shown.
highlight_style	The styled box specification for the highlighted region of the progress bar. This box will be used to paint just the highlighted region of the bar, and will be increased in size as the bar nears completion. Example: “progress_hl_*.png“. If the value is equal to “bar_style“ then no styled boxes will be shown.
highlight_overlay	If this option is set to “true“ then the highlight box side slices (every slice except the center slice) will overlay the frame box side slices. And the center slice of the highlight box can move all the way (from top to bottom), being drawn on the center slice of the frame box. That way we can make a progress bar with round-shaped edges so there won’t be a free space from the highlight to the frame in top and bottom scrollbar positions. Default is “false“.
font	The font to use for progress bar.
text	The text to display on the progress bar. If the progress bar’s ID is set to “__timeout__“ and the value of this property is set to “@TIMEOUT_NOTIFICATION_SHORT@“, “@TIMEOUT_NOTIFICATION_MIDDLE@“ or “@TIMEOUT_NOTIFICATION_LONG@“, then GRUB will update this property with an informative message as the timeout approaches.

circular_progress Displays a circular progress indicator. The appearance of this component is determined by two images: the *center* image and the *tick* image. The center image is generally larger and will be drawn in the center of the component. Around the circumference of a circle within the component, the tick image will be drawn a certain number of times, depending on the properties of the component.

Properties:

id	Set to “__timeout__“ to display the time elapsed to an automatical boot of the default entry.
center_bitmap	The file name of the image to draw in the center of the component.
tick_bitmap	The file name of the image to draw for the tick marks.
num_ticks	The number of ticks that make up a full circle.
ticks_disappear	Boolean value indicating whether tick marks should progressively appear, or progressively disappear as value approaches end. Specify “true“ or “false“. Default is “false“.
start_angle	The position of the first tick mark to appear or disappear. Measured in "parrots", 1 "parrot" = 1 / 256 of the full circle. Use values “xxx deg“ or “xxx \xc2\xb0“ to set the angle in degrees.

boot_menu Displays the GRUB boot menu. It allows selecting items and executing them.

Properties:

item_font	The font to use for the menu item titles.
selected_item_font	The font to use for the selected menu item, or “inherit“ (the default) to use “item_font“ for the selected menu item as well.
item_color	The color to use for the menu item titles.
selected_item_color	The color to use for the selected menu item, or “inherit“ (the default) to use “item_color“ for the selected menu item as well.
icon_width	The width of menu item icons. Icons are scaled to the specified size.
icon_height	The height of menu item icons.
item_height	The height of each menu item in pixels.
item_padding	The amount of space in pixels to leave on each side of the menu item contents.
item_icon_space	The space between an item’s icon and the title text, in pixels.
item_spacing	The amount of space to leave between menu items, in pixels.
menu_pixmap_style	The image file pattern for the menu frame styled box. Example: “menu_*.png“ (this will use images such as “menu_c.png“, “menu_w.png“, ‘menu_nw.png“, etc.)
item_pixmap_style	The image file pattern for the item styled box.
selected_item_pixmap_style	The image file pattern for the selected item highlight styled box.
scrollbar	Boolean value indicating whether the scroll bar should be drawn if the frame and thumb styled boxes are configured.
scrollbar_frame	The image file pattern for the entire scroll bar. Example: “scrollbar_*.png“
scrollbar_thumb	The image file pattern for the scroll bar thumb (the part of the scroll bar that moves as scrolling occurs). Example: “scrollbar_thumb_*.png“
scrollbar_thumb_overlay	If this option is set to “true“ then the scrollbar thumb side slices (every slice except the center slice) will overlay the scrollbar frame side slices. And the center slice of the scrollbar_thumb can move all the way (from top to bottom), being drawn on the center slice of the scrollbar frame. That way we can make a scrollbar with round-shaped edges so there won’t be a free space from the thumb to the frame in top and bottom scrollbar positions. Default is “false“.
scrollbar_slice	The menu frame styled box’s slice in which the scrollbar will be drawn. Possible values are “west“, “center“, “east“ (default). “west“ - the scrollbar will be drawn in the west slice (right-aligned). “east“ - the scrollbar will be drawn in the east slice (left-aligned). “center“ - the scrollbar will be drawn in the center slice. Note: in case of “center“ slice: a) If the scrollbar should be drawn then boot menu entry’s width is decreased by the scrollbar’s width and the scrollbar is drawn at the right side of the center slice. b) If the scrollbar won’t be drawn then the boot menu entry’s width is the width of the center slice. c) We don’t necessary need the menu pixmap box to display the scrollbar.
scrollbar_left_pad	The left scrollbar padding in pixels. Unused if “scrollbar_slice“ is “west“.
scrollbar_right_pad	The right scrollbar padding in pixels. Unused if “scrollbar_slice“ is “east“.
scrollbar_top_pad	The top scrollbar padding in pixels.
scrollbar_bottom_pad	The bottom scrollbar padding in pixels.
visible	Set to “false“ to hide the boot menu.

canvas Canvas is a container that allows manual placement of components within it. It does not alter the positions of its child components. It assigns all child components their preferred sizes.
hbox The *hbox* container lays out its children from left to right, giving each one its preferred width. The height of each child is set to the maximum of the preferred heights of all children.
vbox The *vbox* container lays out its children from top to bottom, giving each one its preferred height. The width of each child is set to the maximum of the preferred widths of all children.

7.3.6 Common properties

The following properties are supported by all components:

‘left’

The distance from the left border of container to left border of the object in either of three formats:

x	Value in pixels
p%	Percentage
p%+x	mixture of both

‘top’

The distance from the left border of container to left border of the object in same format.

‘width’

The width of object in same format.

‘height’

The height of object in same format.

‘id’

The identifier for the component. This can be any arbitrary string. The ID can be used by scripts to refer to various components in the GUI component tree. Currently, there is one special ID value that GRUB recognizes:

“__timeout__“

Component with this ID will be updated by GRUB and will indicate time elapsed to an automatical boot of the default entry. Affected components: “label“, “circular_progress“, “progress_bar“.

8 Booting GRUB from the network

The following instructions don’t work for *-emu, i386-qemu, i386-coreboot, i386-multiboot, mips_loongson, mips-arc and mips_qemu_mips

To generate a netbootable directory, run:

grub-mknetdir --net-directory=/srv/tftp --subdir=/boot/grub -d /usr/lib/grub/<platform>

E.g. for i386-pc:

grub-mknetdir --net-directory=/srv/tftp --subdir=/boot/grub -d /usr/lib/grub/i386-pc

Then follow instructions printed out by grub-mknetdir on configuring your DHCP server.

The grub.cfg file is placed in the same directory as the path output by grub-mknetdir hereafter referred to as FWPATH. GRUB will search for its configuration files in order using the following rules where the appended value corresponds to a value on the client machine.

‘(FWPATH)’/grub.cfg-‘(UUID OF MACHINE)’
‘(FWPATH)’/grub.cfg-01-‘(MAC ADDRESS OF NIC)’
‘(FWPATH)’/grub.cfg-‘(IPv4 OR IPv6 ADDRESS)’
‘(FWPATH)’/grub.cfg

The UUID is the Client Machine Identifier Option Definition as specified in RFC 4578. The client will only attempt to look up a UUID config file if it was provided by the DHCP server.

The client will only attempt to look up an IPv6 address config once, however, it will try the IPv4 multiple times. The concrete example below shows what would happen under the IPv4 case.

UUID: 7726a678-7fc0-4853-a4f6-c85ac36a120a
MAC:  52:54:00:ec:33:81
IPV4: 10.0.0.130 (0A000082)

‘(FWPATH)’/grub.cfg-7726a678-7fc0-4853-a4f6-c85ac36a120a
‘(FWPATH)’/grub.cfg-01-52-54-00-ec-33-81
‘(FWPATH)’/grub.cfg-0A000082
‘(FWPATH)’/grub.cfg-0A00008
‘(FWPATH)’/grub.cfg-0A0000
‘(FWPATH)’/grub.cfg-0A000
‘(FWPATH)’/grub.cfg-0A00
‘(FWPATH)’/grub.cfg-0A0
‘(FWPATH)’/grub.cfg-0A
‘(FWPATH)’/grub.cfg-0
‘(FWPATH)’/grub.cfg

This feature is enabled by default but it can be disabled by setting the ‘feature_net_search_cfg’ to ‘n’. Since this happens before the configuration file is read by GRUB, this option has to be disabled in an embedded configuration file (see Embedding a configuration file into GRUB).

After GRUB has started, files on the TFTP server will be accessible via the ‘(tftp)’ device.

The server IP address can be controlled by changing the ‘(tftp)’ device name to ‘(tftp,server-ip)’. Note that this should be changed both in the prefix and in any references to the device name in the configuration file.

GRUB provides several environment variables which may be used to inspect or change the behaviour of the PXE device. In the following description <interface> is placeholder for the name of network interface (platform dependent):

‘net_<interface>_ip’: The network interface’s IP address. Read-only.
‘net_<interface>_mac’: The network interface’s MAC address. Read-only.
‘net_<interface>_clientid’: The client id provided by DHCP. Read-only.
‘net_<interface>_clientuuid’: The client uuid provided by DHCP. Read-only.
‘net_<interface>_hostname’: The client host name provided by DHCP. Read-only.
‘net_<interface>_domain’: The client domain name provided by DHCP. Read-only.
‘net_<interface>_rootpath’: The path to the client’s root disk provided by DHCP. Read-only.
‘net_<interface>_extensionspath’: The path to additional DHCP vendor extensions provided by DHCP. Read-only.
‘net_<interface>_boot_file’: The boot file name provided by DHCP. Read-only.
‘net_<interface>_dhcp_server_name’: The name of the DHCP server responsible for these boot parameters. Read-only.
‘net_<interface>_next_server’: The IP address of the next (usually, TFTP) server provided by DHCP. Read-only.
‘net_default_interface’: Initially set to name of network interface that was used to load grub. Read-write, although setting it affects only interpretation of ‘net_default_ip’ and ‘net_default_mac’
‘net_default_ip’: The IP address of default interface. Read-only. This is alias for the ‘net_${net_default_interface}_ip’.
‘net_default_mac’: The default interface’s MAC address. Read-only. This is alias for the ‘net_${net_default_interface}_mac’.
‘net_default_server’: The default server used by network drives (see How to specify devices). Read-write, although setting this is only useful before opening a network device.
‘pxe_default_server’: This performs the same function as ‘net_default_server’.

9 Using GRUB via a serial line

This chapter describes how to use the serial terminal support in GRUB.

If you have many computers or computers with no display/keyboard, it could be very useful to control the computers through serial communications. To connect one computer with another via a serial line, you need to prepare a null-modem (cross) serial cable, and you may need to have multiport serial boards, if your computer doesn’t have extra serial ports. In addition, a terminal emulator is also required, such as minicom. Refer to a manual of your operating system, for more information.

As for GRUB, the instruction to set up a serial terminal is quite simple. Here is an example:

grub> serial --unit=0 --speed=9600
grub> terminal_input serial; terminal_output serial

The command serial initializes the serial unit 0 with the speed 9600bps. The serial unit 0 is usually called ‘COM1’, so, if you want to use COM2, you must specify ‘--unit=1’ instead. This command accepts many other options, see serial for more details.

Without argument or with ‘--port=auto’, GRUB will attempt to use ACPI when available to auto-detect the default serial port and its configuration.

The commands terminal_input (see terminal_input) and terminal_output (see terminal_output) choose which type of terminal you want to use. In the case above, the terminal will be a serial terminal, but you can also pass console to the command, as ‘terminal_input serial console’. In this case, a terminal in which you press any key will be selected as a GRUB terminal. In the example above, note that you need to put both commands on the same command line, as you will lose the ability to type commands on the console after the first command.

However, note that GRUB assumes that your terminal emulator is compatible with VT100 by default. This is true for most terminal emulators nowadays. However if your terminal emulator is not VT100-compatible or implements few VT100 escape sequences, you shoud tell GRUB that the terminal is dumb using the terminfo (see terminfo) command. This will have GRUB provide you with an alternative menu interface, because the normal menu requires several fancy features of your terminal.

10 Using GRUB with vendor power-on keys

Some laptop vendors provide an additional power-on button which boots another OS. GRUB supports such buttons with the ‘GRUB_TIMEOUT_BUTTON’, ‘GRUB_TIMEOUT_STYLE_BUTTON’, ‘GRUB_DEFAULT_BUTTON’, and ‘GRUB_BUTTON_CMOS_ADDRESS’ variables in default/grub (see Simple configuration handling). ‘GRUB_TIMEOUT_BUTTON’, ‘GRUB_TIMEOUT_STYLE_BUTTON’, and ‘GRUB_DEFAULT_BUTTON’ are used instead of the corresponding variables without the ‘_BUTTON’ suffix when powered on using the special button. ‘GRUB_BUTTON_CMOS_ADDRESS’ is vendor-specific and partially model-specific. Values known to the GRUB team are:

Dell XPS M1330M: 121:3
Dell XPS M1530: 85:3
Dell Latitude E4300: 85:3
Asus EeePC 1005PE: 84:1 (unconfirmed)
LENOVO ThinkPad T410s (2912W1C): 101:3

To take full advantage of this function, install GRUB into the MBR (see Installing GRUB using grub-install).

If you have a laptop which has a similar feature and not in the above list could you figure your address and contribute? To discover the address do the following:

boot normally

sudo modprobe nvram
sudo cat /dev/nvram | xxd > normal_button.txt

boot using vendor button

sudo modprobe nvram
sudo cat /dev/nvram | xxd > normal_vendor.txt

Then compare these text files and find where a bit was toggled. E.g. in case of Dell XPS it was:

byte 0x47: 20 --> 28

It’s a bit number 3 as seen from following table:

0	01
1	02
2	04
3	08
4	10
5	20
6	40
7	80

0x47 is decimal 71. Linux nvram implementation cuts first 14 bytes of CMOS. So the real byte address in CMOS is 71+14=85 So complete address is 85:3

11 GRUB image files

GRUB consists of several images: a variety of bootstrap images for starting GRUB in various ways, a kernel image, and a set of modules which are combined with the kernel image to form a core image. Here is a short overview of them.

boot.img

On PC BIOS systems, this image is the first part of GRUB to start. It is written to a master boot record (MBR) or to the boot sector of a partition. Because a PC boot sector is 512 bytes, the size of this image is exactly 512 bytes.

The sole function of boot.img is to read the first sector of the core image from a local disk and jump to it. Because of the size restriction, boot.img cannot understand any file system structure, so grub-install hardcodes the location of the first sector of the core image into boot.img when installing GRUB.

diskboot.img

This image is used as the first sector of the core image when booting from a hard disk. It reads the rest of the core image into memory and starts the kernel. Since file system handling is not yet available, it encodes the location of the core image using a block list format.

cdboot.img

This image is used as the first sector of the core image when booting from a CD-ROM drive. It performs a similar function to diskboot.img.

pxeboot.img

This image is used as the start of the core image when booting from the network using PXE. See Booting GRUB from the network.

lnxboot.img

This image may be placed at the start of the core image in order to make GRUB look enough like a Linux kernel that it can be booted by LILO using an ‘image=’ section.

kernel.img

This image contains GRUB’s basic run-time facilities: frameworks for device and file handling, environment variables, the rescue mode command-line parser, and so on. It is rarely used directly, but is built into all core images.

core.img

This is the core image of GRUB. It is built dynamically from the kernel image and an arbitrary list of modules by the grub-mkimage program. Usually, it contains enough modules to access /boot/grub, and loads everything else (including menu handling, the ability to load target operating systems, and so on) from the file system at run-time. The modular design allows the core image to be kept small, since the areas of disk where it must be installed are often as small as 32KB.

See BIOS installation, for details on where the core image can be installed on PC systems.

*.mod

Everything else in GRUB resides in dynamically loadable modules. These are often loaded automatically, or built into the core image if they are essential, but may also be loaded manually using the insmod command (see insmod).

For GRUB Legacy users

GRUB 2 has a different design from GRUB Legacy, and so correspondences with the images it used cannot be exact. Nevertheless, GRUB Legacy users often ask questions in the terms they are familiar with, and so here is a brief guide to how GRUB 2’s images relate to that.

stage1

Stage 1 from GRUB Legacy was very similar to boot.img in GRUB 2, and they serve the same function.

*_stage1_5

In GRUB Legacy, Stage 1.5’s function was to include enough filesystem code to allow the much larger Stage 2 to be read from an ordinary filesystem. In this respect, its function was similar to core.img in GRUB 2. However, core.img is much more capable than Stage 1.5 was; since it offers a rescue shell, it is sometimes possible to recover manually in the event that it is unable to load any other modules, for example if partition numbers have changed. core.img is built in a more flexible way, allowing GRUB 2 to support reading modules from advanced disk types such as LVM and RAID.

GRUB Legacy could run with only Stage 1 and Stage 2 in some limited configurations, while GRUB 2 requires core.img and cannot work without it.

stage2

GRUB 2 has no single Stage 2 image. Instead, it loads modules from /boot/grub at run-time.

stage2_eltorito

In GRUB 2, images for booting from CD-ROM drives are now constructed using cdboot.img and core.img, making sure that the core image contains the ‘iso9660’ module. It is usually best to use the grub-mkrescue program for this.

nbgrub

There is as yet no equivalent for nbgrub in GRUB 2; it was used by Etherboot and some other network boot loaders.

pxegrub

In GRUB 2, images for PXE network booting are now constructed using pxeboot.img and core.img, making sure that the core image contains the ‘pxe’ and ‘pxecmd’ modules. See Booting GRUB from the network.

12 Core image size limitation

Heavily limited platforms:

i386-pc (normal and PXE): the core image size (compressed) is limited by 458240 bytes. kernel.img (.text + .data + .bss, uncompressed) is limited by 392704 bytes. module size (uncompressed) + kernel.img (.text + .data, uncompressed) is limited by the size of contiguous chunk at 1M address.
sparc64-ieee1275: kernel.img (.text + .data + .bss) + modules + 256K (stack) + 2M (heap) is limited by space available at 0x4400. On most platforms it’s just 3 or 4M since ieee1275 maps only so much.
i386-ieee1275: kernel.img (.text + .data + .bss) + modules is limited by memory available at 0x10000, at most 596K

Lightly limited platforms:

*-xen: limited only by address space and RAM size.
i386-qemu: kernel.img (.text + .data + .bss) is limited by 392704 bytes. (core.img would be limited by ROM size but it’s unlimited on qemu
All EFI platforms: limited by contiguous RAM size and possibly firmware bugs
Coreboot and multiboot. kernel.img (.text + .data + .bss) is limited by 392704 bytes. module size is limited by the size of contiguous chunk at 1M address.
mipsel-loongson (ELF), mips(el)-qemu_mips (ELF): if uncompressed: kernel.img (.text + .data) + modules is limited by the space from 80200000 forward if compressed: kernel.img (.text + .data, uncompressed) + modules (uncompressed) + (modules + kernel.img (.text + .data)) (compressed) + decompressor is limited by the space from 80200000 forward
mipsel-loongson (Flash), mips(el)-qemu_mips (Flash): kernel.img (.text + .data) + modules is limited by the space from 80200000 forward core.img (final) is limited by flash size (512K on yeeloong and fulooong)
mips-arc: if uncompressed: kernel.img (.text + .data) is limited by the space from 8bd00000 forward modules + dummy decompressor is limited by the space from 8bd00000 backward if compressed: kernel.img (.text + .data, uncompressed) is limited by the space from 8bd00000 forward modules (uncompressed) + (modules + kernel.img (.text + .data)) (compressed, aligned to 1M) + 1M (decompressor + scratch space) is limited by the space from 8bd00000 backward
powerpc-ieee1275: kernel.img (.text + .data + .bss) + modules is limited by space available at 0x200000

13 Filesystem syntax and semantics

GRUB uses a special syntax for specifying disk drives which can be accessed by BIOS. Because of BIOS limitations, GRUB cannot distinguish between IDE, ESDI, SCSI, or others. You must know yourself which BIOS device is equivalent to which OS device. Normally, that will be clear if you see the files in a device or use the command search (see search).

13.1 How to specify devices

The device syntax is like this:

(device[,partmap-name1part-num1[,partmap-name2part-num2[,...]]])

‘[]’ means the parameter is optional. device depends on the disk driver in use. BIOS and EFI disks use either ‘fd’ or ‘hd’ followed by a digit, like ‘fd0’, or ‘cd’. AHCI, PATA (ata), crypto, USB use the name of driver followed by a number. Memdisk and host are limited to one disk and so it’s referred just by driver name. RAID (md), ofdisk (ieee1275 and nand), LVM (lvm), LDM, virtio (vdsk) and arcdisk (arc) use intrinsic name of disk prefixed by driver name. Additionally just “nand” refers to the disk aliased as “nand”. Conflicts are solved by suffixing a number if necessary. Commas need to be escaped. Loopback uses whatever name specified to loopback command. Hostdisk uses names specified in device.map as long as it’s of the form [fhc]d[0-9]* or hostdisk/<OS DEVICE>. For crypto and RAID (md) additionally you can use the syntax <driver name>uuid/<uuid>. For LVM additionally you can use the syntax lvmid/<volume-group-uuid>/<volume-uuid>.

(fd0)
(hd0)
(cd)
(ahci0)
(ata0)
(crypto0)
(usb0)
(cryptouuid/123456789abcdef0123456789abcdef0)
(mduuid/123456789abcdef0123456789abcdef0)
(lvm/system-root)
(lvmid/F1ikgD-2RES-306G-il9M-7iwa-4NKW-EbV1NV/eLGuCQ-L4Ka-XUgR-sjtJ-ffch-bajr-fCNfz5)
(md/myraid)
(md/0)
(ieee1275/disk2)
(ieee1275//pci@1f\,0/ide@d/disk@2)
(nand)
(memdisk)
(host)
(myloop)
(hostdisk//dev/sda)

part-num represents the partition number of device, starting from one. partname is optional but is recommended since disk may have several top-level partmaps. Specifying third and later component you can access to subpartitions.

The syntax ‘(hd0)’ represents using the entire disk (or the MBR when installing GRUB), while the syntax ‘(hd0,1)’ represents using the first partition of the disk (or the boot sector of the partition when installing GRUB).

(hd0,msdos1)
(hd0,msdos1,msdos5)
(hd0,msdos1,bsd3)
(hd0,netbsd1)
(hd0,gpt1)
(hd0,1,3)

If you enabled the network support, the special drives (protocol[,server]) are also available. Supported protocols are ‘http’ and ‘tftp’. If server is omitted, value of environment variable ‘net_default_server’ is used. Before using the network drive, you must initialize the network. See Booting GRUB from the network, for more information.

When using ‘http’ or ‘tftp’, ports other than ‘80’ can be specified using a colon (‘:’) after the address. To avoid parsing conflicts, when using IPv6 addresses with custom ports, the addresses must be enclosed with square brackets (‘[]’), as is standard practice.

(http,grub.example.com:31337)
(http,192.0.2.1:339)
(http,[2001:db8::1]:11235)

If you boot GRUB from a CD-ROM, ‘(cd)’ is available. See Making a GRUB bootable CD-ROM, for details.

13.2 How to specify files

There are two ways to specify files, by absolute file name and by block list.

An absolute file name resembles a Unix absolute file name, using ‘/’ for the directory separator (not ‘\’ as in DOS). One example is ‘(hd0,1)/boot/grub/grub.cfg’. This means the file /boot/grub/grub.cfg in the first partition of the first hard disk. If you omit the device name in an absolute file name, GRUB uses GRUB’s root device implicitly. So if you set the root device to, say, ‘(hd1,1)’ by the command ‘set root=(hd1,1)’ (see set), then /boot/kernel is the same as (hd1,1)/boot/kernel.

On ZFS filesystem the first path component must be volume‘@’[snapshot]. So ‘/rootvol@snap-129/boot/grub/grub.cfg’ refers to file ‘/boot/grub/grub.cfg’ in snapshot of volume ‘rootvol’ with name ‘snap-129’. Trailing ‘@’ after volume name is mandatory even if snapshot name is omitted.

13.3 How to specify block lists

A block list is used for specifying a file that doesn’t appear in the filesystem, like a chainloader. The syntax is [offset]+[length][,[offset]+[length]]…. Here is an example:

0+100,200+1,300+300,800+

This represents that GRUB should read blocks 0 through 99, block 200, blocks 300 through 599, and blocks 800 until the end of the device. If you omit an offset, then GRUB assumes the offset is zero. If the length is omitted, then GRUB assumes the block list extends until the end of the device.

Like the file name syntax (see How to specify files), if a blocklist does not contain a device name, then GRUB uses GRUB’s root device. So (hd0,2)+1 is the same as +1 when the root device is ‘(hd0,2)’.

14 GRUB’s user interface

GRUB has both a simple menu interface for choosing preset entries from a configuration file, and a highly flexible command-line for performing any desired combination of boot commands.

GRUB looks for its configuration file as soon as it is loaded. If one is found, then the full menu interface is activated using whatever entries were found in the file. If you choose the command-line menu option, or if the configuration file was not found, then GRUB drops to the command-line interface.

14.1 The flexible command-line interface

The command-line interface provides a prompt and after it an editable text area much like a command-line in Unix or DOS. Each command is immediately executed after it is entered⁸. The commands (see Available commands) are a subset of those available in the configuration file, used with exactly the same syntax.

Cursor movement and editing of the text on the line can be done via a subset of the functions available in the Bash shell:

C-f
PC right key: Move forward one character.
C-b
PC left key: Move back one character.
C-a
HOME: Move to the start of the line.
C-e
END: Move the the end of the line.
C-d
DEL: Delete the character underneath the cursor.
C-h
BS: Delete the character to the left of the cursor.
C-k: Kill the text from the current cursor position to the end of the line.
C-u: Kill backward from the cursor to the beginning of the line.
C-y: Yank the killed text back into the buffer at the cursor.
C-p
PC up key: Move up through the history list.
C-n
PC down key: Move down through the history list.

When typing commands interactively, if the cursor is within or before the first word in the command-line, pressing the TAB key (or C-i ) will display a listing of the available commands, and if the cursor is after the first word, the TAB will provide a completion listing of disks, partitions, and file names depending on the context. Note that to obtain a list of drives, one must open a parenthesis, as root (.

Note that you cannot use the completion functionality in the TFTP filesystem. This is because TFTP doesn’t support file name listing for the security.

15 GRUB environment variables

GRUB supports environment variables which are rather like those offered by all Unix-like systems. Environment variables have a name, which is unique and is usually a short identifier, and a value, which is an arbitrary string of characters. They may be set (see set), unset (see unset), or looked up (see Writing full configuration files directly) by name.

A number of environment variables have special meanings to various parts of GRUB. Others may be used freely in GRUB configuration files.

15.1 Special environment variables

These variables have special meaning to GRUB.

15.1.1 appendedsig_key_mgmt

This variable controls whether GRUB enforces appended signature validation using either static or dynamic key management. It is automatically set by GRUB to either static or dynamic based on the ’ibm,secure-boot’ device tree property and Platform KeyStore (PKS). Also, it can be explicitly set to either static or dynamic by setting the appendedsig_key_mgmt variable from the GRUB console when the GRUB is not locked down.

See Using appended signatures in GRUB for more information.

15.1.2 biosnum

When chain-loading another boot loader (see Chain-loading an OS), GRUB may need to know what BIOS drive number corresponds to the root device (see root) so that it can set up registers properly. If the biosnum variable is set, it overrides GRUB’s own means of guessing this.

For an alternative approach which also changes BIOS drive mappings for the chain-loaded system, see drivemap.

15.1.3 blsuki_save_default

If this variable is set, menu entries generated from BLS config files (see blscfg) or UKI files (see uki) will be set as the default boot entry when selected.

15.1.4 check_appended_signatures

This variable controls whether GRUB enforces appended signature validation on loaded kernel and GRUB module files. It is automatically set by GRUB to either no or yes based on the ’ibm,secure-boot’ device tree property. Also, it can be explicitly set to either no or yes by setting the check_appended_signatures variable from the GRUB console when the GRUB is not locked down.

See Using appended signatures in GRUB for more information.

15.1.5 check_signatures

This variable controls whether GRUB enforces GPG-style digital signature validation on loaded files. See Using GPG-style digital signatures in GRUB.

15.1.6 chosen

When executing a menu entry, GRUB sets the chosen variable to the title of the entry being executed.

If the menu entry is in one or more submenus, then chosen is set to the titles of each of the submenus starting from the top level followed by the title of the menu entry itself, separated by ‘>’.

15.1.7 cmdpath

The location from which core.img was loaded as an absolute directory name (see How to specify files). This is set by GRUB at startup based on information returned by platform firmware. Not every platform provides this information and some may return only device without path name.

15.1.8 color_highlight

This variable contains the “highlight” foreground and background terminal colors, separated by a slash (‘/’). Setting this variable changes those colors. For the available color names, see color_normal.

The default is ‘black/light-gray’.

15.1.9 color_normal

This variable contains the “normal” foreground and background terminal colors, separated by a slash (‘/’). Setting this variable changes those colors. Each color must be a name from the following list:

black
blue
green
cyan
red
magenta
brown
light-gray
dark-gray
light-blue
light-green
light-cyan
light-red
light-magenta
yellow
white

The default is ‘light-gray/black’.

The color support support varies from terminal to terminal.

‘morse’ has no color support at all.

‘mda_text’ color support is limited to highlighting by black/white reversal.

‘console’ on ARC, EMU and IEEE1275, ‘serial_*’ and ‘spkmodem’ are governed by terminfo and support only 8 colors if in modes ‘vt100-color’ (default for console on emu), ‘arc’ (default for console on ARC), ‘ieee1275’ (default for console on IEEE1275). When in mode ‘vt100’ then the color support is limited to highlighting by black/white reversal. When in mode ‘dumb’ there is no color support.

When console supports no colors this setting is ignored. When console supports 8 colors, then the colors from the second half of the previous list are mapped to the matching colors of first half.

‘console’ on EFI and BIOS and ‘vga_text’ support all 16 colors.

‘gfxterm’ supports all 16 colors and would be theoretically extendable to support whole rgb24 palette but currently there is no compelling reason to go beyond the current 16 colors.

15.1.10 config_directory

This variable is automatically set by GRUB to the directory part of current configuration file name (see config_file).

15.1.11 config_file

This variable is automatically set by GRUB to the name of configuration file that is being processed by commands configfile (see configfile) or normal (see normal). It is restored to the previous value when command completes.

15.1.12 cryptodisk_passphrase_tries

When prompting the user for a cryptodisk passphrase, allow this many attempts before giving up. Defaults to ‘3’ if unset or set to an invalid value. (The user can give up early by entering an empty passphrase.)

15.1.13 debug

This variable may be set to enable debugging output from various components of GRUB. The value is an ordered list of debug facility names separated by whitespace or ‘,’. If the special facility named ‘all’ is present then debugging output of all facility names is enabled at the start of processing the value of this variable. A facility’s debug output can then be disabled by prefixing its name with a ‘-’. The last occurence facility name with or without a leading ‘-’ takes precendent over any previous occurence. This allows the easy enabling or disabling of facilities by appending a ‘,’ and then the facility name with or without the leading ‘-’, which will preserve the state of the rest of the facilities. The facility names are the first argument to grub_dprintf. Consult the source for more details.

15.1.14 default

If this variable is set, it identifies a menu entry that should be selected by default, possibly after a timeout (see timeout). The entry may be identified by number (starting from 0 at each level of the hierarchy), by title, or by id.

For example, if you have:

menuentry 'Example GNU/Linux distribution' --class gnu-linux --id example-gnu-linux {
	...
}

then you can make this the default using:

default=example-gnu-linux

If the entry is in a submenu, then it must be identified using the number, title, or id of each of the submenus starting from the top level, followed by the number, title, or id of the menu entry itself, with each element separated by ‘>’. For example, take the following menu structure:

GNU/Hurd --id gnu-hurd
  Standard Boot --id=gnu-hurd-std
  Rescue shell --id=gnu-hurd-rescue
Other platforms --id=other
  Minix --id=minix
    Version 3.4.0 --id=minix-3.4.0
    Version 3.3.0 --id=minix-3.3.0
  GRUB Invaders --id=grub-invaders

The more recent release of Minix would then be identified as ‘Other platforms>Minix>Version 3.4.0’, or as ‘1>0>0’, or as ‘other>minix>minix-3.4.0’.

This variable is often set by ‘GRUB_DEFAULT’ (see Simple configuration handling), grub-set-default, or grub-reboot.

15.1.15 fallback

If this variable is set, it identifies a menu entry that should be selected if the default menu entry fails to boot. Entries are identified in the same way as for ‘default’ (see default).

15.1.16 gfxmode

If this variable is set, it sets the resolution used on the ‘gfxterm’ graphical terminal. Note that you can only use modes which your graphics card supports via VESA BIOS Extensions (VBE), so for example native LCD panel resolutions may not be available. The default is ‘auto’, which selects a platform-specific default that should look reasonable. Supported modes can be listed by ‘videoinfo’ command in GRUB.

The resolution may be specified as a sequence of one or more modes, separated by commas (‘,’) or semicolons (‘;’); each will be tried in turn until one is found. Each mode should be either ‘auto’, ‘widthxheight’, or ‘widthxheightxdepth’.

15.1.17 gfxpayload

If this variable is set, it controls the video mode in which the Linux kernel starts up, replacing the ‘vga=’ boot option (see linux). It may be set to ‘text’ to force the Linux kernel to boot in normal text mode, ‘keep’ to preserve the graphics mode set using ‘gfxmode’, or any of the permitted values for ‘gfxmode’ to set a particular graphics mode (see gfxmode).

Depending on your kernel, your distribution, your graphics card, and the phase of the moon, note that using this option may cause GNU/Linux to suffer from various display problems, particularly during the early part of the boot sequence. If you have problems, set this variable to ‘text’ and GRUB will tell Linux to boot in normal text mode.

The default is platform-specific. On platforms with a native text mode (such as PC BIOS platforms), the default is ‘text’. Otherwise the default may be ‘auto’ or a specific video mode.

This variable is often set by ‘GRUB_GFXPAYLOAD_LINUX’ (see Simple configuration handling).

15.1.18 gfxterm_font

If this variable is set, it names a font to use for text on the ‘gfxterm’ graphical terminal. Otherwise, ‘gfxterm’ may use any available font.

15.1.19 grub_cpu

In normal mode (see normal), GRUB sets the ‘grub_cpu’ variable to the CPU type for which GRUB was built (e.g. ‘i386’ or ‘powerpc’).

15.1.20 grub_platform

In normal mode (see normal), GRUB sets the ‘grub_platform’ variable to the platform for which GRUB was built (e.g. ‘pc’ or ‘efi’).

15.1.21 icondir

If this variable is set, it names a directory in which the GRUB graphical menu should look for icons after looking in the theme’s ‘icons’ directory. See Theme file format.

15.1.22 lang

If this variable is set, it names the language code that the gettext command (see gettext) uses to translate strings. For example, French would be named as ‘fr’, and Simplified Chinese as ‘zh_CN’.

grub-mkconfig (see Simple configuration handling) will try to set a reasonable default for this variable based on the system locale.

15.1.23 locale_dir

If this variable is set, it names the directory where translation files may be found (see gettext), usually /boot/grub/locale. Otherwise, internationalization is disabled.

grub-mkconfig (see Simple configuration handling) will set a reasonable default for this variable if internationalization is needed and any translation files are available.

15.1.24 lockdown

If this variable is set to ‘y’, it means that GRUB has entered see Lockdown when booting on a secure setup mode.

15.1.43 prefix

The location of the ‘/boot/grub’ directory as an absolute file name (see How to specify files). This is normally set by GRUB at startup based on information provided by grub-install. GRUB modules are dynamically loaded from this directory, so it must be set correctly in order for many parts of GRUB to work.

15.1.44 pxe_default_server

See Booting GRUB from the network.

15.1.45 root

The root device name (see How to specify devices). Any file names that do not specify an explicit device name are read from this device. The default is normally set by GRUB at startup based on the value of ‘prefix’ (see prefix).

For example, if GRUB was installed to the first partition of the first hard disk, then ‘prefix’ might be set to ‘(hd0,msdos1)/boot/grub’ and ‘root’ to ‘hd0,msdos1’.

15.1.46 shim_lock

If this variable is set to ‘y’, it means that the shim_lock verifier is registered (see see UEFI secure boot and shim support).

15.1.47 superusers

This variable may be set to a list of superuser names to enable authentication support. See Security.

15.1.48 theme

This variable may be set to a directory containing a GRUB graphical menu theme. See Theme file format.

This variable is often set by ‘GRUB_THEME’ (see Simple configuration handling).

15.1.49 timeout

If this variable is set, it specifies the time in seconds to wait for keyboard input before booting the default menu entry. A timeout of ‘0’ means to boot the default entry immediately without displaying the menu; a timeout of ‘-1’ (or unset) means to wait indefinitely.

If ‘timeout_style’ (see timeout_style) is set to ‘countdown’ or ‘hidden’, the timeout is instead counted before the menu is displayed.

This variable is often set by ‘GRUB_TIMEOUT’ (see Simple configuration handling).

15.1.50 timeout_style

This variable may be set to ‘menu’, ‘countdown’, or ‘hidden’ to control the way in which the timeout (see timeout) interacts with displaying the menu. See the documentation of ‘GRUB_TIMEOUT_STYLE’ (see Simple configuration handling) for details.

15.1.51 tpm_fail_fatal

If this variable is set and true (i.e., not set to “0”, “false”, “disable”, or “no”), TPM measurements that fail will be treated as fatal. Otherwise, they will merely be debug-logged and boot will continue.

Call to EFI firmware, like hash_log_extend_event(), can return an unknown error, i.e. due to bug present in firmware. When this variable is set and true (same values as with TPM measurements) this situation will be considered to be fatal and error-logged as “unknown TPM error”. If not set, booting the OS will be enabled.

15.2 The GRUB environment block

It is often useful to be able to remember a small amount of information from one boot to the next. For example, you might want to set the default menu entry based on what was selected the last time. GRUB deliberately does not implement support for writing files in order to minimise the possibility of the boot loader being responsible for file system corruption, so a GRUB configuration file cannot just create a file in the ordinary way. However, GRUB provides an “environment block” which can be used to save a small amount of state.

The environment block is a preallocated 1024-byte file, which normally lives in /boot/grub/grubenv (although you should not assume this). At boot time, the load_env command (see load_env) loads environment variables from it, and the save_env (see save_env) command saves environment variables to it. From a running system, the grub-editenv utility can be used to edit the environment block.

For safety reasons, this storage is only available when installed on a plain disk (no LVM or RAID), using a non-checksumming filesystem (no ZFS), and using BIOS or EFI functions (no ATA, USB or IEEE1275).

On Btrfs filesystems, a reserved area in the filesystem header may be used to store the environment block. This static block avoids the problems of updating a normal file on a copy-on-write filesystem, where writing raw block is not stable and requires metadata update. The reserved area provides a fixed location that GRUB can update directly, allowing commands such as grub-reboot and ‘GRUB_SAVEDEFAULT’ to function correctly on Btrfs volumes.

grub-mkconfig uses this facility to implement ‘GRUB_SAVEDEFAULT’ (see Simple configuration handling).

15.3 Special environment block variables

These special variables are usually written to the environment block (see The GRUB environment block) to customize the behavior of grub.cfg generated by grub-mkconfig.

15.3.1 saved_entry

The saved_entry variable sets the default boot entry in grub.cfg created by grub-mkconfig. It can be set with grub-set-default to choose a default entry, or at runtime with the savedefault function in grub.cfg to save the current entry as the new default. This may require write access by GRUB.

15.3.2 next_entry

The next_entry variable sets the boot entry for the next boot only. After it is used, GRUB clears the value so it is not reused. This requires write access to the environment block (see The GRUB environment block) at runtime. The grub-reboot command is usually used instead of changing this variable directly.

15.3.3 env_block

If the filesystem is Btrfs and the disk is not an abstracted device such as LVM, RAID, or encryption, the reserved space in the Btrfs header can be used as the environment block (see The GRUB environment block). This provides a fixed raw block that GRUB can reliably write to. The env_block records this location in GRUB blocklist syntax (see How to specify block lists) so that grub-editenv and grub.cfg know how to access and use the external raw block.

This variable is initialized when grubenv is first created by grub-editenv and is treated as read-only to avoid being overwritten with an unpredictable value.

15.4 Passing environment variables through Xen

If you are using a GRUB image as the kernel for a PV or PVH Xen virtual machine, you can pass environment variables from Xen’s dom0 to the VM through the Xen-provided kernel command line. When combined with a properly configured guest, this can be used to customize the guest’s behavior on bootup via the VM’s Xen configuration file.

GRUB will parse the kernel command line passed to it by Xen during bootup. The command line will be split into space-delimited words. Single and double quotes may be used to quote words or portions of words that contain spaces. Single quotes will be considered part of a word if inside double quotes, and vice versa. Arbitrary characters may be backslash-escaped to make them a literal component of a word rather than being parsed as quotes or word separators. The command line must consist entirely of printable 7-bit ASCII characters and spaces. If a non-printing ASCII character is found anywhere in the command line, the entire command line will be ignored by GRUB. (This splitter algorithm is meant to behave somewhat like Bash’s word splitting.)

Each word should be a variable assignment in the format “variable” or “variable=value”. Variable names must contain only the characters A-Z, a-z, and underscore (“_”). Variable names must begin with the string “xen_grub_env_”. Variable values can contain arbitrary printable 7-bit ASCII characters and space. If any variable contains an illegal name, that variable will be ignored.

If a variable name and value are both specified, the variable will be set to the specified value. If only a variable name is specified, the variable’s value will be set to “1”.

The following is a simple example of how to use this functionality to append arbitrary variables to a guest’s kernel command line:

# In the Xen configuration file for the guest
name = "linux_vm"
type = "pvh"
kernel = "/path/to/grub-i386-xen_pvh.bin"
extra = "xen_grub_env_linux_append='loglevel=3'"
memory = 1024
disk = [ "file:/srv/vms/linux_vm.img,sda,w" ]
# In the guest's GRUB configuration file
menuentry "Linux VM with dom0-specified kernel parameters" {
    search --set=root --label linux_vm --hint hd0,msdos1
    linux /boot/vmlinuz root=LABEL=linux_vm ${xen_grub_env_linux_append}
    initrd /boot/initrd.img
}

16 Modules

In this chapter, we list all modules that are available in GRUB.

Modules can be loaded via the insmod (see insmod) command.

16.1 acpi

This module provides the command acpi for loading / replacing Advanced Configuration and Power Interface (ACPI) tables. Please see acpi for more information.

16.2 adler32

This module provides the library implementation for the adler32 checksum. This is used as part of LZO decompression / compression.

16.3 affs

This module provides support for the Amiga Fast FileSystem (AFFS). Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.4 afs

This module provides support for the AtheOS File System (AFS). Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.5 afsplitter

This module provides library support for the Anti forensic information splitter (AFS) operation AF_merge. This is used by LUKS and LUKS2.

16.6 ahci

This module provides support for the Advanced Host Controller Interface protocol to access disks supporting this standard. AHCI is often an option for Serial ATA (SATA) controllers (meant to replace the older IDE protocol).

16.7 all_video

This is a "dummy module" with no actual function except to load all other video modules as dependencies (a convenient way to load all video modules).

16.8 aout

This module provides support for loading files packaged in the "a.out" format. The "a.out" format is considered to be an older format than some alternatives such as "ELF", for example support for the "a.out" format was removed from the Linux kernel in 5.18.

16.9 appleldr

This module provides support for loading files on a BIOS / EFI based Apple Mac computer (Intel based Macs).

16.10 archelp

This module provides Archive Helper functions for archive based file systems such as TAR and CPIO archives.

16.11 argon2

This module provides support for the Argon2 key derivation function.

16.12 argon2_test

This module is intended for performing a functional test of the Argon2 operation in GRUB.

16.13 at_keyboard

This module provides support for the AT keyboard input for the GRUB terminal.

16.14 ata

This modules provides support for direct ATA and ATAPI access to compatible disks.

16.15 backtrace

This module provides the command backtrace for printing a backtrace to the terminal for the current call stack.

16.16 bfs

This module provides support for the BeOS "Be File System" (BFS). Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.17 biosdisk

This module provides support for booting from a bootable removable disk such as a CD-ROM, BD-ROM, etc.

16.18 bitmap

This module provides support for reading and interacting with bitmap image files.

16.19 bitmap_scale

This module provides support for scaling bitmap image files.

16.20 bli

This module provides basic support for the Boot Loader Interface. The Boot Loader Interface specifies a set of EFI variables that are used to communicate boot-time information between the bootloader and the operating system.

The following variables are placed under the vendor UUID 4a67b082-0a4c-41cf-b6c7-440b29bb8c4f when the module is loaded:

The GPT partition UUID of the EFI System Partition used during boot is published via the LoaderDevicePartUUID variable. The Boot Loader Interface specification requires GPT formatted drives. The bli module ignores drives/partitions in any other format. If GRUB is loaded from a non-GPT partition, e.g. from an MSDOS formatted drive or network, this variable will not be set.

A string identifying GRUB as the active bootloader including the version number is stored in LoaderInfo.

This module is only available on UEFI platforms.

16.21 blocklist

This module provides support for the command blocklist to list blocks for a given file. Please see blocklist for more information.

16.22 boot

This module provides support for the command boot to boot an operating system. Please see boot for more information.

16.23 boottime

This module provides support for the command boottime to display time taken to perform various GRUB operations. This module is only available when GRUB is built with the conditional compile option BOOT_TIME_STATS.

16.24 bsd

This module provides support for loading BSD operating system images via commands such as: kfreebsd_loadenv, kfreebsd_module_elf, kfreebsd_module, kfreebsd, knetbsd_module_elf, knetbsd_module, knetbsd, kopenbsd, and kopenbsd_ramdisk. Please see Various loader commands for more info.

16.25 bswap_test

This module is intended for performing a functional test of the byte swapping functionality of GRUB.

16.26 btrfs

This module provides support for the B-Tree File System (BTRFS).

16.27 bufio

This module is a library module for support buffered I/O of files to support file reads performed in other modules.

16.28 cacheinfo

This module provides support for the command cacheinfo which provides statistics on disk cache accesses. This module is only built if DISK_CACHE_STATS is enabled.

16.29 cat

This module provides support for the command cat which outputs the content of a file to the terminal. Please see cat for more info.

16.30 cbfs

This module provides support for the Coreboot File System (CBFS) which is an archive based file system. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.31 cbls

This module provides support for the command lscoreboot to list the Coreboot tables.

16.32 cbmemc

This module provides support for the command cbmemc to show the content of the Coreboot Memory console.

16.33 cbtable

This module provides support for accessing the Coreboot tables.

16.34 cbtime

This module provides support for the command coreboot_boottime to show the Coreboot boot time statistics.

16.35 chain

This module provides support for the command chainloader to boot another bootloader. Please see chainloader for more information.

16.36 cmdline_cat_test

This module is intended for performing a functional test of the cat command of GRUB.

16.37 cmosdump

This module provides support for the command cmosdump to show a raw dump of the CMOS contents. Please see cmosdump for more information.

16.38 cmostest

This module provides support for the commands cmostest, cmosclean, and cmosset to interact with a CMOS. See cmostest / see cmosclean for more information.

16.39 cmp

This module provides support for the command cmp to compare the content of two files. See cmp for more information.

16.40 cmp_test

This module is intended for performing a functional test of relational operations in GRUB. Note that this module is *not* associated with the cmp command and does not test the cmp command.

16.41 configfile

This module provides support for the commands: configfile, source, extract_entries_source, extract_entries_configfile, . (dot command). See configfile / see source.

16.42 cpio

This module provides support for the CPIO archive file format. This module is for the "bin" version of CPIO (default of GNU CPIO) supporting around 2GB.

16.43 cpio_be

This module provides support for the CPIO archive file format in big-endian format. This module is for the "bin" version of CPIO (default of GNU CPIO) supporting around 2GB.

16.44 cpuid

This module provides support for the command cpuid to test for various CPU features. See cpuid for more information.

16.45 crc64

This module provides support for the CRC64 operation.

16.46 crypto_cipher_mode_test

This module performs various cipher mode encryption/decryption tests

16.47 crypto

This module provides library support for various base cryptography operations in GRUB.

16.48 cryptodisk

This module provides support for the command cryptomount to interact with encrypted file systems. See cryptomount for more information.

16.49 cs5536

This module provides support for the AMD Geode CS5536 companion device.

16.50 ctz_test

This module is intended for performing a functional test of the ctz functions in GRUB used to Count Trailing Zeros.

16.51 date

This module provides support for the command date to get the date/time or set the date/time. See date for more information.

16.52 datehook

This module provides support for populating / providing the environment variables YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, WEEKDAY.

16.53 datetime

This module provides library support for getting and setting the date / time from / to a hardware clock device.

16.54 disk

This module provides library support for writing to a storage disk.

16.55 diskfilter

This module provides library support for reading a disk RAID array. It also provides support for the command cryptocheck. See cryptocheck for more information.

16.56 div

This module provides library support for some operations such as divmod.

16.57 div_test

This module is intended for performing a functional test of the divmod function in GRUB.

16.58 dm_nv

This module provides support for handling some Nvidia "fakeraid" disk devices.

16.59 drivemap

This module provides support for the drivemap to manage BIOS drive mappings. See drivemap for more information.

16.60 dsa_sexp_test

This module provides a test of the libgcrypt DSA functionality in GRUB.

16.61 echo

This module provides support for the echo to display a line of text. See echo for more information.

16.62 efi_gop

This module provides support for the UEFI video output protocol "Graphics Output Protocol" (GOP).

16.63 efi_uga

This module provides support for the EFI video protocol "Universal Graphic Adapter" (UGA).

16.64 efiemu

This module provides support for the commands efiemu_loadcore, efiemu_prepare, and efiemu_unload. This provides an EFI emulation.

16.65 efifwsetup

This modules provides support for the command fwsetup to reboot into the firmware setup menu. See fwsetup for more information.

16.66 efinet

This module provides support for UEFI Network Booting for loading images and data from the network.

16.67 efitextmode

This module provides support for command efitextmode to get and set output mode resolution. See efitextmode for more information.

16.68 ehci

This module provides support for the USB Enhanced Host Controller Interface (EHCI) specification (USB 2.0).

16.69 elf

This module provides support for loading Executable and Linkable Format (ELF) files.

16.70 emunet

This module provides support for networking in GRUB on the emu platform.

16.71 emupci

This module provides support for accessing the PCI bus in GRUB on the emu platform.

16.72 erofs

This module provides support for the Enhanced Read Only File System (EROFS).

16.73 escc

This module provides support for the "mac-io" terminal device on PowerPC.

16.74 eval

This module provides support for command eval to evaluate the provided input as a sequence of GRUB commands. See eval for more information.

16.75 exfat

This module provides support for the Extensible File Allocation Table (exFAT) file system in GRUB.

16.76 exfctest

This module is intended to provide an Example Functional Test of GRUB functions to use as a template for developing other GRUB functional tests.

16.77 ext2

This module provides support for the Extended File System versions 2, 3, and 4 (ext2, ext3, and ext4) file systems in GRUB.

16.78 extcmd

This module is a support module to provide wrapper functions for registering other module commands depending on the state of the lockdown variable.

16.79 f2fs

This module provides support for the Flash-Friendly File System (F2FS) in GRUB.

16.80 fat

This module provides support for the File Allocation Table 12-bit, 16-bit, and 32-bit (FAT12, FAT16, and FAT32) file systems in GRUB.

16.81 fdt

This module provides support for the commands fdtdump and devicetree to dump the contents of a device tree blob (.dtb) to the console and to load a device tree blob (.dtb) from a filesystem, for later use by a Linux kernel, respectively. See devicetree and see fdtdump for more information.

16.82 file

This module provides support for the command file to test if the provided filename is of the specified type. See file for more information.

16.83 fixvideo

This module provides support for the command fix_video to fix video problems in specific PCIe video devices by "patching" specific device register settings. Currently supports Intel 945GM (PCI ID 0x27a28086) and Intel 965GM (PCI ID 0x2a028086).

16.84 font

This module provides support for the commands loadfont and lsfonts to load a given font or list the loaded fonts. See loadfont and see lsfonts for more information.

16.85 freedos

This module provides support for command freedos for loading a FreeDOS kernel.

16.86 fshelp

This module provides support functions (helper functions) for file systems.

16.87 functional_test

This module provides support for running the GRUB functional tests using commands functional_test and all_functional_test.

16.88 gcry_arcfour

This module provides support for the arcfour stream cipher also known as RC4. If security is a concern, RC4 / arcfour cipher is consider broken (multiple known vulnerabilities make this insecure). This GRUB module is based on libgcrypt.

16.89 gcry_aria

This module provides support for the ARIA cipher. This GRUB module is based on libgcrypt.

16.90 gcry_blake2

This module provides support for the BLAKE2b and BLAKE2s message digests. This GRUB module is based on libgcrypt.

16.91 gcry_blowfish

This module provides support for the Blowfish cipher. This GRUB module is based on libgcrypt.

16.92 gcry_camellia

This module provides support for the Camellia cipher. This GRUB module is based on libgcrypt.

16.93 gcry_cast5

This module provides support for the CAST5 (RFC2144, also known as CAST-128) cipher. This GRUB module is based on libgcrypt.

16.94 gcry_crc

This module provides support for the CRC32, CRC32 RFC1510, and CRC24 RFC2440 cyclic redundancy checks. This GRUB module is based on libgcrypt.

16.95 gcry_des

This module provides support for the Data Encryption Standard (DES) and Triple-DES ciphers. If security is a concern, DES has known vulnerabilities and is not recommended, and Triple-DES is no longer recommended by NIST. This GRUB module is based on libgcrypt.

16.96 gcry_dsa

This module provides support for the Digital Signature Algorithm (DSA) cipher. This GRUB module is based on libgcrypt.

16.97 gcry_gost28147

This module provides support for the GOST 28147-89 cipher. This GRUB module is based on libgcrypt.

16.98 gcry_gostr3411_94

This module provides support for the GOST R 34.11-94 message digest. This GRUB module is based on libgcrypt.

16.99 gcry_idea

This module provides support for the International Data Encryption Algorithm (IDEA) cipher. This GRUB module is based on libgcrypt.

16.100 gcry_keccak

This module provides support for the SHA3 hash message digests (including SHAKE128 and SHAKE256). This GRUB module is based on libgcrypt.

16.101 gcry_md4

This module provides support for the Message Digest 4 (MD4) message digest. If security is a concern, MD4 has known vulnerabilities and is not recommended. This GRUB module is based on libgcrypt.

16.102 gcry_md5

This module provides support for the Message Digest 5 (MD5) message digest. If security is a concern, MD5 has known vulnerabilities and is not recommended. This GRUB module is based on libgcrypt.

16.103 gcry_rfc2268

This module provides support for the RFC2268 (RC2 / Ron’s Cipher 2) cipher. If security is a concern, RC2 has known vulnerabilities and is not recommended. This GRUB module is based on libgcrypt.

16.104 gcry_rijndael

This module provides support for the Advanced Encryption Standard (AES-128, AES-192, and AES-256) ciphers. This GRUB module is based on libgcrypt.

16.105 gcry_rmd160

This module provides support for the RIPEMD-160 message digest. This GRUB module is based on libgcrypt.

16.106 gcry_rsa

This module provides support for the Rivest–Shamir–Adleman (RSA) cipher. This GRUB module is based on libgcrypt.

16.107 gcry_salsa20

This module provides support for the Salsa20 cipher. This GRUB module is based on libgcrypt.

16.108 gcry_seed

This module provides support for the SEED cipher. This GRUB module is based on libgcrypt.

16.109 gcry_serpent

This module provides support for the Serpent (128, 192, and 256) ciphers. This GRUB module is based on libgcrypt.

16.110 gcry_sha1

This module provides support for the Secure Hash Algorithm 1 (SHA-1) message digest. If security is a concern, SHA-1 has known vulnerabilities and is not recommended. This GRUB module is based on libgcrypt.

16.111 gcry_sha256

This module provides support for the Secure Hash Algorithm 2 (224 and 256 bit) (SHA-224 / SHA-256) message digests. This GRUB module is based on libgcrypt.

16.112 gcry_sha512

This module provides support for the Secure Hash Algorithm 2 (384 and 512 bit) (SHA-384 / SHA-512) message digests. This GRUB module is based on libgcrypt.

16.113 gcry_sm3

This module provides support for the SM3 message digest. This GRUB module is based on libgcrypt.

16.114 gcry_sm4

This module provides support for the SM4 cipher. This GRUB module is based on libgcrypt.

16.115 gcry_stribog

This module provides support for the GOST R 34.11-2012 (Stribog) message digest. This GRUB module is based on libgcrypt.

16.116 gcry_tiger

This module provides support for the Tiger, Tiger 1, and Tiger 2 message digests. This GRUB module is based on libgcrypt.

16.117 gcry_twofish

This module provides support for the Twofish (128 and 256) ciphers. This GRUB module is based on libgcrypt.

16.118 gcry_whirlpool

This module provides support for the Whirlpool message digest. This GRUB module is based on libgcrypt.

16.119 gdb

This module provides support for remotely debugging GRUB using the GNU Debugger (GDB) over serial. This is typically done when troubleshooting GRUB during development and not required for normal GRUB operation. This module adds support for commands required by the GDB remote debug function including gdbstub to start GDB stub on given serial port, gdbstub_break to break into GDB, gdbstub_stop to stop the GDB stub.

16.120 geli

This module provides support for the GEOM ELI (GELI) disk encryption / decryption protocol used by FreeBSD. This module supports the following ciphers using the associated "gcry" modules: DES, Triple-DES, Blowfish, CAST5, AES, and Camellia 128.

16.121 gettext

This module provides support for the gettext command to support translating information displayed / output by GRUB. See gettext for more information.

16.123 gfxterm

This module provides support for displaying a terminal and menu interface from GRUB using graphics mode.

16.124 gfxterm_background

This module provides support for setting the gfxterm background color and background image using commands background_color and background_image. See background_color and see background_image for more information.

16.126 gptsync

This module provides support for the gptsync command.. See gptsync for more information.

16.127 gzio

This module provides support for decompression (inflate) of files compressed with the GZ compression algorithm. This supports only the "DEFLATE" method for GZIP. Unsupported flags (will result in failure to inflate) include: GRUB_GZ_CONTINUATION, GRUB_GZ_ENCRYPTED, GRUB_GZ_RESERVED, and GRUB_GZ_EXTRA_FIELD.

16.128 halt

This module provides support for the halt command to shutdown / halt the system. See halt for more information.

16.129 hashsum

This module provide support for the commands hashsum, md5sum, sha1sum, sha256sum, sha512sum, and crc to calculate or check hashes of files using various methods. See hashsum, see md5sum see sha1sum, see sha256sum, see sha512sum, and see crc.

16.130 hdparm

This module provides support for the hdparm command to get or set various ATA disk parameters. This includes controlling Advanced Power Management (APM), displaying power mode, freezing ATA security settings until reset, displaying SMART status, controlling automatic acoustic management, setting standby timeout, setting the drive to standby mode, setting the drive to sleep mode, displaying the drive identification and settings, and enable/disable SMART.

16.131 hello

This provides support for the hello command to simply output "Hello World". This is intended for testing GRUB module loading / functionality.

16.132 help

This module provides support for the help command to output help text. See help for more information.

16.133 hexdump

This module provides support for the hexdump command to dump the contents of a file in hexadecimal. See hexdump for more information.

16.134 hfs

This module provides support for the Hierarchical File System (HFS) file system in GRUB. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.135 hfsplus

This module provides support for the Hierarchical File System Plus (HFS+) file system in GRUB.

16.136 hfspluscomp

This module provides support for the Hierarchical File System Plus Compressed (HFS+ Compressed) file system in GRUB.

16.137 http

This module provides support for getting data over the HTTP network protocol in GRUB (using the HTTP GET method). This may be used, for example, to obtain an operating system over HTTP (network boot).

16.138 ieee1275_fb

This module provides support for the IEEE1275 video driver output for PowerPC with a IEEE-1275 platform.

16.139 iorw

This module provides support for commands inb, inw, inl, outb, outw, and outl to read / write data to physical I/O ports. The "in" commands accept one parameter to specify the source port. The "out" commands require either two or three parameters, with the order: port, value, <optional mask>.

16.140 iso9660

This module provides support for the ISO9660 file system (often associated with optical disks such as CD-ROMs and DVD-ROMs, with extensions: System Use Sharing Protocol (SUSP), Rock Ridge (UNIX style permissions and longer names)

16.141 jfs

This module provides support for the Journaled File System (JFS) file system. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.142 jpeg

This module provides support for reading JPEG image files in GRUB, such as to support displaying a JPEG image as a background image of the gfxmenu.

16.143 json

This module provides library support for parsing / processing JavaScript Object Notation (JSON) formatted data. This is used, for example, to support LUKS2 disk encryption / decryption as metadata is encoded in JSON.

16.144 keylayouts

This module provides support for the keymap command. This command accepts one parameter to specify either the layout_name or the filename. When specifying the layout_name, this command will attempt to open the GRUB keymap file based on the following logic:

Get the "prefix" from environment variable prefix

Open keymap file prefix/layouts/layout_name.gkb

When specifying the filename, the full path to the ".gkb" file should be provided. The ".gkb" file can be generated by grub-kbdcomp.

16.145 keystatus

This module provides support for the keystatus command to check key modifier status. See keystatus for more information.

16.146 ldm

This module provides support for the Logical Disk Manager (LDM) disk format. LDM is used to add support for logical volumes most often with Microsoft Windows systems. A logical volume can be defined to span more than one physical disk.

16.147 legacy_password_test

This module is intended for performing a functional test of the legacy password function in GRUB.

16.148 legacycfg

This module provides support for commands legacy_source, legacy_configfile, extract_legacy_entries_source, extract_legacy_entries_configfile, legacy_kernel, legacy_initrd, legacy_initrd_nounzip, legacy_password, and legacy_check_password. For new uses / configurations of GRUB other commands / modules offer the modern equivalents.

16.149 linux

This module provides support for the commands linux and initrd to load Linux and an Initial RAM Disk respectively. See linux and see initrd for more information.

16.150 linux16

This module provides support for the commands linux16 and initrd16 to load Linux in 16-bit mode and an Initial RAM Disk in 16-bit mode respectively. See linux16 and see initrd16 for more information.

16.151 loadbios

This module provides support for the commands fakebios and loadbios. These commands may only be useful on platforms with issues requiring work-arounds. Command fakebios is used to create BIOS-like structures for backward compatibility with existing OS. Command loadbios is used to load a BIOS dump.

16.152 loadenv

This module provides support for commands load_env, list_env, and save_env. These commands can be used to load environment variables from a file, list environment variables in a file, and save environment variables to a file. See load_env, see list_env, and see save_env.

16.153 loopback

This module provides support for the loopback command. See loopback for more information.

16.154 ls

This module provides support for the ls command. See ls for more information.

16.155 lsacpi

This module provides support for the lsacpi command. This command can be used to display Advanced Configuration and Power Interface (ACPI) tables.

16.156 lsapm

This module provides support for the lsapm command. This command can be used to display Advanced power management (APM) information.

16.157 lsdev

This module provides support for the lsdev command. This command can be used on MIPS Advanced RISC Computing (ARC) platforms to display devices.

16.158 lsefi

This module provides support for the lsefi command. This command can be used on EFI platforms to display EFI handles.

16.159 lsefimmap

This module provides support for the lsefimmap command. This command can be used on EFI platforms to display the EFI memory map.

16.160 lsefisystab

This module provides support for the lsefisystab command. This command can be used on EFI platforms to display the EFI system tables.

16.161 lsmmap

This module provides support for the lsmmap command. This command can be used to display the memory map provided by firmware.

16.162 lspci

This module provides support for the lspci command. This command can be used to display the PCI / PCIe devices.

16.163 lssal

This module provides support for the lsefisystab command. This command can be used on Itanium (IA-64) EFI platforms to display the EFI System Abstraction Layer system table.

16.164 lsspd

This module provides support for the lsspd command. This command can be used on MIPS Loongson platforms to display the DDR RAM Serial Presence Detect (SPD) EEPROM data.

16.165 lsxen

This module provides support for the commands xen_ls and xen_cat on Xen platforms to list Xen storage.

16.166 luks

This module provides support for the Linux Unified Key Setup (LUKS) (version 1) disk encryption / decryption protocol.

16.167 luks2

This module provides support for the Linux Unified Key Setup 2 (LUKS2) disk encryption / decryption protocol.

16.168 lvm

This module provides support for reading Logical Volume Management "logical" disks. For example, a single "logical" disk may be mapped to span more than one physical disk. This would be used when booting from a LVM formatted disk as may be setup in Linux.

16.169 lzopio

This module provides support for decompressing LZO / LZOP compressed files / archives.

16.170 macbless

This module provides support for commands mactelbless and macppcbless for "blessing" a bootloader on Intel / PPC based MACs using the HFS or HFS+ file system. On HFS / HFS+ - "blessing" makes a file run as the bootloader.

16.171 macho

This module provides support for Mach Object (Mach-O) object / executable files in GRUB often used in MacOS.

16.172 mda_text

This module provides support for the Monochrome Display Adapter (MDA) terminal output device. MDA is a predecessor to VGA.

16.173 mdraid09

This module provides support for handling Linux compatible "version 0.9" software-based RAID disks in little-endian format. The "version 0.9" format was largely replaced around the year 2009 with the "version 1.x" format (see mdraid1x for more information).

16.174 mdraid09_be

This module provides support for handling Linux compatible "version 0.9" software-based RAID disks in bid-endian format. The "version 0.9" format was largely replaced around the year 2009 with the "version 1.x" format (see mdraid1x for more information).

16.175 mdraid1x

This module provides support for handling Linux compatible "version 1.x" software-based RAID disks. This includes the current version used by Linux at the time of writing.

16.176 memdisk

This module provides support for a memdisk device. A memdisk is a memory mapped emulated disk.

16.177 memrw

This module provides support for commands read_byte, read_word, read_dword, write_byte, write_word, and write_dword to read / write data to physical memory (addresses). The "read" commands accept one parameter to specify the source address. The "write" commands require either two or three parameters, with the order: address, value, <optional mask>. Note: The commands provided by this module are not allowed when lockdown is enforced (see Lockdown when booting on a secure setup).

16.178 memtools

This module provides support for GRUB development / debugging commands lsmem, lsfreemem, lsmemregions, and stress_big_allocs.

16.179 minicmd

This module provides support for a subset of commands for GRUB rescue mode including: cat, help, dump, rmmod, lsmod, and exit. The version of the commands in this module are similar to their full-fledged counterparts implemented in other GRUB modules. Note: The dump command is not allowed when lockdown is enforced (see Lockdown when booting on a secure setup).

16.180 minix

This module provides support for the Minix filesystem, version 1. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.181 minix2

This module provides support for the Minix filesystem, version 2. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.182 minix2_be

This module provides support for the Minix filesystem, version 2 big-endian. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.183 minix3

This module provides support for the Minix filesystem, version 3. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.184 minix3_be

This module provides support for the Minix filesystem, version 3 big-endian. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.185 minix_be

This module provides support for the Minix filesystem, version 1 big-endian. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.186 mmap

This module provides support for mapping or unmapping devices or files into memory as well as commands badram and cutmem. See badram and cutmem.

16.187 morse

This module provides support for outputting terminal output via Morse code to an audio speaker output.

16.188 mpi

This module provides support for multi-precision-integers (MPIs) in GRUB. MPIs are used by the crypto functions as many depend on mathematics of large numbers. This GRUB module is based on libgcrypt.

16.189 msdospart

This module provides support for modifying MSDOS formatted disk partitions through the separate parttool command.

16.190 mul_test

This module is intended for performing a functional test of the multiplication operations in GRUB.

16.191 multiboot

This module provides support for commands multiboot and module to load a multiboot kernel and load a multiboot module, respectively. See multiboot and module for more information. This is for loading data formatted per the GNU Multiboot specification.

16.192 multiboot2

This module provides support for commands multiboot2 and module2 to load a multiboot kernel and load a multiboot module, respectively. This is for loading data formatted per the GNU Multiboot specification.

16.193 nand

This module provides support for accessing an IEEE-1275 compliant NAND disk from GRUB.

16.194 nativedisk

This module provides support for the nativedisk command. See nativedisk for more information.

16.195 net

This module provides support for networking protocols including ARP, BOOTP, DNS, Ethernet, ICMPv6, ICMP, IP, TCP, and UDP. Support is included for both IPv4 and IPv6. This includes the following commands:

net_bootp - see net_bootp
net_dhcp - see net_dhcp
net_get_dhcp_option - see net_get_dhcp_option
net_nslookup - see net_nslookup
net_add_dns - see net_add_dns
net_del_dns - see net_del_dns
net_ls_dns - see net_ls_dns
net_add_addr - see net_add_addr
net_ipv6_autoconf - see net_ipv6_autoconf
net_del_addr - see net_del_addr
net_add_route - see net_add_route
net_del_route - see net_del_route
net_set_vlan - see net_set_vlan
net_ls_routes - see net_ls_routes
net_ls_cards - see net_ls_cards
net_ls_addr - see net_ls_addr

16.196 newc

This module provides support for accessing a CPIO archive as a file system from GRUB. This module is for the following newer variants of the CPIO archive supported by GNU CPIO (but GNU CPIO defaults to the "bin" format which is handled by the module cpio).

These are the variants supported by this module:

"newc" - SVR4 portable format without CRC. GNU file utility will identify these as something like "ASCII cpio archive (SVR4 with no CRC)"
‘crc’ - SVR4 portable format with CRC. GNU file utility will identify these as something like "ASCII cpio archive (SVR4 with CRC)"

16.197 nilfs2

This module provides support for the New Implementation of Log filesystem (nilfs2). Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.198 normal

This module provides support for the normal mode in GRUB. See normal for more information.

16.199 ntfs

This module provides support for the New Technology File System (NTFS) in GRUB. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.200 ntfscomp

This module provides support for compression with the New Technology File System (NTFS) in GRUB. Note: This module is not allowed in lockdown mode, see Lockdown when booting on a secure setup for more information.

16.201 ntldr

This module provides support for the ntldr command. This is may be used to boot a Windows boot loader such as NTLDR or BootMGR.

16.202 odc

This module provides support for accessing a CPIO archive as a file system from GRUB. This module is for "odc" variant of the CPIO archive supported by GNU CPIO (but GNU CPIO defaults to the "bin" format which is handled by the module cpio).

GNU file utility will identify these as something like "ASCII cpio archive (pre-SVR4 or odc)"

16.203 offsetio

This module provides support for reading from a file / archive at specified offsets in GRUB.

16.204 ofnet

This module provides support for the Open Firmware (IEEE-1275) network device support in GRUB.

16.205 ohci

This module provides support for the Open Host Controller Interface (OHCI) for USB 1 / USB 1.1 support in GRUB.

16.206 part_acorn

This module provides support for reading from disks partitioned with the Acorn Disc Filing System (ADFS) used on RiscOS.

16.207 part_amiga

This module provides support for reading from disks partitioned with the Amiga partition table.

16.208 part_apple

This module provides support for reading from disks partitioned with the Macintosh partition table.

16.209 part_bsd

This module provides support for reading from disks partitioned with BSD style partition tables.

16.210 part_dfly

This module provides support for reading from disks partitioned with the DragonFly BSD partition table.

16.211 part_dvh

This module provides support for reading from disks partitioned with the SGI Disk Volume Header partition table.

16.212 part_gpt

This module provides support for reading from disks partitioned with the GUID Partition Tables (GPT) partition table.

16.213 part_msdos

This module provides support for reading from disks partitioned with the MSDOS (Master Boot Record / MBR) style partition tables.

16.214 part_plan

This module provides support for reading from disk partitioned with the Plan9 style partition table.

16.215 part_sun

This module provides support for reading from disk partitioned with the Sun style partition table.

16.216 part_sunpc

This module provides support for reading from disk partitioned with the Sun PC style partition table.

16.217 parttool

This module provides support for the parttool command. See parttool for more information.

16.218 password

This module provides support for the password command. Please note that this uses the password in plain text, if security is a concern consider using password_pbkdf2 instead. See password for more information.

16.219 password_pbkdf2

This module provides support for the password_pbkdf2 command. See password_pbkdf2 for more information.

16.220 pata

This module provides support for Parallel ATA (PATA) disk device interfaces.

16.221 pbkdf2

This module provides support for the Password-Based Key Derivation Function 2 (PBKDF2) / PKCS#5 PBKDF2 as per RFC 2898.

16.222 pbkdf2_test

This module is intended for performing a functional test of the PBKDF2 operation in GRUB.

16.223 pci

This module provides support for generic Peripheral Component Interconnect (PCI) bus in GRUB.

16.224 pcidump

This module provides support for the pcidump command in GRUB to dump the PCI configuration registers in hexadecimal of a specified PCI device (vendor / device ID) or by position on the bus.

16.225 pgp

This module provides support for the commands: verify_detached, trust, list_trusted, distrust associated with digital signature checking via the "Open Pretty Good Privacy" (PGP) protocol / RFC 4880 using a provided public key. This module also uses / sets environment variable check_signatures. See verify_detached, trust, list_trusted, distrust, and check_signatures.

16.226 plainmount

This module provides support for accessing / mounting partitions encrypted by "cryptsetup" operating in "plain mode". See plainmount for more information.

16.227 plan9

This module provides support for the plan9 command to load a Plan9 kernel.

16.228 play

This module provides support for the play command to play a tune through the PC speaker. See play for more information.

16.229 png

This module provides support for reading Portable Network Graphics (PNG) image files in GRUB.

16.230 priority_queue

This module provides support for a priority queue function within GRUB such as to support networking functions.

16.231 probe

This module provides support for the probe command to retrieve device information

GNU GRUB manual

Table of Contents

1 Introduction to GRUB

1.1 Overview

1.2 History of GRUB

1.3 Differences from previous versions

1.4 GRUB features

1.5 The role of a boot loader

2 Naming convention

3 OS-specific notes about grub tools

4 Installation

4.1 Installing GRUB using grub-install

4.2 Making a GRUB bootable CD-ROM

4.3 The map between BIOS drives and OS devices

4.4 BIOS installation

MBR

GPT

5 Booting

5.1 How to boot operating systems

5.1.1 How to boot an OS directly with GRUB

5.1.2 Kexec with grub2-emu

5.1.3 Chain-loading an OS

5.2 Loopback booting

5.3 Booting from LVM cache logical volume

5.4 Some caveats on OS-specific issues

5.4.1 GNU/Hurd

5.4.2 GNU/Linux

5.4.3 NetBSD

5.4.4 DOS/Windows

6 Writing your own configuration file

6.1 Simple configuration handling

6.2 Root Identification Heuristics

6.3 Writing full configuration files directly

Words

Reserved words

Quoting

Variable expansion

Comments

Simple commands

Compound commands

Built-in Commands

6.4 Multi-boot manual config

6.5 Embedding a configuration file into GRUB

7 Theme file format

7.1 Introduction

7.2 Theme Elements

7.2.1 Colors

7.2.2 Fonts

7.2.3 Progress Bar

7.2.4 Circular Progress Indicator

7.2.5 Labels

7.2.7 Styled Boxes

7.2.8 Creating Styled Box Images

7.3 Theme File Manual

7.3.1 Global Properties

7.3.2 Format

7.3.3 Global Property List

7.3.4 Component Construction

7.3.5 Component List

7.3.6 Common properties

8 Booting GRUB from the network

9 Using GRUB via a serial line

10 Using GRUB with vendor power-on keys

11 GRUB image files

For GRUB Legacy users

12 Core image size limitation

13 Filesystem syntax and semantics

13.1 How to specify devices

13.2 How to specify files

13.3 How to specify block lists

14 GRUB’s user interface

14.1 The flexible command-line interface

15 GRUB environment variables

15.1 Special environment variables

15.1.1 appendedsig_key_mgmt

15.1.2 biosnum

15.1.3 blsuki_save_default

15.1.4 check_appended_signatures

15.1.5 check_signatures

15.1.6 chosen

15.1.27 net_`<interface>`_boot_file

15.1.28 net_`<interface>`_clientid

15.1.29 net_`<interface>`_clientuuid

15.1.30 net_`<interface>`_dhcp_server_name

15.1.31 net_`<interface>`_domain

15.1.32 net_`<interface>`_extensionspath

15.1.33 net_`<interface>`_hostname

15.1.34 net_`<interface>`_ip

15.1.35 net_`<interface>`_mac

15.1.36 net_`<interface>`_next_server

15.1.37 net_`<interface>`_rootpath