GNU GRUB Manual 2.03

Table of Contents

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 2.03.

This manual is for GNU GRUB (version 2.03, 10 April 2024).

Copyright © 1999,2000,2001,2002,2004,2006,2008,2009,2010,2011,2012,2013 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.

1 Introduction to GRUB

1.1 Overview

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

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

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

When booting with GRUB, you can use either a command-line interface (see 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.

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.

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.

• The configuration file has a new name (grub.cfg rather than menu.lst or grub.conf), new syntax (see Configuration) and many new commands (see Commands). Configuration cannot be copied over directly, although most GRUB Legacy users should not find the syntax too surprising.
• grub.cfg is typically automatically generated by grub2-mkconfig (see Simple configuration). This makes it easier to handle versioned kernel upgrades.
• Partition numbers in GRUB device names now start at 1, not 0 (see Naming convention).
• The configuration file is now written in something closer to a full scripting language: variables, conditionals, and loops are available.
• A small amount of persistent storage is available across reboots, using the save_env and load_env commands in GRUB and the grub2-editenv utility. This is not available in all configurations (see Environment block).
• GRUB 2 has more reliable ways to find its own files and those of target kernels on multiple-disk systems, and has commands (see search) to find devices using file system labels or Universally Unique Identifiers (UUIDs).
• GRUB 2 is available for several other types of system in addition to the PC BIOS systems supported by GRUB Legacy: PC EFI, PC coreboot, PowerPC, SPARC, and MIPS Lemote Yeeloong are all supported.
• Many more file systems are supported, including but not limited to ext4, HFS+, and NTFS.
• GRUB 2 can read files directly from LVM and RAID devices.
• A graphical terminal and a graphical menu system are available.
• GRUB 2’s interface can be translated, including menu entry names.
• The image files (see Images) that make up GRUB have been reorganised; Stage 1, Stage 1.5, and Stage 2 are no more.
• GRUB 2 puts many facilities in dynamically loaded modules, allowing the core image to be smaller, and allowing the core image to be built in more flexible ways.

1.4 GRUB features

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

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

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

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

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

Recognize multiple executable formats

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

Support non-Multiboot kernels

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

Load multiples modules

Fully support the Multiboot feature of loading multiple modules.

Load a configuration file

Support a human-readable text configuration file with preset boot commands. You can also load another configuration file dynamically and embed a preset configuration file in a GRUB image file. The list of commands (see 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 Editing in Bash Features), with TAB-completion of commands, devices, partitions, and files in a directory depending on context.

Support multiple filesystem types

Support multiple filesystem types transparently, plus a useful explicit blocklist notation. The currently supported filesystem types are Amiga Fast FileSystem (AFFS), AtheOS fs, BeFS, BtrFS (including raid0, raid1, raid10, gzip and lzo), cpio (little- and big-endian bin, odc and newc variants), Linux ext2/ext3/ext4, DOS FAT12/FAT16/FAT32, exFAT, F2FS, HFS, HFS+, ISO9660 (including Joliet, Rock-ridge and multi-chunk files), JFS, Minix fs (versions 1, 2 and 3), nilfs2, NTFS (including compression), ReiserFS, ROMFS, Amiga Smart FileSystem (SFS), Squash4, tar, UDF, BSD UFS/UFS2, XFS, and ZFS (including lzjb, gzip, zle, mirror, stripe, raidz1/2/3 and encryption in AES-CCM and AES-GCM). See Filesystem, for more information.

Support automatic decompression

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

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

Access data on any installed device

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

Be independent of drive geometry translations

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

Detect all installed RAM

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

Support Logical Block Address mode

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

Support network booting

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

Support remote terminals

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

1.5 The role of a boot loader

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

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

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

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

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

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

2 Naming convention

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

Look at the following examples and explanations:

(fd0)

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

(hd0,msdos2)

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

(hd0,msdos5)

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

(hd1,msdos1,bsd1)

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

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

set root=(

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

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

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

(hd0,msdos1)/vmlinuz

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

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

3 OS-specific notes about grub tools

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

# grub2-install /dev/sda

On AROS we use another syntax. For volumes:

//:<volume name>

E.g.

//:DH0

For disks we use syntax:

//:<driver name>/unit/flags

E.g.

# grub2-install //:ata.device/0/0

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

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

E.g.

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

For disks it’s

\\?\PhysicalDrive<number>

E.g.

# grub2-install \\?\PhysicalDrive0

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

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

# grub2-install /dev/sda

4 Installation

In order to install GRUB as your boot loader, you need to first install the GRUB system and utilities under your UNIX-like operating system (see 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 grub2-install (see Invoking grub2-install) on a UNIX-like OS.

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

4.1 Installing GRUB using grub2-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 grub2-install (see Invoking grub2-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:

# grub2-install /dev/sda

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

# grub2-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
# grub2-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
# grub2-install --boot-directory=/mnt/usb/bugbios --force --allow-floppy /dev/loop0

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

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

# grub2-install

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

# grub2-install --efi-directory=/mnt/efi

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

# grub2-install --efi-directory=/mnt/usb --boot-directory=/mnt/usb/boot --removable

4.2 Making a GRUB bootable CD-ROM

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

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

To make a simple generic GRUB rescue CD, you can use the grub2-mkrescue program (see Invoking grub2-mkrescue):

$ grub2-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:

$ grub2-mkrescue -o grub.iso iso

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

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

4.3 The map between BIOS drives and OS devices

If the device map file exists, the GRUB utilities (grub2-probe, etc.) read it to map BIOS drives to OS devices. This file consists of lines like this:

(device) file

device is a drive specified in the GRUB syntax (see 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 ‘#’.

4.4 BIOS installation

MBR

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

GPT

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

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

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

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

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

5 Booting

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

5.1 How to boot operating systems

GRUB has 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.

5.1.1 How to boot an OS directly with GRUB

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

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.

5.1.2 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.

5.2 Loopback booting

GRUB is able to read from an image (be it one of CD or HDD) stored on any of its accessible storages (refer to see loopback command). However the OS itself should be able to find its root. This usually involves running a userspace program running before the real root is discovered. This is achieved by GRUB loading a specially made small image and passing it as ramdisk to the kernel. This is achieved by commands kfreebsd_module, knetbsd_module_elf, kopenbsd_ramdisk, initrd (see initrd), initrd16 (see 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

5.3 Some caveats on OS-specific issues

Here, we describe some caveats on several operating systems.

5.3.1 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)' '$(task-resume)'
   grub> module /lib/ld.so.1 exec /hurd/exec '$(exec-task=task-create)'
   

3. Finally, run the command boot (see boot).

5.3.2 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.

5.3.3 NetBSD

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

1. Set GRUB’s root device to the partition holding the NetBSD root file system. For a disk with a NetBSD disk label, this is usually the first partition (a:). In that case, and assuming that the partition is on the first hard disk, set GRUB’s root device as follows:

   grub> insmod part_bsd
   grub> set root=(hd0,netbsd1)
   

   For a disk with a GUID Partition Table (GPT), and assuming that the NetBSD root partition is the third GPT partition, do this:

   grub> insmod part_gpt
   grub> set root=(hd0,gpt3)
   

2. Load the kernel using the command knetbsd:

   grub> knetbsd /netbsd
   

   Various options may be given to knetbsd. These options are, for the most part, the same as in the NetBSD boot loader. For instance, to boot the system in single-user mode and with verbose messages, do this:

   grub> knetbsd /netbsd -s -v
   

3. If needed, load kernel modules with the command knetbsd_module_elf. A typical example is the module for the root file system:

   grub> knetbsd_module_elf /stand/amd64/6.0/modules/ffs/ffs.kmod
   

4. Finally, run the command boot (see boot).

5.3.4 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+
boot

6 Writing your own configuration file

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

6.1 Simple configuration handling

The program grub2-mkconfig (see Invoking grub2-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.

grub2-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/grub2/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 grub2-mkconfig.

The file /etc/default/grub controls the operation of grub2-mkconfig. It is sourced by a shell script, and so must be valid POSIX shell input; normally, it will just be a sequence of ‘KEY=value’ lines, but if the value contains spaces or other special characters then it must be quoted. For example:

GRUB_TERMINAL_INPUT="console serial"

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

‘GRUB_DEFAULT’

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

For example, if you have:

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

then you can make this the default using:

GRUB_DEFAULT=example-gnu-linux

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

If you set this to ‘saved’, then the default menu entry will be that saved by ‘GRUB_SAVEDEFAULT’ or grub2-set-default. This relies on the environment block, which may not be available in all situations (see Environment block).

The default is ‘0’.

‘GRUB_SAVEDEFAULT’

If this option is set to ‘true’, then, when an entry is selected, save it as a new default entry for use by future runs of GRUB. This is only useful if ‘GRUB_DEFAULT=saved’; it is a separate option because ‘GRUB_DEFAULT=saved’ is useful without this option, in conjunction with grub2-set-default. Unset by default. This option relies on the environment block, which may not be available in all situations (see Environment block).

‘GRUB_TIMEOUT’

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

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

‘GRUB_TIMEOUT_STYLE’

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

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

‘GRUB_DEFAULT_BUTTON’

‘GRUB_TIMEOUT_BUTTON’

‘GRUB_TIMEOUT_STYLE_BUTTON’

‘GRUB_BUTTON_CMOS_ADDRESS’

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

‘GRUB_DISTRIBUTOR’

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

‘GRUB_TERMINAL_INPUT’

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

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

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

‘GRUB_TERMINAL_OUTPUT’

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

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

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

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

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

‘GRUB_TERMINAL’

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

‘GRUB_SERIAL_COMMAND’

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

‘GRUB_CMDLINE_LINUX’

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

‘GRUB_CMDLINE_LINUX_DEFAULT’

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

‘GRUB_CMDLINE_NETBSD’

‘GRUB_CMDLINE_NETBSD_DEFAULT’

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

‘GRUB_CMDLINE_GNUMACH’

As ‘GRUB_CMDLINE_LINUX’, but for GNU Mach.

‘GRUB_CMDLINE_XEN’

‘GRUB_CMDLINE_XEN_DEFAULT’

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

‘GRUB_CMDLINE_LINUX_XEN_REPLACE’

‘GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT’

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

‘GRUB_EARLY_INITRD_LINUX_CUSTOM’

‘GRUB_EARLY_INITRD_LINUX_STOCK’

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

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

‘GRUB_EARLY_INITRD_LINUX_CUSTOM’ is for your custom created images.

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

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

‘GRUB_DISABLE_LINUX_UUID’

Normally, grub2-mkconfig will generate menu entries that use universally-unique identifiers (UUIDs) to identify the root filesystem to the Linux kernel, using a ‘root=UUID=...’ kernel parameter. This is usually more reliable, but in some cases it may not be appropriate. To disable the use of UUIDs, set this option to ‘true’.

‘GRUB_DISABLE_LINUX_PARTUUID’

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

‘GRUB_DISABLE_RECOVERY’

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

‘GRUB_DISABLE_UUID’

Normally, grub2-mkconfig will generate menu entries that use universally-unique identifiers (UUIDs) to identify various filesystems to search for files. This is usually more reliable, but in some cases it may not be appropriate. To disable this use of UUIDs, set this option to ‘true’.

‘GRUB_VIDEO_BACKEND’

If graphical video support is required, either because the ‘gfxterm’ graphical terminal is in use or because ‘GRUB_GFXPAYLOAD_LINUX’ is set, then grub2-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 grub2-install has been run, the available video drivers are listed in /boot/grub2/video.lst.

‘GRUB_GFXMODE’

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

‘GRUB_BACKGROUND’

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

‘GRUB_THEME’

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

‘GRUB_GFXPAYLOAD_LINUX’

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

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

‘GRUB_DISABLE_OS_PROBER’

Normally, grub2-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.

‘GRUB_OS_PROBER_SKIP_LIST’

List of space-separated FS UUIDs of filesystems to be ignored from os-prober output. For efi chainloaders it’s <UUID>@<EFI FILE>

‘GRUB_DISABLE_SUBMENU’

Normally, grub2-mkconfig will generate top level menu entry for the kernel with highest version number and put all other found kernels or alternative menu entries for recovery mode in submenu. For entries returned by os-prober first entry will be put on top level and all others in submenu. If this option is set to ‘y’, flat menu with all entries on top level will be generated instead. Changing this option will require changing existing values of ‘GRUB_DEFAULT’, ‘fallback’ (see fallback) and ‘default’ (see default) environment variables as well as saved default entry using grub2-set-default and value used with grub2-reboot.

‘GRUB_ENABLE_CRYPTODISK’

If set to ‘y’, grub2-mkconfig and grub2-install will check for encrypted disks and generate additional commands needed to access them during boot. Note that in this case unattended boot is not possible because GRUB will wait for passphrase to unlock encrypted container.

‘GRUB_INIT_TUNE’

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

‘GRUB_BADRAM’

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

‘GRUB_PRELOAD_MODULES’

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

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

‘GRUB_HIDDEN_TIMEOUT’

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

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

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

‘GRUB_HIDDEN_TIMEOUT_QUIET’

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

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

‘GRUB_HIDDEN_TIMEOUT_BUTTON’

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

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

For more detailed customisation of grub2-mkconfig’s output, you may edit the scripts in /etc/grub.d directly. /etc/grub.d/40_custom is particularly useful for adding entire custom menu entries; simply type the menu entries you want to add at the end of that file, making sure to leave at least the first two lines intact.

6.2 Root Identifcation Heuristics

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

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

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

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

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

6.3 Writing full configuration files directly

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

Words

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

{ } | & $ ; < >

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

Reserved words

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

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

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

Quoting

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

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

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

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

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

Variable expansion

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

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

Comments

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

Simple commands

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

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

Compound commands

A compound command is one of the following:

for name in word …; do list; done

The list of words following in is expanded, generating a list of items. The variable name is set to each element of this list in turn, and list is executed each time. The return value is the exit status of the last command that executes. If the expansion of the items following in results in an empty list, no commands are executed, and the return status is 0.

if list; then list; [elif list; then list;] … [else list;] fi

The if list is executed. 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] [--id=id] { command; … }

See menuentry.

Built-in Commands

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

break [n]

Exit from within a for, while, or until loop. If n is specified, break n levels. n must be greater than or equal to 1. If n is greater than the number of enclosing loops, all enclosing loops are exited. The return value is 0 unless n is not greater than or equal to 1.

continue [n]

Resume the next iteration of the enclosing for, while or until loop. If n is specified, resume at the nth enclosing loop. n must be greater than or equal to 1. If n is greater than the number of enclosing loops, the last enclosing loop (the top-level loop) is resumed. The return value is 0 unless n is not greater than or equal to 1.

return [n]

Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed in the function body. If used outside a function the return status is false.

setparams [arg] …

Replace positional parameters starting with $1 with arguments to setparams.

shift [n]

The positional parameters from n+1 … are renamed to $1…. Parameters represented by the numbers $# down to $#-n+1 are unset. n must be a non-negative number less than or equal to $#. If n is 0, no parameters are changed. If n is not given, it is assumed to be 1. If n is greater than $#, the positional parameters are not changed. The return status is greater than zero if n is greater than $# or less than zero; otherwise 0.

6.4 Multi-boot manual config

Currently autogenerating config files for multi-boot environments depends on os-prober and has several shortcomings. 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:

grub2-install --boot-directory=/mnt/boot /dev/sda

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

GRUB_DISABLE_OS_PROBER=true

in /etc/default/grub

Then write a grub.cfg (/mnt/boot/grub2/grub.cfg):

menuentry "OS using grub2" {
   insmod xfs
   search --set=root --label OS1 --hint hd0,msdos8
   configfile /boot/grub2/grub.cfg
}

menuentry "OS using grub2-legacy" {
   insmod ext2
   search --set=root --label OS2 --hint hd0,msdos6
   legacy_configfile /boot/grub2/menu.lst
}

menuentry "Windows XP" {
   insmod ntfs
   search --set=root --label WINDOWS_XP --hint hd0,msdos1
   ntldr /ntldr
}

menuentry "Windows 7" {
   insmod ntfs
   search --set=root --label WINDOWS_7 --hint hd0,msdos2
   ntldr /bootmgr
}

menuentry "FreeBSD" {
          insmod zfs
          search --set=root --label freepool --hint hd0,msdos7
          kfreebsd /freebsd@/boot/kernel/kernel
          kfreebsd_module_elf /freebsd@/boot/kernel/opensolaris.ko
          kfreebsd_module_elf /freebsd@/boot/kernel/zfs.ko
          kfreebsd_module /freebsd@/boot/zfs/zpool.cache type=/boot/zfs/zpool.cache
          set kFreeBSD.vfs.root.mountfrom=zfs:freepool/freebsd
          set kFreeBSD.hw.psm.synaptics_support=1
}

menuentry "experimental GRUB" {
          search --set=root --label GRUB --hint hd0,msdos5
          multiboot /experimental/grub/i386-pc/core.img
}

menuentry "Fedora 16 installer" {
          search --set=root --label GRUB --hint hd0,msdos5
          linux /fedora/vmlinuz lang=en_US keymap=sg resolution=1280x800
          initrd /fedora/initrd.img
}

menuentry "Fedora rawhide installer" {
          search --set=root --label GRUB --hint hd0,msdos5
          linux /fedora/vmlinuz repo=ftp://mirror.switch.ch/mirror/fedora/linux/development/rawhide/x86_64 lang=en_US keymap=sg resolution=1280x800
          initrd /fedora/initrd.img
}

menuentry "Debian sid installer" {
          search --set=root --label GRUB --hint hd0,msdos5
          linux /debian/dists/sid/main/installer-amd64/current/images/hd-media/vmlinuz
          initrd /debian/dists/sid/main/installer-amd64/current/images/hd-media/initrd.gz
}

Notes:

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

6.5 Embedding a configuration file into GRUB

GRUB supports embedding a configuration file directly into the core image, so that it is loaded before entering normal mode. This is useful, for example, when it is not straightforward to find the real configuration file, or when you need to debug problems with loading that file. grub2-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 grub2-mkimage. The file is copied into the core image, so it may reside anywhere on the file system, and may be removed after running grub2-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/grub2/example/test1.cfg ]; then
    set prefix=($root)/boot/grub
    configfile /boot/grub2/example/test1.cfg
else
    if [ -e /boot/grub2/example/test2.cfg ]; then
        set prefix=($root)/boot/grub
        configfile /boot/grub2/example/test2.cfg
    else
        echo "Could not find an example configuration file!"
    fi
fi

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

7 Theme file format

7.1 Introduction

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

7.2 Theme Elements

7.2.1 Colors

Colors can be specified in several ways:

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

7.2.2 Fonts

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

7.2.3 Progress Bar

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

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

7.2.4 Circular Progress Indicator

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

7.2.5 Labels

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

7.2.6 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.

7.2.7 Styled Boxes

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

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.

7.2.8 Creating Styled Box Images

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

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.

7.3 Theme File Manual

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

The theme file contains two types of statements:

1. Global properties.
2. Component construction.

7.3.1 Global Properties

7.3.2 Format

Global properties are specified with the simple format:

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

In this example, name3 is assigned a color value.

7.3.3 Global Property List

7.3.4 Component Construction

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

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

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

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

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

7.3.5 Component List

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

• label A label displays a line of text.

  Properties:

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

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

  Properties:

  file	The full path to the image file to load.

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

  Properties:

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

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

  Properties:

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

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

  Properties:

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

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

7.3.6 Common properties

The following properties are supported by all components:

‘left’

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

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

‘top’

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

‘width’

The width of object in same format.

‘height’

The height of object in same format.

‘id’

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

“__timeout__“

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

8 Booting GRUB from the network

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

To generate a netbootable directory, run:

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

E.g. for i386-pc:

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

Then follow instructions printed out by grub2-mknetdir on configuring your DHCP server.

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

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

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

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

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

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

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

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

‘net_<interface>_ip’

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

‘net_<interface>_mac’

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

‘net_<interface>_hostname’

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

‘net_<interface>_domain’

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

‘net_<interface>_rootpath’

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

‘net_<interface>_extensionspath’

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

‘net_<interface>_boot_file’

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

‘net_<interface>_dhcp_server_name’

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

‘net_<interface>_next_server’

The IP address of the next (usually, TFTP) server provided by DHCP. Read-only.

‘net_default_interface’

Initially set to name of network interface that was used to load grub. Read-write, although setting it affects only interpretation of ‘net_default_ip’ and ‘net_default_mac’

‘net_default_ip’

The IP address of default interface. Read-only. This is alias for the ‘net_${net_default_interface}_ip’.

‘net_default_mac’

The default interface’s MAC address. Read-only. This is alias for the ‘net_${net_default_interface}_mac’.

‘net_default_server’

The default server used by network drives (see Device syntax). Read-write, although setting this is only useful before opening a network device.

9 Using GRUB via a serial line

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

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

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

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

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

10 Using GRUB with vendor power-on keys

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

Dell XPS M1330M

121:3

Dell XPS M1530

85:3

Dell Latitude E4300

85:3

Asus EeePC 1005PE

84:1 (unconfirmed)

LENOVO ThinkPad T410s (2912W1C)

101:3

To take full advantage of this function, install GRUB into the MBR (see Installing GRUB using grub2-install).

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

• boot normally

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

• boot using vendor button

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

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

byte 0x47: 20 --> 28

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

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

11 GRUB image files

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

boot.img

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

The sole function of boot.img is to read the first sector of the core image from a local disk and jump to it. Because of the size restriction, boot.img cannot understand any file system structure, so grub2-install hardcodes the location of the first sector of the core image into boot.img when installing GRUB.

diskboot.img

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

cdboot.img

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

pxeboot.img

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

lnxboot.img

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

kernel.img

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

core.img

This is the core image of GRUB. It is built dynamically from the kernel image and an arbitrary list of modules by the grub2-mkimage program. Usually, it contains enough modules to access /boot/grub, and loads everything else (including menu handling, the ability to load target operating systems, and so on) from the file system at run-time. The modular design allows the core image to be kept small, since the areas of disk where it must be installed are often as small as 32KB.

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

*.mod

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

For GRUB Legacy users

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

stage1

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

*_stage1_5

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

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

stage2

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

stage2_eltorito

In GRUB 2, images for booting from CD-ROM drives are now constructed using cdboot.img and core.img, making sure that the core image contains the ‘iso9660’ module. It is usually best to use the grub2-mkrescue program for this.

nbgrub

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

pxegrub

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

12 Core image size limitation

Heavily limited platforms:

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

Lightly limited platforms:

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

13 Filesystem syntax and semantics

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

13.1 How to specify devices

The device syntax is like this:

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

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

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

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

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

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

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

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

13.2 How to specify files

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

An absolute file name resembles a Unix absolute file name, using ‘/’ for the directory separator (not ‘\’ as in DOS). One example is ‘(hd0,1)/boot/grub2/grub.cfg’. This means the file /boot/grub2/grub.cfg in the first partition of the first hard disk. If you omit the device name in an absolute file name, GRUB uses GRUB’s root device implicitly. So if you set the root device to, say, ‘(hd1,1)’ by the command ‘set root=(hd1,1)’ (see set), then /boot/kernel is the same as (hd1,1)/boot/kernel.

On ZFS filesystem the first path component must be volume‘@’[snapshot]. So ‘/rootvol@snap-129/boot/grub2/grub.cfg’ refers to file ‘/boot/grub2/grub.cfg’ in snapshot of volume ‘rootvol’ with name ‘snap-129’. Trailing ‘@’ after volume name is mandatory even if snapshot name is omitted.

13.3 How to specify block lists

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

0+100,200+1,300+300

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)’.

14 GRUB’s user interface

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

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

14.1 The flexible command-line interface

The command-line interface provides a prompt and after it an editable text area much like a command-line in Unix or DOS. Each command is immediately executed after it is entered⁸. The commands (see 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:

C-f

PC right key

Move forward one character.

C-b

PC left key

Move back one character.

C-a

HOME

Move to the start of the line.

C-e

END

Move the the end of the line.

C-d

DEL

Delete the character underneath the cursor.

C-h

BS

Delete the character to the left of the cursor.

C-k

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

C-u

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

C-y

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

C-p

PC up key

Move up through the history list.

C-n

PC down key

Move down through the history list.

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

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

14.2 The simple menu interface

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

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

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

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

14.3 Editing a menu entry

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

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

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

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

15 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.

15.1 Special environment variables

These variables have special meaning to GRUB.

15.1.1 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.

15.1.2 check_appended_signatures

This variable controls whether GRUB enforces appended signature validation on certain loaded files. See Using appended signatures.

15.1.3 check_signatures

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

15.1.4 chosen

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

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

15.1.5 cmdpath

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

15.1.6 color_highlight

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

The default is ‘black/light-gray’.

15.1.7 color_normal

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

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

The default is ‘light-gray/black’.

The color support support varies from terminal to terminal.

‘morse’ has no color support at all.

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

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

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

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

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

15.1.8 config_directory

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

15.1.9 config_file

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

15.1.10 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. The facility names are the first argument to grub_dprintf. Consult source for more details.

15.1.11 default

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

For example, if you have:

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

then you can make this the default using:

default=example-gnu-linux

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

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

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

This variable is often set by ‘GRUB_DEFAULT’ (see Simple configuration), grub2-set-default, or grub2-reboot.

15.1.12 fallback

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

15.1.13 gfxmode

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

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

15.1.14 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).

15.1.15 gfxterm_font

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

15.1.16 grub_cpu

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

15.1.17 grub_platform

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

15.1.18 icondir

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

15.1.19 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’.

grub2-mkconfig (see Simple configuration) will try to set a reasonable default for this variable based on the system locale.

15.1.20 locale_dir

If this variable is set, it names the directory where translation files may be found (see gettext), usually /boot/grub2/locale. Otherwise, internationalization is disabled.

grub2-mkconfig (see Simple configuration) will set a reasonable default for this variable if internationalization is needed and any translation files are available.

15.1.21 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).

15.1.22 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).

15.1.23 net_<interface>_boot_file

See Network.

15.1.24 net_<interface>_dhcp_server_name

See Network.

15.1.25 net_<interface>_domain

See Network.

15.1.26 net_<interface>_extensionspath

See Network.

15.1.27 net_<interface>_hostname

See Network.

15.1.28 net_<interface>_ip

See Network.

15.1.29 net_<interface>_mac

See Network.

15.1.30 net_<interface>_next_server

See Network.

15.1.31 net_<interface>_rootpath

See Network.

15.1.32 net_default_interface

See Network.

15.1.33 net_default_ip

See Network.

15.1.34 net_default_mac

See Network.

15.1.35 net_default_server

See Network.

15.1.36 pager

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

15.1.37 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 grub2-install. GRUB modules are dynamically loaded from this directory, so it must be set correctly in order for many parts of GRUB to work.

15.1.38 pxe_blksize

See Network.

15.1.39 pxe_default_gateway

See Network.

15.1.40 pxe_default_server

See Network.

15.1.41 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’.

15.1.42 superusers

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

15.1.43 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).

15.1.44 timeout

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

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

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

15.1.45 timeout_style

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

15.2 The GRUB environment block

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

The environment block is a preallocated 1024-byte file, which normally lives in /boot/grub2/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 grub2-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).

grub2-mkconfig uses this facility to implement ‘GRUB_SAVEDEFAULT’ (see Simple configuration).

16 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.

16.1 The list of commands for the menu only

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

• The files must be in plain-text format.
• ‘#’ at the beginning of a line in a configuration file means it is only a comment.
• Options are separated by spaces.
• All numbers can be either decimal or hexadecimal. A hexadecimal number must be preceded by ‘0x’, and is case-insensitive.

These commands can only be used in the menu:

16.1.1 menuentry

Command: menuentry title [--class=class …] [--users=users] [--unrestricted] [--hotkey=key] [--id=id] [arg …] { 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 value of --id if --id is given, 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’.

The --id may be used to associate unique identifier with a menu entry. id is string of ASCII aphanumeric characters, underscore and hyphen and should not start with a digit.

All other arguments including title are passed as positional parameters when list of commands is executed with title always assigned to $1.

16.1.2 submenu

Command: submenu title [--class=class …] [--users=users] [--unrestricted] [--hotkey=key] [--id=id] { 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).

16.2 The list of general commands

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

16.2.1 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.

16.2.2 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.

16.2.3 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.

16.2.4 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.

16.3 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).

16.3.1 [

Command: [ expression ]

Alias for test expression (see test).

16.3.2 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.

Note: The command is not allowed when lockdown is enforced (see Lockdown). Otherwise an attacker can instruct the GRUB to load an SSDT table to overwrite the kernel lockdown configuration and later load and execute unsigned code.

16.3.3 authenticate

Command: authenticate [userlist]

Check whether user is in userlist or listed in the value of variable ‘superusers’. See see superusers for valid user list format. If ‘superusers’ is empty, this command returns true. See Security.

16.3.4 background_color

Command: background_color color

Set background color for active terminal. For valid color specifications see see Colors. Background color can be changed only when using ‘gfxterm’ for terminal output.

This command sets color of empty areas without text. Text background color is controlled by environment variables color_normal, color_highlight, menu_color_normal, menu_color_highlight. See Special environment variables.

16.3.5 background_image

Command: background_image [[--mode ‘stretch’|‘normal’] file]

Load background image for active terminal from file. Image is stretched to fill up entire screen unless option --mode ‘normal’ is given. Without arguments remove currently loaded background image. Background image can be changed only when using ‘gfxterm’ for terminal output.

16.3.6 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.

The command is similar to cutmem command.

Note: The command is not allowed when lockdown is enforced (see Lockdown). This prevents removing EFI memory regions to potentially subvert the security mechanisms provided by the UEFI secure boot.

16.3.7 blocklist

Command: blocklist file

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

16.3.8 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).

16.3.9 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.

16.3.10 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.

16.3.11 clear

Command: clear

Clear the screen.

16.3.12 cmosclean

Command: cmosclean byte:bit

Clear value of bit in CMOS at location byte:bit. This command is available only on platforms that support CMOS.

16.3.13 cmosdump

Dump: CMOS contents

Dump full CMOS contents as hexadecimal values. This command is available only on platforms that support CMOS.

16.3.14 cmostest

Command: cmostest byte:bit

Test value of bit in CMOS at location byte:bit. Exit status is zero if bit is set, non zero otherwise. This command is available only on platforms that support CMOS.

16.3.15 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.

16.3.16 configfile

Command: configfile file

Load file as a configuration file. If file defines any menu entries, then show a menu containing them immediately. Any environment variable changes made by the commands in file will not be preserved after configfile returns.

16.3.17 cpuid

Command: cpuid [-l] [-p]

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).

With the -p option, return true if the CPU supports Physical Address Extension (PAE).

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

16.3.18 crc

Command: crc arg …

Alias for hashsum --hash crc32 arg …. See command hashsum (see hashsum) for full description.

16.3.19 cryptomount

Command: cryptomount device|-u uuid|-a|-b

Setup access to encrypted device. If necessary, passphrase is requested interactively. Option device configures specific grub device (see Naming convention); option -u uuid configures device with specified uuid; option -a configures all detected encrypted devices; option -b configures all geli containers that have boot flag set.

GRUB suports devices encrypted using LUKS and geli. Note that necessary modules (luks and geli) have to be loaded manually before this command can be used.

16.3.20 cutmem

Command: cutmem from[K|M|G] to[K|M|G]

Remove any memory regions in specified range.

This command notifies the memory manager that specified regions of RAM ought to be filtered out. 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.

The command is similar to badram command.

Note: The command is not allowed when lockdown is enforced (see Lockdown). This prevents removing EFI memory regions to potentially subvert the security mechanisms provided by the UEFI secure boot.

16.3.21 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.

16.3.22 devicetree

Command: devicetree file

Load a device tree blob (.dtb) from a filesystem, for later use by a Linux kernel. Does not perform merging with any device tree supplied by firmware, but rather replaces it completely.

Note: The command is not allowed when lockdown is enforced (see Lockdown). This is done to prevent subverting various security mechanisms.

16.3.23 distrust

Command: distrust pubkey_id

Remove public key pubkey_id from GRUB’s keyring of trusted keys. pubkey_id is the last four bytes (eight hexadecimal digits) of the GPG v4 key id, which is also the output of list_trusted (see list_trusted). Outside of GRUB, the key id can be obtained using gpg --fingerprint). These keys are used to validate signatures when environment variable check_signatures is set to enforce (see check_signatures), and by some invocations of verify_detached (see verify_detached). See Using GPG-style digital signatures, for more information.

16.3.24 distrust_certificate

Command: distrust_certificate cert_number

Remove the x509 certificate numbered cert_number from GRUB’s keyring of trusted x509 certificates for verifying appended signatures.

cert_number is the certificate number as listed by list_certificates (see list_certificates).

These certificates are used to validate appended signatures when environment variable check_appended_signatures is set to enforce (see check_appended_signatures), and by verify_appended (see verify_appended). See See Using appended signatures for more information.

16.3.25 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)

16.3.26 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:

\\

backslash

\a

alert (BEL)

\c

suppress trailing new line

\f

form feed

\n

new line

\r

carriage return

\t

horizontal tab

\v

vertical tab

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

16.3.27 eval

Command: eval string ...

Concatenate arguments together using single space as separator and evaluate result as sequence of GRUB commands.

16.3.28 export

Command: export envvar

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

16.3.29 false

Command: false

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

16.3.30 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/grub2/locale.

16.3.31 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.

16.3.32 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.

16.3.33 hashsum

Command: hashsum --hash hash --keep-going --uncompress --check file [--prefix dir]|file …

Compute or verify file hashes. Hash type is selected with option --hash. Supported hashes are: ‘adler32’, ‘crc64’, ‘crc32’, ‘crc32rfc1510’, ‘crc24rfc2440’, ‘md4’, ‘md5’, ‘ripemd160’, ‘sha1’, ‘sha224’, ‘sha256’, ‘sha512’, ‘sha384’, ‘tiger192’, ‘tiger’, ‘tiger2’, ‘whirlpool’. Option --uncompress uncompresses files before computing hash.

When list of files is given, hash of each file is computed and printed, followed by file name, each file on a new line.

When option --check is given, it points to a file that contains list of hash name pairs in the same format as used by UNIX md5sum command. Option --prefix may be used to give directory where files are located. Hash verification stops after the first mismatch was found unless option --keep-going was given. The exit code $? is set to 0 if hash verification is successful. If it fails, $? is set to a nonzero value.

16.3.34 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.

16.3.35 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.

16.3.36 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.

16.3.37 insmod

Command: insmod module

Insert the dynamic GRUB module called module.

16.3.38 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.

16.3.39 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.

16.3.40 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.

16.3.41 list_certificates

Command: list_certificates

List all x509 certificates trusted by GRUB for validating appended signatures. The output is a numbered list of certificates, showing the certificate’s serial number and Common Name.

The certificate number can be used as an argument to distrust_certificate (see distrust_certificate).

See See Using appended signatures for more information.

16.3.42 list_env

Command: list_env [--file file]

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

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

16.3.43 list_trusted

Command: list_trusted

List all public keys trusted by GRUB for validating signatures. The output is in GPG’s v4 key fingerprint format (i.e., the output of gpg --fingerprint). The least significant four bytes (last eight hexadecimal digits) can be used as an argument to distrust (see distrust). See Using GPG-style digital signatures, for more information about uses for these keys.

16.3.44 load_env

Command: load_env [--file file] [--skip-sig] [whitelisted_variable_name] …

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

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

The --skip-sig option skips signature checking even when the value of environment variable check_signatures is set to enforce (see check_signatures).

If one or more variable names are provided as arguments, they are interpreted as a whitelist of variables to load from the environment block file. Variables set in the file but not present in the whitelist are ignored.

The --skip-sig option should be used with care, and should always be used in concert with a whitelist of acceptable variables whose values should be set. Failure to employ a carefully constructed whitelist could result in reading a malicious value into critical environment variables from the file, such as setting check_signatures=no, modifying prefix to boot from an unexpected location or not at all, etc.

When used with care, --skip-sig and the whitelist enable an administrator to configure a system to boot only signed configurations, but to allow the user to select from among multiple configurations, and to enable “one-shot” boot attempts and “savedefault” behavior. See Using GPG-style digital signatures, for more information.

Extra care should be taken when combining this command with appended signatures (see Using appended signatures), as this file is not validated by an appended signature and could set check_appended_signatures=no.

16.3.45 loadfont

Command: loadfont file …

Load specified font files. Unless absolute pathname is given, file is assumed to be in directory ‘$prefix/fonts’ with suffix ‘.pf2’ appended. See Fonts.

16.3.46 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.

16.3.47 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 print the name of the filesystem 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.

16.3.48 lsfonts

Command: lsfonts

List loaded fonts.

16.3.49 lsmod

Command: lsmod

Show list of loaded modules.

16.3.50 md5sum

Command: md5sum arg …

Alias for hashsum --hash md5 arg …. See command hashsum (see hashsum) for full description.

16.3.51 module

Command: module [--nounzip] file [arguments]

Load a module for multiboot kernel image. The rest of the line is passed verbatim as the module command line.

16.3.52 multiboot

Command: multiboot [--quirk-bad-kludge] [--quirk-modules-after-kernel] file …

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

Some kernels have known problems. You need to specify –quirk-* for those. –quirk-bad-kludge is a problem seen in several products that they include loading kludge information with invalid data in ELF file. GRUB prior to 0.97 and some custom builds prefered ELF information while 0.97 and GRUB 2 use kludge. Use this option to ignore kludge. Known affected systems: old Solaris, SkyOS.

–quirk-modules-after-kernel is needed for kernels which load at relatively high address e.g. 16MiB mark and can’t cope with modules stuffed between 1MiB mark and beginning of the kernel. Known afftected systems: VMWare.

16.3.53 nativedisk

Command: nativedisk

Switch from firmware disk drivers to native ones. Really useful only on platforms where both firmware and native disk drives are available. Currently i386-pc, i386-efi, i386-ieee1275 and x86_64-efi.

16.3.54 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.

16.3.55 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.

16.3.56 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 Windows and multiple primary FAT partitions exist in one disk. See also DOS/Windows.

16.3.57 password

Command: password user clear-password

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

16.3.58 password_pbkdf2

Command: password_pbkdf2 user hashed-password

Define a user named user with password hash hashed-password. Use grub2-mkpasswd-pbkdf2 (see Invoking grub2-mkpasswd-pbkdf2) to generate password hashes. See Security.

16.3.59 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.

16.3.60 probe

Command: probe [--set var] --driver|--partmap|--fs|--fs-uuid|--label device

Retrieve device information. If option --set is given, assign result to variable var, otherwise print information on the screen.

16.3.61 pxe_unload

Command: pxe_unload

Unload the PXE environment (see Network).

This command is only available on PC BIOS systems.

16.3.62 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.

16.3.63 reboot

Command: reboot

Reboot the computer.

16.3.64 regexp

Command: regexp [--set [number:]var] regexp string

Test if regular expression regexp matches string. Supported regular expressions are POSIX.2 Extended Regular Expressions. If option --set is given, store numberth matched subexpression in variable var. Subexpressions are numbered in order of their opening parentheses starting from ‘1’. number defaults to ‘1’.

16.3.65 rmmod

Command: rmmod module

Remove a loaded module.

16.3.66 save_env

Command: save_env [--file file] var …

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

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

This command will operate successfully even when environment variable check_signatures is set to enforce (see check_signatures), since it writes to disk and does not alter the behavior of GRUB based on any contents of disk that have been read. It is possible to modify a digitally signed environment block file from within GRUB using this command, such that its signature will no longer be valid on subsequent boots. Care should be taken in such advanced configurations to avoid rendering the system unbootable. See Using GPG-style digital signatures, for more information.

16.3.67 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.

16.3.68 sendkey

Command: sendkey [--num|--caps|--scroll|--insert|--pause|--left-shift|--right-shift|--sysrq|--numkey|--capskey|--scrollkey|--insertkey|--left-alt|--right-alt|--left-ctrl|--right-ctrl ‘on’|‘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:

Name	Key
escape	Escape
exclam	!
at	@
numbersign	#
dollar	$
percent	%
caret	^
ampersand	&
asterisk	*
parenleft	(
parenright	)
minus	-
underscore	_
equal	=
plus	+
backspace	Backspace
tab	Tab
bracketleft	[
braceleft	{
bracketright	]
braceright	}
enter	Enter
control	press and release Control
semicolon	;
colon	:
quote	’
doublequote	"
backquote	‘
tilde	~
shift	press and release left Shift
backslash	\
bar	|
comma	,
less	<
period	.
greater	>
slash	/
question	?
rshift	press and release right Shift
alt	press and release Alt
space	space bar
capslock	Caps Lock
F1	F1
F2	F2
F3	F3
F4	F4
F5	F5
F6	F6
F7	F7
F8	F8
F9	F9
F10	F10
F11	F11
F12	F12
num1	1 (numeric keypad)
num2	2 (numeric keypad)
num3	3 (numeric keypad)
num4	4 (numeric keypad)
num5	5 (numeric keypad)
num6	6 (numeric keypad)
num7	7 (numeric keypad)
num8	8 (numeric keypad)
num9	9 (numeric keypad)
num0	0 (numeric keypad)
numperiod	. (numeric keypad)
numend	End (numeric keypad)
numdown	Down (numeric keypad)
numpgdown	Page Down (numeric keypad)
numleft	Left (numeric keypad)
numcenter	5 with Num Lock inactive (numeric keypad)
numright	Right (numeric keypad)
numhome	Home (numeric keypad)
numup	Up (numeric keypad)
numpgup	Page Up (numeric keypad)
numinsert	Insert (numeric keypad)
numdelete	Delete (numeric keypad)
numasterisk	* (numeric keypad)
numminus	- (numeric keypad)
numplus	+ (numeric keypad)
numslash	/ (numeric keypad)
numenter	Enter (numeric keypad)
delete	Delete
insert	Insert
home	Home
end	End
pgdown	Page Down
pgup	Page Up
down	Down
up	Up
left	Left
right	Right

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.

16.3.69 set

Command: set [envvar=value]

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

16.3.70 sha1sum

Command: sha1sum arg …

Alias for hashsum --hash sha1 arg …. See command hashsum (see hashsum) for full description.

16.3.71 sha256sum

Command: sha256sum arg …

Alias for hashsum --hash sha256 arg …. See command hashsum (see hashsum) for full description.

16.3.72 sha512sum

Command: sha512sum arg …

Alias for hashsum --hash sha512 arg …. See command hashsum (see hashsum) for full description.

16.3.73 sleep

Command: sleep [--verbose] [--interruptible] count

Sleep for count seconds. If option --interruptible is given, allow ESC to interrupt sleep. With --verbose show countdown of remaining seconds. Exit code is set to 0 if timeout expired and to 1 if timeout was interrupted by ESC.

16.3.74 source

Command: source file

Read file as a configuration file, as if its contents had been incorporated directly into the sourcing file. Unlike configfile (see configfile), this executes the contents of file without changing context: any environment variable changes made by the commands in file will be preserved after source returns, and the menu will not be shown immediately.

16.3.75 test

Command: test expression

Evaluate expression and return zero exit status if result is true, non zero status otherwise.

expression is one of:

string1 == string2

the strings are equal

string1 != string2

the strings are not equal

string1 < string2

string1 is lexicographically less than string2

string1 <= string2

string1 is lexicographically less or equal than string2

string1 > string2

string1 is lexicographically greater than string2

string1 >= string2

string1 is lexicographically greater or equal than string2

integer1 -eq integer2

integer1 is equal to integer2

integer1 -ge integer2

integer1 is greater than or equal to integer2

integer1 -gt integer2

integer1 is greater than integer2

integer1 -le integer2

integer1 is less than or equal to integer2

integer1 -lt integer2

integer1 is less than integer2

integer1 -ne integer2

integer1 is not equal to integer2

prefixinteger1 -pgt prefixinteger2

integer1 is greater than integer2 after stripping off common non-numeric prefix.

prefixinteger1 -plt prefixinteger2

integer1 is less than integer2 after stripping off common non-numeric prefix.

file1 -nt file2

file1 is newer than file2 (modification time). Optionally numeric bias may be directly appended to -nt in which case it is added to the first file modification time.

file1 -ot file2

file1 is older than file2 (modification time). Optionally numeric bias may be directly appended to -ot in which case it is added to the first file modification time.

-d file

file exists and is a directory

-e file

file exists

-f file

file exists and is not a directory

-s file

file exists and has a size greater than zero

-n string

the length of string is nonzero

string

string is equivalent to -n string

-z string

the length of string is zero

( expression )

expression is true

! expression

expression is false

expression1 -a expression2

both expression1 and expression2 are true

expression1 expression2

both expression1 and expression2 are true. This syntax is not POSIX-compliant and is not recommended.

expression1 -o expression2

either expression1 or expression2 is true

16.3.76 true

Command: true

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

16.3.77 trust

Command: trust [--skip-sig] pubkey_file

Read public key from pubkey_file and add it to GRUB’s internal list of trusted public keys. These keys are used to validate digital signatures when environment variable check_signatures is set to enforce. Note that if check_signatures is set to enforce when trust executes, then pubkey_file must itself be properly signed. The --skip-sig option can be used to disable signature-checking when reading pubkey_file itself. It is expected that --skip-sig is useful for testing and manual booting. See Using GPG-style digital signatures, for more information.

16.3.78 trust_certificate

Command: trust_certificate x509_certificate

Read an DER-formatted x509 certificate from the file x509_certificate and add it to GRUB’s internal list of trusted x509 certificates. These certificates are used to validate appended signatures when the environment variable check_appended_signatures is set to enforce.

Note that if check_appended_signatures is set to enforce when trust_certificate is executed, then x509_certificate must itself bear an appended signature. (It is not sufficient that x509_certificate be signed by a trusted certificate according to the x509 rules: grub does not include support for validating signatures within x509 certificates themselves.)

See See Using appended signatures for more information.

16.3.79 unset

Command: unset envvar

Unset the environment variable envvar.

16.3.80 uppermem

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

16.3.81 verify_appended

Command: verify_appended file

Verifies an appended signature on file against the trusted certificates known to GRUB (See see list_certificates, see trust_certificate, and see distrust_certificate).

Exit code $? is set to 0 if the signature validates successfully. If validation fails, it is set to a non-zero value. See See Using appended signatures, for more information.

16.3.82 verify_detached

Command: verify_detached [--skip-sig] file signature_file [pubkey_file]

Verifies a GPG-style detached signature, where the signed file is file, and the signature itself is in file signature_file. Optionally, a specific public key to use can be specified using pubkey_file. When environment variable check_signatures is set to enforce, then pubkey_file must itself be properly signed by an already-trusted key. An unsigned pubkey_file can be loaded by specifying --skip-sig. If pubkey_file is omitted, then public keys from GRUB’s trusted keys (see list_trusted, see trust, and see distrust) are tried.

Exit code $? is set to 0 if the signature validates successfully. If validation fails, it is set to a non-zero value. See Using GPG-style digital signatures, for more information.

16.3.83 videoinfo

Command: videoinfo [[WxH]xD]

List available video modes. If resolution is given, show only matching modes.

16.3.84 xen_hypervisor

Command: xen_hypervisor file [arguments] …

Load a Xen hypervisor binary from file. The rest of the line is passed verbatim as the kernel command-line. Any other binaries must be reloaded after using this command. This command is only available on AArch64 systems.

16.3.85 xen_module

Command: xen_module [--nounzip] file [arguments]

Load a module for xen hypervisor at the booting process of xen. The rest of the line is passed verbatim as the module command line. Modules should be loaded in the following order: - dom0 kernel image - dom0 ramdisk if present - XSM policy if present This command is only available on AArch64 systems.

16.4 The list of networking commands

16.4.1 net_add_addr

Command: net_add_addr interface card address

Configure additional network interface with address on a network card. address can be either IP in dotted decimal notation, or symbolic name which is resolved using DNS lookup. If successful, this command also adds local link routing entry to the default subnet of address with name interface‘:local’ via interface.

16.4.2 net_add_dns

Command: net_add_dns server

Resolve server IP address and add to the list of DNS servers used during name lookup.

16.4.3 net_add_route

Command: net_add_route shortname ip[/prefix] [interface | ‘gw’ gateway]

Add route to network with address ip as modified by prefix via either local interface or gateway. prefix is optional and defaults to 32 for IPv4 address and 128 for IPv6 address. Route is identified by shortname which can be used to remove it (see net_del_route).

16.4.4 net_bootp

Command: net_bootp [card]

Perform configuration of card using DHCP protocol. If no card name is specified, try to configure all existing cards. If configuration was successful, interface with name card‘:dhcp’ and configured address is added to card. Additionally the following DHCP options are recognized and processed:

‘1 (Subnet Mask)’

Used to calculate network local routing entry for interface card‘:dhcp’.

‘3 (Router)’

Adds default route entry with the name card‘:dhcp:default’ via gateway from DHCP option. Note that only option with single route is accepted.

‘6 (Domain Name Server)’

Adds all servers from option value to the list of servers used during name resolution.

‘12 (Host Name)’

Sets environment variable ‘net_’<card>‘_dhcp_hostname’ (see net_<interface>_hostname) to the value of option.

‘15 (Domain Name)’

Sets environment variable ‘net_’<card>‘_dhcp_domain’ (see net_<interface>_domain) to the value of option.

‘17 (Root Path)’

Sets environment variable ‘net_’<card>‘_dhcp_rootpath’ (see net_<interface>_rootpath) to the value of option.

‘18 (Extensions Path)’

Sets environment variable ‘net_’<card>‘_dhcp_extensionspath’ (see net_<interface>_extensionspath) to the value of option.