GNU GRUB Manual 2.00

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GNU GRUB manual

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

This edition documents version 2.00.

This manual is for GNU GRUB (version 2.00, 23 June 2012).

Copyright © 1999,2000,2001,2002,2004,2006,2008,2009,2010,2011,2012 Free Software Foundation, Inc.

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

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1 Introduction to GRUB

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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 Command-line interface), or a menu interface (see 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 Configuration). 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.

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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 Obtaining and Building 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.

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1.3 Differences from previous versions

GRUB 2 is a rewrite of GRUB (see History), 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.

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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:

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, NetBSD, 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 Commands) are a superset of those supported on the command-line. An example configuration file is provided in Configuration.

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 Commands) are a subset of those supported for configuration files. Editing commands closely resembles the Bash command-line (see Command-line interface), 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), Linux ext2/ext3/ext4, DOS FAT12/FAT16/FAT32, exFAT, 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, for more information.

Support automatic decompression

Can decompress files which were compressed by gzip or xz2. 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.3

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.

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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”4 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.

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2 Naming convention

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

Look at the following examples and explanations:


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


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


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


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

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

set root=(

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

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

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


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

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

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3 Installation

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

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

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

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3.1 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/hda

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.

Note that grub-install is actually just a shell script and the real task is done by grub-mkimage and grub-setup. Therefore, you may run those commands directly to install GRUB, without using grub-install. Don’t do that, however, unless you are very familiar with the internals of GRUB. Installing a boot loader on a running OS may be extremely dangerous.

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3.2 Making a GRUB bootable CD-ROM

GRUB supports the no emulation mode in the El Torito specification5. 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 Configuration), 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.

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3.3 The map between BIOS drives and OS devices

If the device map file exists, the GRUB utilities (grub-probe, grub-setup, 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 Device syntax), 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 ‘#’.

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3.4 BIOS installation


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 31 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.

Each of these has different 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; and systems are sometimes partitioned without leaving enough space before the first partition. On the other hand, 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 31 KiB (63 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.


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.

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4 Booting

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

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4.1 How to boot operating systems

GRUB has two distinct boot methods. One of the two is to load an operating system directly, and the other is to chain-load another boot loader which then will load an operating system actually. Generally speaking, the former is 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, the latter is sometimes required, since GRUB doesn’t support all the existing operating systems natively.

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4.1.1 How to boot an OS directly with GRUB

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

FIXME: this section is incomplete.

  1. Run the command boot (see boot).

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

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4.1.2 Chain-loading an OS

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

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

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

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

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

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4.2 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 initrd), 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. Aditionally 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 and plan9 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

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4.3 Some caveats on OS-specific issues

Here, we describe some caveats on several operating systems.

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4.3.1 GNU/Hurd

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

  1. Set GRUB’s root device to the same drive as GNU/Hurd’s. The command search --set=root --file /boot/gnumach.gz or similar may help you (see search).
  2. Load the kernel and the modules, like this:
    grub> multiboot /boot/gnumach.gz root=device:hd0s1
    grub> module  /hurd/ext2fs.static ext2fs --readonly \
                       --multiboot-command-line='${kernel-command-line}' \
                       --host-priv-port='${host-port}' \
                       --device-master-port='${device-port}' \
                       --exec-server-task='${exec-task}' -T typed '${root}' \
                       '$(task-create)' '$(task-resume)'
    grub> module /lib/ exec /hurd/exec '$(exec-task=task-create)'
  3. Finally, run the command boot (see boot).

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4.3.2 GNU/Linux

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

  1. Set GRUB’s root device to the same drive as GNU/Linux’s. The command search --set=root --file /vmlinuz or similar may help you (see search).
  2. Load the kernel using the command linux (see linux):
    grub> linux /vmlinuz root=/dev/sda1

    If you need to specify some kernel parameters, just append them to the command. For example, to set acpi to ‘off’, do this:

    grub> linux /vmlinuz root=/dev/sda1 acpi=off

    See the documentation in the Linux source tree for complete information on the available options.

    With linux GRUB uses 32-bit protocol. Some BIOS services like APM or EDD aren’t available with this protocol. In this case you need to use linux16

    grub> linux16 /vmlinuz root=/dev/sda1 acpi=off
  3. If you use an initrd, execute the command initrd (see initrd) after linux:
    grub> initrd /initrd

    If you used linux16 you need to use initrd16:

    grub> initrd16 /initrd
  4. Finally, run the command boot (see boot).

Caution: If you use an initrd and specify the ‘mem=’ option to the kernel to let it use less than actual memory size, you will also have to specify the same memory size to GRUB. To let GRUB know the size, run the command uppermem before loading the kernel. See uppermem, for more information.

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4.3.3 DOS/Windows

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

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

drivemap -s (hd0) (hd1)

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

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

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

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

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

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5 Writing your own configuration file

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

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5.1 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 Shell-like scripting), 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:


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 title 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 {

then you can make this the default using:

GRUB_DEFAULT='Example GNU/Linux distribution'

If you set this to ‘saved’, then the default menu entry will be that saved by ‘GRUB_SAVEDEFAULT’, grub-set-default, or grub-reboot.

The default is ‘0’.


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 or grub-reboot. Unset by default. This option relies on the environment block, which may not be available in all situations (see Environment block).


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.


Wait this many seconds for a key to be pressed before displaying the menu. If no key is pressed during that time, display the menu for the number of seconds specified in GRUB_TIMEOUT before booting the default entry. We expect that most people who use GRUB_HIDDEN_TIMEOUT will want to have GRUB_TIMEOUT set to ‘0’ so that the menu is not displayed at all unless a key is pressed. Unset by default.


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. Unset by default.


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


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


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’ (PC BIOS and EFI consoles), ‘serial’ (serial terminal), ‘ofconsole’ (Open Firmware console), ‘at_keyboard’ (PC AT keyboard, mainly useful with Coreboot), 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.


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’ (PC BIOS and EFI consoles), ‘serial’ (serial terminal), ‘gfxterm’ (graphics-mode output), ‘ofconsole’ (Open Firmware console), or ‘vga_text’ (VGA text output, mainly useful with Coreboot).

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


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


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


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


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’.




As ‘GRUB_CMDLINE_LINUX’, but for GNU Mach.


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


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


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’.


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


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.


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.


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.


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


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.


Normally, grub-mkconfig will try to use the external os-prober program, if installed, to discover other operating systems installed on the same system and generate appropriate menu entries for them. Set this option to ‘true’ to disable this.


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.


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


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.

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.

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5.2 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.


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 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 ignored6). 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 Environment).

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.


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. If its exit status is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its 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. 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] { 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.

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.

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5.3 Multi-boot manual config

Currently autogenerating config files for multi-boot environments depends on os-prober and has several shortcomings. While fixing it is scheduled for the next release, meanwhile you can make use of 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:


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= 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


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

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

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6 Theme file format

6.1 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

6.2 Theme Elements

6.2.1 Colors

Colors can be specified in several ways:

6.2.2 Fonts

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

6.2.3 Progress Bar

Figure 6.1

Figure 6.2

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

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

6.2.4 Circular Progress Indicator

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

6.2.5 Labels

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

6.2.6 Boot Menu

The boot menu where GRUB displays the menu entries from the “grub.cfg” file. It is a list of items, where each item has a title and an optional icon. The icon is selected based on the *classes* specified for the menu entry. If there is a PNG file named “myclass.png” in the “grub/themes/icons” directory, it will be displayed for items which have the class *myclass*. The boot menu can be customized in several ways, such as the font and color used for the menu entry title, and by specifying styled boxes for the menu itself and for the selected item highlight.

6.2.7 Styled Boxes

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

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

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

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

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

Figure 6.3

6.2.8 Creating Styled Box Images

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

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

6.3 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.

6.3.1 Global Properties

6.3.2 Format

Global properties are specified with the simple format:

In this example, name3 is assigned a color value.

6.3.3 Global Property List

title-textSpecifies the text to display at the top center of the screen as a title.
title-fontDefines the font used for the title message at the top of the screen.
title-colorDefines the color of the title message.
message-fontDefines the font used for messages, such as when GRUB is unable to automatically boot an entry.
message-colorDefines the color of the message text.
message-bg-colorDefines the background color of the message text area.
desktop-imageSpecifies the image to use as the background. It will be scaled to fit the screen size.
desktop-colorSpecifies the color for the background if *desktop-image* is not specified.
terminal-boxSpecifies the file name pattern for the styled box slices used for the command line terminal window. For example, “terminal-box: terminal_*.png” will use the images “terminal_c.png“ as the center area, “terminal_n.png“ as the north (top) edge, “terminal_nw.png“ as the northwest (upper left) corner, and so on. If the image for any slice is not found, it will simply be left empty.

6.3.4 Component Construction

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

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

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

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

6.3.5 Component List

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

6.3.6 Common properties

The following properties are supported by all components:


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

xValue in pixels
p%+xmixture of both

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


The width of object in same format.


The height of object in same format.


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

“__timeout__“Any component with this ID will have its *text*, *start*, *end*, *value*, and *visible* properties set by GRUB when it is counting down to an automatic boot of the default menu entry.

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7 Booting GRUB from the network

The following instructions only work on PC BIOS systems where the Preboot eXecution Environment (PXE) is available.

To generate a PXE boot image, run:

grub-mkimage --format=i386-pc-pxe --output=grub.pxe --prefix='(pxe)/boot/grub' pxe pxecmd

Copy grub.pxe, /boot/grub/*.mod, and /boot/grub/*.lst to the PXE (TFTP) server, ensuring that *.mod and *.lst are accessible via the /boot/grub/ path from the TFTP server root. Set the DHCP server configuration to offer grub.pxe as the boot file (the ‘filename’ option in ISC dhcpd).

You can also use the grub-mknetdir utility to generate an image and a GRUB directory tree, rather than copying files around manually.

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

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

GRUB provides several environment variables which may be used to inspect or change the behaviour of the PXE device:


The IP address of this machine. Read-only.


The network interface’s MAC address. Read-only.


The client host name provided by DHCP. Read-only.


The client domain name provided by DHCP. Read-only.


The path to the client’s root disk provided by DHCP. Read-only.


The path to additional DHCP vendor extensions provided by DHCP. Read-only.


The boot file name provided by DHCP. Read-only.


The name of the DHCP server responsible for these boot parameters. Read-only.


The default server. Read-write, although setting this is only useful before opening a network device.

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8 Using GRUB via a serial line

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

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

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

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

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

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

However, note that GRUB assumes that your terminal emulator is compatible with VT100 by default. This is true for most terminal emulators nowadays, but you should pass the option --dumb to the command if your terminal emulator is not VT100-compatible or implements few VT100 escape sequences. If you specify this option then GRUB provides you with an alternative menu interface, because the normal menu requires several fancy features of your terminal.

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9 Using GRUB with vendor power-on keys

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

Dell XPS M1530


Asus EeePC 1005PE

84:1 (unconfirmed)

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

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

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

byte 0x47: 20 --> 28

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


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

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10 GRUB image files

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


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

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


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


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


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


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


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


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

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


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

For GRUB Legacy users

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


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


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

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


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


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


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


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

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11 Filesystem syntax and semantics

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

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11.1 How to specify devices

The device syntax is like this:


[]’ 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 refered just by driver name. RAID (md), ofdisk (ieee1275 and nand), LVM (lv), LDM 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 necessarry. Commas need to be escaped. Loopback uses whatever name specified to loopback command. Hostdisk uses names specified in 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>.


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).


If you enabled the network support, the special drives ‘(tftp)’, ‘(http)’ and so on ars also available. Before using the network drive, you must initialize the network. See Network, for more information.

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

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11.2 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.

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11.3 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:


This represents that GRUB should read blocks 0 through 99, block 200, and blocks 300 through 599. If you omit an offset, then GRUB assumes the offset is zero.

Like the file name syntax (see File name syntax), 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)’.

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12 GRUB’s user interface

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

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

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12.1 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 entered7. The commands (see Command-line and menu entry 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:

PC right key

Move forward one character.

PC left key

Move back one character.


Move to the start of the line.


Move the the end of the line.


Delete the character underneath the cursor.


Delete the character to the left of the cursor.


Kill the text from the current cursor position to the end of the line.


Kill backward from the cursor to the beginning of the line.


Yank the killed text back into the buffer at the cursor.

PC up key

Move up through the history list.

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.

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12.2 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.

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12.3 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.

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13 GRUB environment variables

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

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

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13.1 Special environment variables

These variables have special meaning to GRUB.

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13.1.1 biosnum

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

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

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13.1.2 chosen

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

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

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13.1.3 color_highlight

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

The default is ‘black/white’.

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13.1.4 color_normal

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

The default is ‘white/black’.

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13.1.5 debug

This variable may be set to enable debugging output from various components of GRUB. The value is a list of debug facility names separated by whitespace or ‘,’, or ‘all’ to enable all available debugging output.

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13.1.6 default

If this variable is set, it identifies a menu entry that should be selected by default, possibly after a timeout (see timeout). The entry may be identified by number or by title.

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

Submenu 1
  Menu Entry 1
  Menu Entry 2
Submenu 2
  Submenu 3
    Menu Entry 3
    Menu Entry 4
  Menu Entry 5

“Menu Entry 3” would then be identified as ‘Submenu 2>Submenu 3>Menu Entry 3’.

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

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13.1.7 fallback

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

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13.1.8 gfxmode

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

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

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13.1.9 gfxpayload

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

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

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

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

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13.1.10 gfxterm_font

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

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13.1.11 icondir

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

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13.1.12 lang

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

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

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13.1.13 locale_dir

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

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

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13.1.14 menu_color_highlight

This variable contains the foreground and background colors to be used for the highlighted menu entry, separated by a slash (‘/’). Setting this variable changes those colors. For the available color names, see color_normal.

The default is the value of ‘color_highlight’ (see color_highlight).

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13.1.15 menu_color_normal

This variable contains the foreground and background colors to be used for non-highlighted menu entries, separated by a slash (‘/’). Setting this variable changes those colors. For the available color names, see color_normal.

The default is the value of ‘color_normal’ (see color_normal).

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13.1.16 net_pxe_boot_file

See Network.

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13.1.17 net_pxe_dhcp_server_name

See Network.

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13.1.18 net_pxe_domain

See Network.

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13.1.19 net_pxe_extensionspath

See Network.

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13.1.20 net_pxe_hostname

See Network.

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13.1.21 net_pxe_ip

See Network.

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13.1.22 net_pxe_mac

See Network.

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13.1.23 net_pxe_rootpath

See Network.

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13.1.24 pager

If set to ‘1’, pause output after each screenful and wait for keyboard input. The default is not to pause output.

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13.1.25 prefix

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

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13.1.26 pxe_blksize

See Network.

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13.1.27 pxe_default_gateway

See Network.

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13.1.28 pxe_default_server

See Network.

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13.1.29 root

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

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

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13.1.30 superusers

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

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13.1.31 theme

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

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

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13.1.32 timeout

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

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

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13.2 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).

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

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14 The list of available commands

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

Commands belong to different groups. A few can only be used in the global section of the configuration file (or “menu”); most of them can be entered on the command-line and can be used either anywhere in the menu or specifically in the menu entries.

In rescue mode, only the insmod (see insmod), ls (see ls), set (see set), and unset (see unset) commands are normally available. If you end up in rescue mode and do not know what to do, then see GRUB only offers a rescue shell.

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14.1 The list of commands for the menu only

The semantics used in parsing the configuration file are the following:

These commands can only be used in the menu:

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14.1.1 menuentry

Command: menuentry title [--class=class …] [--users=users] [--unrestricted] [--hotkey=key] { command; … }

This defines a GRUB menu entry named title. When this entry is selected from the menu, GRUB will set the chosen environment variable to title, execute the list of commands given within braces, and if the last command in the list returned successfully and a kernel was loaded it will execute the boot command.

The --class option may be used any number of times to group menu entries into classes. Menu themes may display different classes using different styles.

The --users option grants specific users access to specific menu entries. See Security.

The --unrestricted option grants all users access to specific menu entries. See Security.

The --hotkey option associates a hotkey with a menu entry. key may be a single letter, or one of the aliases ‘backspace’, ‘tab’, or ‘delete’.

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14.1.2 submenu

Command: submenu title [--class=class …] [--users=users] [--unrestricted] [--hotkey=key] { menu entries … }

This defines a submenu. An entry called title will be added to the menu; when that entry is selected, a new menu will be displayed showing all the entries within this submenu.

All options are the same as in the menuentry command (see menuentry).

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14.2 The list of general commands

Commands usable anywhere in the menu and in the command-line.

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14.2.1 serial

Command: serial [--unit=unit] [--port=port] [--speed=speed] [--word=word] [--parity=parity] [--stop=stop]

Initialize a serial device. unit is a number in the range 0-3 specifying which serial port to use; default is 0, which corresponds to the port often called COM1. port is the I/O port where the UART is to be found; if specified it takes precedence over unit. speed is the transmission speed; default is 9600. word and stop are the number of data bits and stop bits. Data bits must be in the range 5-8 and stop bits must be 1 or 2. Default is 8 data bits and one stop bit. parity is one of ‘no’, ‘odd’, ‘even’ and defaults to ‘no’.

The serial port is not used as a communication channel unless the terminal_input or terminal_output command is used (see terminal_input, see terminal_output).

See also Serial terminal.

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14.2.2 terminal_input

Command: terminal_input [--append|--remove] [terminal1] [terminal2] …

List or select an input terminal.

With no arguments, list the active and available input terminals.

With --append, add the named terminals to the list of active input terminals; any of these may be used to provide input to GRUB.

With --remove, remove the named terminals from the active list.

With no options but a list of terminal names, make only the listed terminal names active.

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14.2.3 terminal_output

Command: terminal_output [--append|--remove] [terminal1] [terminal2] …

List or select an output terminal.

With no arguments, list the active and available output terminals.

With --append, add the named terminals to the list of active output terminals; all of these will receive output from GRUB.

With --remove, remove the named terminals from the active list.

With no options but a list of terminal names, make only the listed terminal names active.

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14.2.4 terminfo

Command: terminfo [-a|-u|-v] [term]

Define the capabilities of your terminal by giving the name of an entry in the terminfo database, which should correspond roughly to a ‘TERM’ environment variable in Unix.

The currently available terminal types are ‘vt100’, ‘vt100-color’, ‘ieee1275’, and ‘dumb’. If you need other terminal types, please contact us to discuss the best way to include support for these in GRUB.

The -a (--ascii), -u (--utf8), and -v (--visual-utf8) options control how non-ASCII text is displayed. -a specifies an ASCII-only terminal; -u specifies logically-ordered UTF-8; and -v specifies "visually-ordered UTF-8" (in other words, arranged such that a terminal emulator without bidirectional text support will display right-to-left text in the proper order; this is not really proper UTF-8, but a workaround).

If no option or terminal type is specified, the current terminal type is printed.

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14.3 The list of command-line and menu entry commands

These commands are usable in the command-line and in menu entries. If you forget a command, you can run the command help (see help).

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14.3.1 acpi

Command: acpi [-1|-2] [--exclude=table1,…|--load-only=table1,…] [--oemid=id] [--oemtable=table] [--oemtablerev=rev] [--oemtablecreator=creator] [--oemtablecreatorrev=rev] [--no-ebda] filename …

Modern BIOS systems normally implement the Advanced Configuration and Power Interface (ACPI), and define various tables that describe the interface between an ACPI-compliant operating system and the firmware. In some cases, the tables provided by default only work well with certain operating systems, and it may be necessary to replace some of them.

Normally, this command will replace the Root System Description Pointer (RSDP) in the Extended BIOS Data Area to point to the new tables. If the --no-ebda option is used, the new tables will be known only to GRUB, but may be used by GRUB’s EFI emulation.

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14.3.2 badram

Command: badram addr,mask[,addr,mask...]

Filter out bad RAM.

This command notifies the memory manager that specified regions of RAM ought to be filtered out (usually, because they’re damaged). This remains in effect after a payload kernel has been loaded by GRUB, as long as the loaded kernel obtains its memory map from GRUB. Kernels that support this include Linux, GNU Mach, the kernel of FreeBSD and Multiboot kernels in general.

Syntax is the same as provided by the Memtest86+ utility: a list of address/mask pairs. Given a page-aligned address and a base address / mask pair, if all the bits of the page-aligned address that are enabled by the mask match with the base address, it means this page is to be filtered. This syntax makes it easy to represent patterns that are often result of memory damage, due to physical distribution of memory cells.

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14.3.3 blocklist

Command: blocklist file

Print a block list (see Block list syntax) for file.

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14.3.4 boot

Command: boot

Boot the OS or chain-loader which has been loaded. Only necessary if running the fully interactive command-line (it is implicit at the end of a menu entry).

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14.3.5 cat

Command: cat [--dos] file

Display the contents of the file file. This command may be useful to remind you of your OS’s root partition:

grub> cat /etc/fstab

If the --dos option is used, then carriage return / new line pairs will be displayed as a simple new line. Otherwise, the carriage return will be displayed as a control character (‘<d>’) to make it easier to see when boot problems are caused by a file formatted using DOS-style line endings.

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14.3.6 chainloader

Command: chainloader [--force] file

Load file as a chain-loader. Like any other file loaded by the filesystem code, it can use the blocklist notation (see Block list syntax) to grab the first sector of the current partition with ‘+1’. If you specify the option --force, then load file forcibly, whether it has a correct signature or not. This is required when you want to load a defective boot loader, such as SCO UnixWare 7.1.

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14.3.7 cmp

Command: cmp file1 file2

Compare the file file1 with the file file2. If they differ in size, print the sizes like this:

Differ in size: 0x1234 [foo], 0x4321 [bar]

If the sizes are equal but the bytes at an offset differ, then print the bytes like this:

Differ at the offset 777: 0xbe [foo], 0xef [bar]

If they are completely identical, nothing will be printed.

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14.3.8 configfile

Command: configfile file

Load file as a configuration file. If file defines any menu entries, then show a menu containing them immediately.

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14.3.9 cpuid

Command: cpuid [-l]

Check for CPU features. This command is only available on x86 systems.

With the -l option, return true if the CPU supports long mode (64-bit).

If invoked without options, this command currently behaves as if it had been invoked with -l. This may change in the future.

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14.3.10 crc

Command: crc file

Display the CRC32 checksum of file.

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14.3.11 date

Command: date [[year-]month-day] [hour:minute[:second]]

With no arguments, print the current date and time.

Otherwise, take the current date and time, change any elements specified as arguments, and set the result as the new date and time. For example, ‘date 01-01’ will set the current month and day to January 1, but leave the year, hour, minute, and second unchanged.

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14.3.12 drivemap

Command: drivemap -l|-r|[-s] from_drive to_drive

Without options, map the drive from_drive to the drive to_drive. This is necessary when you chain-load some operating systems, such as DOS, if such an OS resides at a non-first drive. For convenience, any partition suffix on the drive is ignored, so you can safely use ${root} as a drive specification.

With the -s option, perform the reverse mapping as well, swapping the two drives.

With the -l option, list the current mappings.

With the -r option, reset all mappings to the default values.

For example:

drivemap -s (hd0) (hd1)

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14.3.13 echo

Command: echo [-n] [-e] string …

Display the requested text and, unless the -n option is used, a trailing new line. If there is more than one string, they are separated by spaces in the output. As usual in GRUB commands, variables may be substituted using ‘${var}’.

The -e option enables interpretation of backslash escapes. The following sequences are recognised:




alert (BEL)


suppress trailing new line


form feed


new line


carriage return


horizontal tab


vertical tab

When interpreting backslash escapes, backslash followed by any other character will print that character.

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14.3.14 export

Command: export envvar

Export the environment variable envvar. Exported variables are visible to subsidiary configuration files loaded using configfile.

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14.3.15 false

Command: false

Do nothing, unsuccessfully. This is mainly useful in control constructs such as if and while (see Shell-like scripting).

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14.3.16 gettext

Command: gettext string

Translate string into the current language.

The current language code is stored in the ‘lang’ variable in GRUB’s environment (see lang). Translation files in MO format are read from ‘locale_dir’ (see locale_dir), usually /boot/grub/locale.

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14.3.17 gptsync

Command: gptsync device [partition[+/-[type]]] …

Disks using the GUID Partition Table (GPT) also have a legacy Master Boot Record (MBR) partition table for compatibility with the BIOS and with older operating systems. The legacy MBR can only represent a limited subset of GPT partition entries.

This command populates the legacy MBR with the specified partition entries on device. Up to three partitions may be used.

type is an MBR partition type code; prefix with ‘0x’ if you want to enter this in hexadecimal. The separator between partition and type may be ‘+’ to make the partition active, or ‘-’ to make it inactive; only one partition may be active. If both the separator and type are omitted, then the partition will be inactive.

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14.3.18 halt

Command: halt --no-apm

The command halts the computer. If the --no-apm option is specified, no APM BIOS call is performed. Otherwise, the computer is shut down using APM.

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14.3.19 help

Command: help [pattern …]

Display helpful information about builtin commands. If you do not specify pattern, this command shows short descriptions of all available commands.

If you specify any patterns, it displays longer information about each of the commands whose names begin with those patterns.

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14.3.20 initrd

Command: initrd file

Load an initial ramdisk for a Linux kernel image, and set the appropriate parameters in the Linux setup area in memory. This may only be used after the linux command (see linux) has been run. See also GNU/Linux.

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14.3.21 initrd16

Command: initrd16 file

Load an initial ramdisk for a Linux kernel image to be booted in 16-bit mode, and set the appropriate parameters in the Linux setup area in memory. This may only be used after the linux16 command (see linux16) has been run. See also GNU/Linux.

This command is only available on x86 systems.

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14.3.22 insmod

Command: insmod module

Insert the dynamic GRUB module called module.

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14.3.23 keystatus

Command: keystatus [--shift] [--ctrl] [--alt]

Return true if the Shift, Control, or Alt modifier keys are held down, as requested by options. This is useful in scripting, to allow some user control over behaviour without having to wait for a keypress.

Checking key modifier status is only supported on some platforms. If invoked without any options, the keystatus command returns true if and only if checking key modifier status is supported.

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14.3.24 linux

Command: linux file …

Load a Linux kernel image from file. The rest of the line is passed verbatim as the kernel command-line. Any initrd must be reloaded after using this command (see initrd).

On x86 systems, the kernel will be booted using the 32-bit boot protocol. Note that this means that the ‘vga=’ boot option will not work; if you want to set a special video mode, you will need to use GRUB commands such as ‘set gfxpayload=1024x768’ or ‘set gfxpayload=keep’ (to keep the same mode as used in GRUB) instead. GRUB can automatically detect some uses of ‘vga=’ and translate them to appropriate settings of ‘gfxpayload’. The linux16 command (see linux16) avoids this restriction.

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14.3.25 linux16

Command: linux16 file …

Load a Linux kernel image from file in 16-bit mode. The rest of the line is passed verbatim as the kernel command-line. Any initrd must be reloaded after using this command (see initrd16).

The kernel will be booted using the traditional 16-bit boot protocol. As well as bypassing problems with ‘vga=’ described in linux, this permits booting some other programs that implement the Linux boot protocol for the sake of convenience.

This command is only available on x86 systems.

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14.3.26 list_env

Command: list_env [-f file]

List all variables in the environment block file. See Environment block.

The -f option overrides the default location of the environment block.

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14.3.27 load_env

Command: load_env [-f file]

Load all variables from the environment block file into the environment. See Environment block.

The -f option overrides the default location of the environment block.

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14.3.28 loopback

Command: loopback [-d] device file

Make the device named device correspond to the contents of the filesystem image in file. For example:

loopback loop0 /path/to/image
ls (loop0)/

With the -d option, delete a device previously created using this command.

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14.3.29 ls

Command: ls [arg …]

List devices or files.

With no arguments, print all devices known to GRUB.

If the argument is a device name enclosed in parentheses (see Device syntax), then list all files at the root directory of that device.

If the argument is a directory given as an absolute file name (see File name syntax), then list the contents of that directory.

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14.3.30 normal

Command: normal [file]

Enter normal mode and display the GRUB menu.

In normal mode, commands, filesystem modules, and cryptography modules are automatically loaded, and the full GRUB script parser is available. Other modules may be explicitly loaded using insmod (see insmod).

If a file is given, then commands will be read from that file. Otherwise, they will be read from $prefix/grub.cfg if it exists.

normal may be called from within normal mode, creating a nested environment. It is more usual to use configfile (see configfile) for this.

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14.3.31 normal_exit

Command: normal_exit

Exit normal mode (see normal). If this instance of normal mode was not nested within another one, then return to rescue mode.

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14.3.32 parttool

Command: parttool partition commands

Make various modifications to partition table entries.

Each command is either a boolean option, in which case it must be followed with ‘+’ or ‘-’ (with no intervening space) to enable or disable that option, or else it takes a value in the form ‘command=value’.

Currently, parttool is only useful on DOS partition tables (also known as Master Boot Record, or MBR). On these partition tables, the following commands are available:

boot’ (boolean)

When enabled, this makes the selected partition be the active (bootable) partition on its disk, clearing the active flag on all other partitions. This command is limited to primary partitions.

type’ (value)

Change the type of an existing partition. The value must be a number in the range 0-0xFF (prefix with ‘0x’ to enter it in hexadecimal).

hidden’ (boolean)

When enabled, this hides the selected partition by setting the hidden bit in its partition type code; when disabled, unhides the selected partition by clearing this bit. This is useful only when booting DOS or Wwindows and multiple primary FAT partitions exist in one disk. See also DOS/Windows.

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14.3.33 password

Command: password user clear-password

Define a user named user with password clear-password. See Security.

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14.3.34 password_pbkdf2

Command: password_pbkdf2 user hashed-password

Define a user named user with password hash hashed-password. Use grub-mkpasswd-pbkdf2 (see Invoking grub-mkpasswd-pbkdf2) to generate password hashes. See Security.

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14.3.35 play

Command: play file | tempo [pitch1 duration1] [pitch2 duration2] ...

Plays a tune

If the argument is a file name (see File name syntax), play the tune recorded in it. The file format is first the tempo as an unsigned 32bit little-endian number, then pairs of unsigned 16bit little-endian numbers for pitch and duration pairs.

If the arguments are a series of numbers, play the inline tune.

The tempo is the base for all note durations. 60 gives a 1-second base, 120 gives a half-second base, etc. Pitches are Hz. Set pitch to 0 to produce a rest.

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14.3.36 pxe_unload

Command: pxe_unload

Unload the PXE environment (see Network).

This command is only available on PC BIOS systems.

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14.3.37 read

Command: read [var]

Read a line of input from the user. If an environment variable var is given, set that environment variable to the line of input that was read, with no terminating newline.

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14.3.38 reboot

Command: reboot

Reboot the computer.

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14.3.39 save_env

Command: save_env [-f file] var …

Save the named variables from the environment to the environment block file. See Environment block.

The -f option overrides the default location of the environment block.

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14.3.40 search

Command: search [--file|--label|--fs-uuid] [--set [var]] [--no-floppy] name

Search devices by file (-f, --file), filesystem label (-l, --label), or filesystem UUID (-u, --fs-uuid).

If the --set option is used, the first device found is set as the value of environment variable var. The default variable is ‘root’.

The --no-floppy option prevents searching floppy devices, which can be slow.

The ‘search.file’, ‘search.fs_label’, and ‘search.fs_uuid’ commands are aliases for ‘search --file’, ‘search --label’, and ‘search --fs-uuid’ respectively.

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14.3.41 sendkey

Command: sendkey [--num|--caps|--scroll|--insert|--pause|--left-shift|--right-shift|--sysrq|--numkey|--capskey|--scrollkey|--insertkey|--left-alt|--right-alt|--left-ctrl|--right-ctrlon’|‘off’]… [no-led] keystroke

Insert keystrokes into the keyboard buffer when booting. Sometimes an operating system or chainloaded boot loader requires particular keys to be pressed: for example, one might need to press a particular key to enter "safe mode", or when chainloading another boot loader one might send keystrokes to it to navigate its menu.

You may provide up to 16 keystrokes (the length of the BIOS keyboard buffer). Keystroke names may be upper-case or lower-case letters, digits, or taken from the following table:

controlpress and release Control
shiftpress and release left Shift
rshiftpress and release right Shift
altpress and release Alt
spacespace bar
capslockCaps Lock
num11 (numeric keypad)
num22 (numeric keypad)
num33 (numeric keypad)
num44 (numeric keypad)
num55 (numeric keypad)
num66 (numeric keypad)
num77 (numeric keypad)
num88 (numeric keypad)
num99 (numeric keypad)
num00 (numeric keypad)
numperiod. (numeric keypad)
numendEnd (numeric keypad)
numdownDown (numeric keypad)
numpgdownPage Down (numeric keypad)
numleftLeft (numeric keypad)
numcenter5 with Num Lock inactive (numeric keypad)
numrightRight (numeric keypad)
numhomeHome (numeric keypad)
numupUp (numeric keypad)
numpgupPage Up (numeric keypad)
numinsertInsert (numeric keypad)
numdeleteDelete (numeric keypad)
numasterisk* (numeric keypad)
numminus- (numeric keypad)
numplus+ (numeric keypad)
numslash/ (numeric keypad)
numenterEnter (numeric keypad)
pgdownPage Down
pgupPage Up

As well as keystrokes, the sendkey command takes various options that affect the BIOS keyboard status flags. These options take an ‘on’ or ‘off’ parameter, specifying that the corresponding status flag be set or unset; omitting the option for a given status flag will leave that flag at its initial state at boot. The --num, --caps, --scroll, and --insert options emulate setting the corresponding mode, while the --numkey, --capskey, --scrollkey, and --insertkey options emulate pressing and holding the corresponding key. The other status flag options are self-explanatory.

If the --no-led option is given, the status flag options will have no effect on keyboard LEDs.

If the sendkey command is given multiple times, then only the last invocation has any effect.

Since sendkey manipulates the BIOS keyboard buffer, it may cause hangs, reboots, or other misbehaviour on some systems. If the operating system or boot loader that runs after GRUB uses its own keyboard driver rather than the BIOS keyboard functions, then sendkey will have no effect.

This command is only available on PC BIOS systems.

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14.3.42 set

Command: set [envvar=value]

Set the environment variable envvar to value. If invoked with no arguments, print all environment variables with their values.

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14.3.43 true

Command: true

Do nothing, successfully. This is mainly useful in control constructs such as if and while (see Shell-like scripting).

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14.3.44 unset

Command: unset envvar

Unset the environment variable envvar.

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14.3.45 uppermem

This command is not yet implemented for GRUB 2, although it is planned.

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15 Charset

GRUB uses UTF-8 internally other than in rendering where some GRUB-specific appropriate representation is used. All text files (including config) are assumed to be encoded in UTF-8.

16 Filesystems

NTFS, JFS, UDF, HFS+, exFAT, long filenames in FAT, Joliet part of ISO9660 are treated as UTF-16 as per specification. AFS and BFS are read as UTF-8, again according to specification. BtrFS, cpio, tar, squash4, minix, minix2, minix3, ROMFS, ReiserFS, XFS, ext2, ext3, ext4, FAT (short names), RockRidge part of ISO9660, nilfs2, UFS1, UFS2 and ZFS are assumed to be UTF-8. This might be false on systems configured with legacy charset but as long as the charset used is superset of ASCII you should be able to access ASCII-named files. And it’s recommended to configure your system to use UTF-8 to access the filesystem, convmv may help with migration. ISO9660 (plain) filenames are specified as being ASCII or being described with unspecified escape sequences. GRUB assumes that the ISO9660 names are UTF-8 (since any ASCII is valid UTF-8). There are some old CD-ROMs which use CP437 in non-compliant way. You’re still able to access files with names containing only ASCII characters on such filesystems though. You’re also able to access any file if the filesystem contains valid Joliet (UTF-16) or RockRidge (UTF-8). AFFS, SFS and HFS never use unicode and GRUB assumes them to be in Latin1, Latin1 and MacRoman respectively. GRUB handles filesystem case-insensitivity however no attempt is performed at case conversion of international characters so e.g. a file named lowercase greek alpha is treated as different from the one named as uppercase alpha. The filesystems in questions are NTFS (except POSIX namespace), HFS+ (configurable at mkfs time, default insensitive), SFS (configurable at mkfs time, default insensitive), JFS (configurable at mkfs time, default sensitive), HFS, AFFS, FAT, exFAT and ZFS (configurable on per-subvolume basis by property “casesensitivity”, default sensitive). On ZFS subvolumes marked as case insensitive files containing lowercase international characters are inaccessible. Also like all supported filesystems except HFS+ and ZFS (configurable on per-subvolume basis by property “normalization”, default none) GRUB makes no attempt at check of canonical equivalence so a file name u-diaresis is treated as distinct from u+combining diaresis. This however means that in order to access file on HFS+ its name must be specified in normalisation form D. On normalized ZFS subvolumes filenames out of normalisation are inaccessible.

17 Output terminal

Firmware output console “console” on ARC and IEEE1275 are limited to ASCII. BIOS firmware console and VGA text are limited to ASCII and some pseudographics. None of above mentioned is appropriate for displaying international and any unsupported character is replaced with question mark except pseudographics which we attempt to approximate with ASCII. EFI console on the other hand nominally supports UTF-16 but actual language coverage depends on firmware and may be very limited. The encoding used on serial can be chosen with terminfo as either ASCII, UTF-8 or “visual UTF-8”. Last one is against the specification but results in correct rendering of right-to-left on some readers which don’t have own bidi implementation. When using gfxterm or gfxmenu GRUB itself is responsible for rendering the text. In this case GRUB is limited by loaded fonts. If fonts contain all required characters then bidirectional text, cursive variants and combining marks other than enclosing, half (e.g. left half tilde or combining overline) and double ones. Ligatures aren’t supported though. This should cover European, Middle Eastern (if you don’t mind lack of lam-alif ligature in Arabic) and East Asian scripts. Notable unsupported scripts are Brahmic family and derived as well as Mongolian, Tifinagh, Korean Jamo (precomposed characters have no problem) and tonal writing (2e5-2e9). GRUB also ignores deprecated (as specified in Unicode) characters (e.g. tags). GRUB also doesn’t handle so called “annotation characters” If you can complete either of two lists or, better, propose a patch to improve rendering, please contact developper team.

18 Input terminal

Firmware console on BIOS, IEEE1275 and ARC doesn’t allow you to enter non-ASCII characters. EFI specification allows for such but author is unaware of any actual implementations. Serial input is currently limited for latin1 (unlikely to change). Own keyboard implementations (at_keyboard and usb_keyboard) supports any key but work on one-char-per-keystroke. So no dead keys or advanced input method. Also there is no keymap change hotkey. In practice it makes difficult to enter any text using non-Latin alphabet. Moreover all current input consumers are limited to ASCII.

19 Gettext

GRUB supports being translated. For this you need to have language *.mo files in $prefix/locale, load gettext module and set “lang” variable.

20 Regexp

Regexps work on unicode characters, however no attempt at checking cannonical equivalence has been made. Moreover the classes like [:alpha:] match only ASCII subset.

21 Other

Currently GRUB always uses YEAR-MONTH-DAY HOUR:MINUTE:SECOND [WEEKDAY] 24-hour datetime format but weekdays are translated. GRUB always uses the decimal number format with [0-9] as digits and . as descimal separator and no group separator. IEEE1275 aliases are matched case-insensitively except non-ASCII which is matched as binary. Similar behaviour is for matching OSBundleRequired. Since IEEE1275 aliases and OSBundleRequired don’t contain any non-ASCII it should never be a problem in practice. Case-sensitive identifiers are matched as raw strings, no canonical equivalence check is performed. Case-insenstive identifiers are matched as RAW but additionally [a-z] is equivalent to [A-Z]. GRUB-defined identifiers use only ASCII and so should user-defined ones. Identifiers containing non-ASCII may work but aren’t supported. Only the ASCII space characters (space U+0020, tab U+000b, CR U+000d and LF U+000a) are recognised. Other unicode space characters aren’t a valid field separator. test tests <, >, <=, >=, -pgt and -plt compare the strings in the lexicographical order of unicode codepoints, replicating the behaviour of test from coreutils. environment variables and commands are listed in the same order.

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22 Authentication and authorisation

By default, the boot loader interface is accessible to anyone with physical access to the console: anyone can select and edit any menu entry, and anyone can get direct access to a GRUB shell prompt. For most systems, this is reasonable since anyone with direct physical access has a variety of other ways to gain full access, and requiring authentication at the boot loader level would only serve to make it difficult to recover broken systems.

However, in some environments, such as kiosks, it may be appropriate to lock down the boot loader to require authentication before performing certain operations.

The ‘password’ (see password) and ‘password_pbkdf2’ (see password_pbkdf2) commands can be used to define users, each of which has an associated password. ‘password’ sets the password in plain text, requiring grub.cfg to be secure; ‘password_pbkdf2’ sets the password hashed using the Password-Based Key Derivation Function (RFC 2898), requiring the use of grub-mkpasswd-pbkdf2 (see Invoking grub-mkpasswd-pbkdf2) to generate password hashes.

In order to enable authentication support, the ‘superusers’ environment variable must be set to a list of usernames, separated by any of spaces, commas, semicolons, pipes, or ampersands. Superusers are permitted to use the GRUB command line, edit menu entries, and execute any menu entry. If ‘superusers’ is set, then use of the command line is automatically restricted to superusers.

Other users may be given access to specific menu entries by giving a list of usernames (as above) using the --users option to the ‘menuentry’ command (see menuentry). If the --unrestricted option is used for a menu entry, then that entry is unrestricted. If the --users option is not used for a menu entry, then that only superusers are able to use it.

Putting this together, a typical grub.cfg fragment might look like this:

set superusers="root"
password_pbkdf2 root grub.pbkdf2.sha512.10000.biglongstring
password user1 insecure

menuentry "May be run by any user" --unrestricted {
	set root=(hd0,1)
	linux /vmlinuz

menuentry "Superusers only" --users "" {
	set root=(hd0,1)
	linux /vmlinuz single

menuentry "May be run by user1 or a superuser" --users user1 {
	set root=(hd0,2)
	chainloader +1

The grub-mkconfig program does not yet have built-in support for generating configuration files with authentication. You can use /etc/grub.d/40_custom to add simple superuser authentication, by adding set superusers= and password or password_pbkdf2 commands.

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23 Platform limitations

GRUB2 is designed to be portable and is actually ported across platforms. We try to keep all platforms at the level. Unfortunately some platforms are better supported than others. This is detailed in current and 2 following sections.

ARC platform is unable to change datetime (firmware doesn’t seem to provide a function for it). EMU has similar limitation.

ARC platform no serial port is available. EMU has similar limitation.

Console charset refers only to firmware-assisted console. gfxterm is always Unicode (see Internationalisation section for its limitations). Serial is configurable to UTF-8 or ASCII (see Internationalisation). In case of qemu and coreboot ports the refered console is vga_text. Loongson always uses gfxterm.

Most limited one is ASCII. CP437 provides additionally pseudographics. GRUB2 doesn’t use any language characters from CP437 as often CP437 is replaced by national encoding compatible only in pseudographics. Unicode is the most versatile charset which supports many languages. However the actual console may be much more limited depending on firmware

On BIOS network is supported only if the image is loaded through network. On sparc64 GRUB is unable to determine which server it was booted from.

On platforms not having direct serial support (as indicated in the line serial) you can still redirect firmware console to serial if it allows so.

Direct ATA/AHCI support allows to circumvent various firmware limitations but isn’t needed for normal operation except on baremetal ports.

AT keyboard support allows keyboard layout remapping and support for keys not available through firmware. It isn’t needed for normal operation except baremetal ports.

USB support provides benefits similar to ATA (for USB disks) or AT (for USB keyboards). In addition it allows USBserial.

Chainloading refers to the ability to load another bootloader through the same protocol

Hints allow faster disk discovery by already knowing in advance which is the disk in question. On some platforms hints are correct unless you move the disk between boots. On other platforms it’s just an educated guess. Note that hint failure results in just reduced performance, not a failure

BadRAM is the ability to mark some of the RAM as “bad”. Note: due to protocol limitations mips-loongson (with Linux protocol) and mips-qemu_mips can use only memory up to first hole.

console charsetCP437CP437CP437CP437
networkyes (*)nonono
AT keyboardyesyesyesyes
ia32 EFIamd64 EFIia32 IEEE1275Itanium
console charsetUnicodeUnicodeASCIIUnicode
AT keyboardyesyesyesno
networknoyes (*)yesno
AT keyboardyesnonono
badramyes (*)nonono
MIPS qemuemu
console charsetCP437ASCII
AT keyboardyesno
badramyes (*)no

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24 Outline

Some platforms have features which allows to implement some commands useless or not implementable on others.

Quick summary:

Information retrieval:

Workarounds for platform-specific issues:

Advanced operations for power users:


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25 Supported boot targets

X86 support is summarised in the following table. “Yes” means that the kernel works on the given platform, “crashes” means an early kernel crash which we hope will be fixed by concerned kernel developers. “no” means GRUB doesn’t load the given kernel on a given platform. “headless” means that the kernel works but lacks console drivers (you can still use serial or network console). In case of “no” and “crashes” the reason is given in footnote.

BIOS chainloadingyesno (1)
NTLDRyesno (1)
Plan9yesno (1)
Freedosyesno (1)
FreeBSD bootloaderyescrashes (1)
32-bit kFreeBSDyescrashes (2,6)
64-bit kFreeBSDyescrashes (2,6)
32-bit kNetBSDyescrashes (1)
64-bit kNetBSDyescrashes (2)
32-bit kOpenBSDyesyes
64-bit kOpenBSDyesyes
32-bit Linux (legacy protocol)yesno (1)
64-bit Linux (legacy protocol)yesno (1)
32-bit Linux (modern protocol)yesyes
64-bit Linux (modern protocol)yesyes
32-bit XNUyes?
64-bit XNUyes?
32-bit EFI chainloaderno (3)no (3)
64-bit EFI chainloaderno (3)no (3)
Appleloaderno (3)no (3)
BIOS chainloadingno (1)no (1)
NTLDRno (1)no (1)
Plan9no (1)no (1)
FreeDOSno (1)no (1)
FreeBSD bootloadercrashes (1)crashes (1)
32-bit kFreeBSDcrashes (6)crashes (6)
64-bit kFreeBSDcrashes (6)crashes (6)
32-bit kNetBSDcrashes (1)crashes (1)
64-bit kNetBSDyesyes
32-bit kOpenBSDyesyes
64-bit kOpenBSDyesyes
32-bit Linux (legacy protocol)no (1)no (1)
64-bit Linux (legacy protocol)no (1)no (1)
32-bit Linux (modern protocol)yesyes
64-bit Linux (modern protocol)yesyes
32-bit XNU??
64-bit XNU??
32-bit EFI chainloaderno (3)no (3)
64-bit EFI chainloaderno (3)no (3)
Appleloaderno (3)no (3)
ia32 EFIamd64 EFI
BIOS chainloadingno (1)no (1)
NTLDRno (1)no (1)
Plan9no (1)no (1)
FreeDOSno (1)no (1)
FreeBSD bootloadercrashes (1)crashes (1)
32-bit kFreeBSDheadlessheadless
64-bit kFreeBSDheadlessheadless
32-bit kNetBSDcrashes (1)crashes (1)
64-bit kNetBSDyesyes
32-bit kOpenBSDheadlessheadless
64-bit kOpenBSDheadlessheadless
32-bit Linux (legacy protocol)no (1)no (1)
64-bit Linux (legacy protocol)no (1)no (1)
32-bit Linux (modern protocol)yesyes
64-bit Linux (modern protocol)yesyes
32-bit XNUyesyes
64-bit XNUyes (5)yes
32-bit EFI chainloaderyesno (4)
64-bit EFI chainloaderno (4)yes
ia32 IEEE1275
BIOS chainloadingno (1)
NTLDRno (1)
Plan9no (1)
FreeDOSno (1)
FreeBSD bootloadercrashes (1)
32-bit kFreeBSDcrashes (6)
64-bit kFreeBSDcrashes (6)
32-bit kNetBSDcrashes (1)
64-bit kNetBSD?
32-bit kOpenBSD?
64-bit kOpenBSD?
32-bit Linux (legacy protocol)no (1)
64-bit Linux (legacy protocol)no (1)
32-bit Linux (modern protocol)?
64-bit Linux (modern protocol)?
32-bit XNU?
64-bit XNU?
32-bit EFI chainloaderno (3)
64-bit EFI chainloaderno (3)
Appleloaderno (3)
  1. Requires BIOS
  2. Crashes because the memory at 0x0-0x1000 isn’t available
  3. EFI only
  4. 32-bit and 64-bit EFI have different structures and work in different CPU modes so it’s not possible to chainload 32-bit bootloader on 64-bit platform and vice-versa
  5. Some modules may need to be disabled
  6. Requires ACPI

PowerPC, IA64 and Sparc64 ports support only Linux. MIPS port supports Linux and multiboot2.

26 Boot tests

As you have seen in previous chapter the support matrix is pretty big and some of the configurations are only rarely used. To ensure the quality bootchecks are available for all x86 targets except EFI chainloader, Appleloader and XNU. All x86 platforms have bootcheck facility except ieee1275. Multiboot, multiboot2, BIOS chainloader, ntldr and freebsd-bootloader boot targets are tested only with a fake kernel images. Only Linux is tested among the payloads using Linux protocols.

Following variables must be defined:

GRUB_PAYLOADS_DIRdirectory containing the required kernels
GRUB_CBFSTOOLcbfstoll from Coreboot package (for coreboot platform only)
GRUB_QEMU_OPTSadditional options to be supplied to QEMU

Required files are:

kfreebsd_env.i38632-bit kFreeBSD device hints
kfreebsd.i38632-bit FreeBSD kernel image
kfreebsd.x86_64, kfreebsd_env.x86_64same from 64-bit kFreeBSD
knetbsd.i38632-bit NetBSD kernel image
knetbsd.miniroot.i38632-bit kNetBSD miniroot.kmod.
knetbsd.x86_64, knetbsd.miniroot.x86_64same from 64-bit kNetBSD
kopenbsd.i38632-bit OpenBSD kernel bsd.rd image
kopenbsd.x86_64same from 64-bit kOpenBSD
linux.i38632-bit Linux
linux.x86_6464-bit Linux

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27 Error messages produced by GRUB

Up: Troubleshooting   [Contents][Index]

27.1 GRUB only offers a rescue shell

GRUB’s normal start-up procedure involves setting the ‘prefix’ environment variable to a value set in the core image by grub-install, setting the ‘root’ variable to match, loading the ‘normal’ module from the prefix, and running the ‘normal’ command (see normal). This command is responsible for reading /boot/grub/grub.cfg, running the menu, and doing all the useful things GRUB is supposed to do.

If, instead, you only get a rescue shell, this usually means that GRUB failed to load the ‘normal’ module for some reason. It may be possible to work around this temporarily: for instance, if the reason for the failure is that ‘prefix’ is wrong (perhaps it refers to the wrong device, or perhaps the path to /boot/grub was not correctly made relative to the device), then you can correct this and enter normal mode manually:

# Inspect the current prefix (and other preset variables):
# Find out which devices are available:
# Set to the correct value, which might be something like this:
set prefix=(hd0,1)/grub
set root=(hd0,1)
insmod normal

However, any problem that leaves you in the rescue shell probably means that GRUB was not correctly installed. It may be more useful to try to reinstall it properly using grub-install device (see Invoking grub-install). When doing this, there are a few things to remember:

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28 Invoking grub-install

The program grub-install installs GRUB on your drive using grub-mkimage and (on some platforms) grub-setup. You must specify the device name on which you want to install GRUB, like this:

grub-install install_device

The device name install_device is an OS device name or a GRUB device name.

grub-install accepts the following options:


Print a summary of the command-line options and exit.


Print the version number of GRUB and exit.


Install GRUB images under the directory dir/grub/ This option is useful when you want to install GRUB into a separate partition or a removable disk. If this option is not specified then it defaults to /boot, so

grub-install /dev/sda

is equivalent to

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

Here is an example in which you have a separate boot partition which is mounted on /mnt/boot:

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

Recheck the device map, even if /boot/grub/ already exists. You should use this option whenever you add/remove a disk into/from your computer.

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29 Invoking grub-mkconfig

The program grub-mkconfig generates a configuration file for GRUB (see Simple configuration).

grub-mkconfig -o /boot/grub/grub.cfg

grub-mkconfig accepts the following options:


Print a summary of the command-line options and exit.


Print the version number of GRUB and exit.

-o file

Send the generated configuration file to file. The default is to send it to standard output.

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30 Invoking grub-mkpasswd-pbkdf2

The program grub-mkpasswd-pbkdf2 generates password hashes for GRUB (see Security).


grub-mkpasswd-pbkdf2 accepts the following options:

-c number

Number of iterations of the underlying pseudo-random function. Defaults to 10000.

-l number

Length of the generated hash. Defaults to 64.

-s number

Length of the salt. Defaults to 64.

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31 Invoking grub-mkrescue

The program grub-mkrescue generates a bootable GRUB rescue image (see Making a GRUB bootable CD-ROM).

grub-mkrescue -o grub.iso

All arguments not explicitly listed as grub-mkrescue options are passed on directly to xorriso in mkisofs emulation mode. Options passed to xorriso will normally be interpreted as mkisofs options; if the option ‘--’ is used, then anything after that will be interpreted as native xorriso options.

Non-option arguments specify additional source directories. This is commonly used to add extra files to the image:

mkdir -p disk/boot/grub
(add extra files to disk/boot/grub)
grub-mkrescue -o grub.iso disk

grub-mkrescue accepts the following options:


Print a summary of the command-line options and exit.


Print the version number of GRUB and exit.

-o file

Save output in file. This "option" is required.


Pre-load the named GRUB modules in the image. Multiple entries in modules should be separated by whitespace (so you will probably need to quote this for your shell).


If generating images for the QEMU or Coreboot platforms, copy the resulting qemu.img or coreboot.elf files respectively to the dir directory as well as including them in the image.


Use file as the xorriso program, rather than the built-in default.


Use file as the grub-mkimage program, rather than the built-in default.

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32 Invoking grub-probe

The program grub-probe probes device information for a given path or device.

grub-probe --target=fs /boot/grub
grub-probe --target=drive --device /dev/sda1

grub-probe must be given a path or device as a non-option argument, and also accepts the following options:


Print a summary of the command-line options and exit.


Print the version number of GRUB and exit.


If this option is given, then the non-option argument is a system device name (such as ‘/dev/sda1’), and grub-probe will print information about that device. If it is not given, then the non-option argument is a filesystem path (such as ‘/boot/grub’), and grub-probe will print information about the device containing that part of the filesystem.

-m file

Use file as the device map (see Device map) rather than the default, usually ‘/boot/grub/’.

-t target

Print information about the given path or device as defined by target. The available targets and their meanings are:


GRUB filesystem module.


Filesystem Universally Unique Identifier (UUID).


Filesystem label.


GRUB device name.


System device name.


GRUB partition map module.


GRUB abstraction module (e.g. ‘lvm’).


Crypto device UUID.


MBR partition type code (two hexadecimal digits).


A string of platform search hints suitable for passing to the search command (see search).


Search hints for the PC BIOS platform.


Search hints for the IEEE1275 platform.


Search hints for platforms where disks are addressed directly rather than via firmware.


Search hints for the EFI platform.


Search hints for the ARC platform.


A guess at a reasonable GRUB drive name for this device, which may be used as a fallback if the search command fails.


System device name for the whole disk.


Print verbose messages.

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Appendix A How to obtain and build GRUB

Caution: GRUB requires binutils- or later because the GNU assembler has been changed so that it can produce real 16bits machine code between 2.9.1 and See, to obtain information on how to get the latest version.

GRUB is available from the GNU alpha archive site or any of its mirrors. The file will be named grub-version.tar.gz. The current version is 2.00, so the file you should grab is:

To unbundle GRUB use the instruction:

zcat grub-2.00.tar.gz | tar xvf -

which will create a directory called grub-2.00 with all the sources. You can look at the file INSTALL for detailed instructions on how to build and install GRUB, but you should be able to just do:

cd grub-2.00
make install

Also, the latest version is available using Bazaar. See for more information.

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Appendix B Reporting bugs

These are the guideline for how to report bugs. Take a look at this list below before you submit bugs:

  1. Before getting unsettled, read this manual through and through. Also, see the GNU GRUB FAQ.
  2. Always mention the information on your GRUB. The version number and the configuration are quite important. If you build it yourself, write the options specified to the configure script and your operating system, including the versions of gcc and binutils.
  3. If you have trouble with the installation, inform us of how you installed GRUB. Don’t omit error messages, if any. Just ‘GRUB hangs up when it boots’ is not enough.

    The information on your hardware is also essential. These are especially important: the geometries and the partition tables of your hard disk drives and your BIOS.

  4. If GRUB cannot boot your operating system, write down everything you see on the screen. Don’t paraphrase them, like ‘The foo OS crashes with GRUB, even though it can boot with the bar boot loader just fine’. Mention the commands you executed, the messages printed by them, and information on your operating system including the version number.
  5. Explain what you wanted to do. It is very useful to know your purpose and your wish, and how GRUB didn’t satisfy you.
  6. If you can investigate the problem yourself, please do. That will give you and us much more information on the problem. Attaching a patch is even better.

    When you attach a patch, make the patch in unified diff format, and write ChangeLog entries. But, even when you make a patch, don’t forget to explain the problem, so that we can understand what your patch is for.

  7. Write down anything that you think might be related. Please understand that we often need to reproduce the same problem you encountered in our environment. So your information should be sufficient for us to do the same thing—Don’t forget that we cannot see your computer directly. If you are not sure whether to state a fact or leave it out, state it! Reporting too many things is much better than omitting something important.

If you follow the guideline above, submit a report to the Bug Tracking System. Alternatively, you can submit a report via electronic mail to, but we strongly recommend that you use the Bug Tracking System, because e-mail can be passed over easily.

Once we get your report, we will try to fix the bugs.

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Appendix C Where GRUB will go

GRUB 2 is now quite stable and used in many production systems. We are currently working towards a 2.0 release.

If you are interested in the development of GRUB 2, take a look at the homepage.

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Appendix D Copying This Manual

Up: Copying This Manual   [Contents][Index]

D.1 GNU Free Documentation License

Version 1.2, November 2002
Copyright © 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

    The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

    We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.


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    You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

    The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

    In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”


    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.


    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.


    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

    If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.


    You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.


    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

D.1.1 ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.2
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts.  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

    with the Invariant Sections being list their titles, with
    the Front-Cover Texts being list, and with the Back-Cover Texts
    being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

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Index Entry  Section

acpi: acpi

badram: badram
blocklist: blocklist
boot: boot

cat: cat
chainloader: chainloader
cmp: cmp
configfile: configfile
cpuid: cpuid
crc: crc

date: date
drivemap: drivemap

echo: echo
export: export

false: false
FDL, GNU Free Documentation License: GNU Free Documentation License

gettext: gettext
gptsync: gptsync

halt: halt
help: help

initrd: initrd
initrd16: initrd16
insmod: insmod

keystatus: keystatus

linux: linux
linux16: linux16
list_env: list_env
load_env: load_env
loopback: loopback
ls: ls

menuentry: menuentry

normal: normal
normal_exit: normal_exit

parttool: parttool
password: password
password_pbkdf2: password_pbkdf2
play: play
pxe_unload: pxe_unload

read: read
reboot: reboot

save_env: save_env
search: search
sendkey: sendkey
serial: serial
set: set
submenu: submenu

terminal_input: terminal_input
terminal_output: terminal_output
terminfo: terminfo
true: true

unset: unset

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chain-load is the mechanism for loading unsupported operating systems by loading another boot loader. It is typically used for loading DOS or Windows.


Only CRC32 data integrity check is supported (xz default is CRC64 so one should use –check=crc32 option). LZMA BCJ filters are supported.


There are a few pathological cases where loading a very badly organized ELF kernel might take longer, but in practice this never happen.


The LInux LOader, a boot loader that everybody uses, but nobody likes.


El Torito is a specification for bootable CD using BIOS functions.


Currently a backslash-newline pair within a variable name is not handled properly, so use this feature with some care.


However, this behavior will be changed in the future version, in a user-invisible way.