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.

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.

5.1 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.

• How to boot an OS directly with GRUB
• Kexec with grub2-emu
• Chain-loading an OS

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

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.

• GNU/Hurd
• GNU/Linux
• NetBSD
• DOS/Windows

drivemap -s (hd0) (hd1)

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

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.

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.

7.1 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

• Colors
• Fonts
• Progress Bar
• Circular Progress Indicator
• Labels
• Boot Menu
• Styled Boxes
• Creating Styled Box Images

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:

1. Global properties.
2. Component construction.

• Global Properties
• Format
• Global Property List
• Component Construction
• Component List
• Common properties

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)’.

14.1 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.

14.2 The simple menu interface

The menu interface is quite easy to use. Its commands are both reasonably intuitive and described on screen.

Basically, the menu interface provides a list of boot entries to the user to choose from. Use the arrow keys to select the entry of choice, then press RET to run it. An optional timeout is available to boot the default entry (the first one if not set), which is aborted by pressing any key.

Commands are available to enter a bare command-line by pressing c (which operates exactly like the non-config-file version of GRUB, but allows one to return to the menu if desired by pressing ESC) or to edit any of the boot entries by pressing e.

If you protect the menu interface with a password (see Security), all you can do is choose an entry by pressing RET, or press p to enter the password.

Pressing Ctrl-l will refresh the menu, which can be useful when connecting via serial after the menu has been drawn.

14.3 Editing a menu entry

The menu entry editor looks much like the main menu interface, but the lines in the menu are individual commands in the selected entry instead of entry names.

If an ESC is pressed in the editor, it aborts all the changes made to the configuration entry and returns to the main menu interface.

Each line in the menu entry can be edited freely, and you can add new lines by pressing RET at the end of a line. To boot the edited entry, press Ctrl-x.

Although GRUB unfortunately does not support undo, you can do almost the same thing by just returning to the main menu using ESC.

15.1 Special environment variables

These variables have special meaning to GRUB.

• appendedsig_key_mgmt
• biosnum
• blsuki_save_default
• check_appended_signatures
• check_signatures
• chosen
• cmdpath
• color_highlight
• color_normal
• config_directory
• config_file
• cryptodisk_passphrase_tries
• debug
• default
• fallback
• gfxmode
• gfxpayload
• gfxterm_font
• grub_cpu
• grub_platform
• icondir
• lang
• locale_dir
• lockdown
• menu_color_highlight
• menu_color_normal
• net_<interface>_boot_file
• net_<interface>_clientid
• net_<interface>_clientuuid
• net_<interface>_dhcp_server_name
• net_<interface>_domain
• net_<interface>_extensionspath
• net_<interface>_hostname
• net_<interface>_ip
• net_<interface>_mac
• net_<interface>_next_server
• net_<interface>_rootpath
• net_default_interface
• net_default_ip
• net_default_mac
• net_default_server
• pager
• prefix
• pxe_default_server
• root
• shim_lock
• superusers
• theme
• timeout
• timeout_style
• tpm_fail_fatal

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

default=example-gnu-linux

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

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.

• saved_entry
• next_entry
• env_block

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
}

16.1 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.

16.122 gfxmenu

This module provides support for displaying a graphical menu / user interface from GRUB. This includes features such as graphical font support, theme support, image support, and icon support.

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.

16.125 gfxterm_menu

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

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.