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 1.98.
This manual is for GNU GRUB (version 1.98, 5 September 2010).
Copyright © 1999,2000,2001,2002,2004,2006,2008,2009,2010 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.
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.
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 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.
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 primary requirement for GRUB is that it be compliant with the Multiboot Specification, which is described in Multiboot Specification.
The other goals, listed in approximate order of importance, are:
Except for specific compatibility modes (chain-loading and the Linux piggyback format), all kernels will be started in much the same state as in the Multiboot Specification. Only kernels loaded at 1 megabyte or above are presently supported. Any attempt to load below that boundary will simply result in immediate failure and an error message reporting the problem.
In addition to the requirements above, GRUB has the following features (note that the Multiboot Specification doesn't require all the features that GRUB supports):
The list of commands (see Commands) are a subset of those supported
for configuration files. Editing commands closely resembles the Bash
command-line (see Bash), with <TAB>-completion of commands,
devices, partitions, and files in a directory depending on context.
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.
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”3 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.
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.
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). There are two ways of doing that - either using the utility grub-install (see Invoking grub-install) on a UNIX-like OS, or by running GRUB itself from a floppy. These are quite similar, however the utility might probe a wrong BIOS drive, so you should be careful.
Also, if you install GRUB on a UNIX-like OS, please make sure that you have an emergency boot disk ready, so that you can rescue your computer if, by any chance, your hard drive becomes unusable (unbootable).
GRUB comes with boot images, which are normally put in the directory /usr/lib/grub/i386-pc. Hereafter, the directory where GRUB images are initially placed (normally /usr/lib/grub/i386-pc) will be called the image directory, and the directory where the boot loader needs to find them (usually /boot/grub) will be called the boot directory.
Caution: This procedure is definitely less safe, because there are several ways in which your computer can become unbootable. For example, most operating systems don't tell GRUB how to map BIOS drives to OS devices correctly—GRUB merely guesses the mapping. This will succeed in most cases, but not always. Therefore, GRUB provides you with a map file called the device map, which you must fix if it is wrong. See Device map, for more details.
If you still do want to install GRUB under a UNIX-like OS (such as gnu), invoke the program grub-install (see Invoking grub-install) as the superuser (root).
The usage is basically very simple. You only need to specify one argument to the program, namely, where to install the boot loader. The argument can be either a device file (like ‘/dev/hda’) or a partition specified in GRUB's notation. For example, under Linux the following will install GRUB into the MBR of the first IDE disk:
# grub-install /dev/hda
Likewise, under GNU/Hurd, this has the same effect:
# grub-install /dev/hd0
If it is the first BIOS drive, this is the same as well:
# grub-install '(hd0)'
Or you can omit the parentheses:
# grub-install hd0
But all the above examples assume that GRUB should use images under the root directory. If you want GRUB to use images under a directory other than the root directory, you need to specify the option --root-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
# grub-install --root-directory=/mnt fd0
# umount /mnt
Another example is when you have a separate boot partition which is mounted at /boot. Since GRUB is a boot loader, it doesn't know anything about mountpoints at all. Thus, you need to run grub-install like this:
# grub-install --root-directory=/boot /dev/hda
By the way, as noted above, it is quite difficult to guess BIOS drives correctly under a UNIX-like OS. Thus, grub-install will prompt you to check if it could really guess the correct mappings, after the installation. The format is defined in Device map. Please be quite careful. If the output is wrong, it is unlikely that your computer will be able to boot with no problem.
Note that grub-install is actually just a shell script and the real task is done by grub-mkimage and grub-setup. Therefore, you may run those commands directly to install GRUB, without using grub-install. Don't do that, however, unless you are very familiar with the internals of GRUB. Installing a boot loader on a running OS may be extremely dangerous.
GRUB supports the no emulation mode in the El Torito specification4. 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 Stage 2 called stage2_eltorito. The only GRUB files you need to have in your bootable CD-ROM are this stage2_eltorito and optionally a config file grub.cfg. You don't need to use stage1 or stage2, because El Torito is quite different from the standard boot process.
Here is an example of procedures to make a bootable CD-ROM image. First, make a top directory for the bootable image, say, ‘iso’:
$ mkdir iso
Make a directory for GRUB:
$ mkdir -p iso/boot/grub
Copy the file stage2_eltorito:
$ cp /usr/lib/grub/i386-pc/stage2_eltorito 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 a ISO9660 image file like this:
$ mkisofs -R -b boot/grub/stage2_eltorito -no-emul-boot \
-boot-load-size 4 -boot-info-table -o grub.iso iso
This produces a file named grub.iso, which then can be burned into a CD (or a DVD). mkisofs has already set up the disc to boot from the boot/grub/stage2_eltorito file, so there is no need to setup GRUB on the disc. (Note that the -boot-load-size 4 bit is required for compatibility with the BIOS on many older machines.)
You can use the device ‘(cd)’ to access a CD-ROM in your config file. This is not required; GRUB automatically sets the root device to ‘(cd)’ when booted from a CD-ROM. It is only necessary to refer to ‘(cd)’ if you want to access other drives as well.
The grub-mkdevicemap program can be used to create the device map file. It is often run automatically by tools such as grub-install if the device map file does not already exist. The file name /boot/grub/device.map is preferred.
If the device map file exists, the GRUB utilities (grub-probe, grub-setup, etc.) read it to map BIOS drives to OS devices. This file consists of lines like this:
device file
device is a drive specified in the GRUB syntax (see Device syntax), and file is an OS file, which is normally a device file.
Historically, the device map file was used because GRUB device names had to be used in the configuration file, and they were derived from BIOS drive numbers. The map between BIOS drives and OS devices cannot always be guessed correctly: for example, GRUB will get the order wrong if you exchange the boot sequence between IDE and SCSI in your BIOS.
Unfortunately, even OS device names are not always stable. Modern versions of the Linux kernel may probe drives in a different order from boot to boot, and the prefix (/dev/hd* versus /dev/sd*) may change depending on the driver subsystem in use. As a result, the device map file required frequent editing on some systems.
GRUB avoids this problem nowadays by using UUIDs or file system labels when generating grub.cfg, and we advise that you do the same for any custom menu entries you write. If the device map file does not exist, then the GRUB utilities will assume a temporary device map on the fly. This is often good enough, particularly in the common case of single-disk systems.
However, the device map file is not entirely obsolete yet, and there are still some situations that require it to exist. If necessary, you may edit the file if grub-mkdevicemap makes a mistake. 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 ‘#’.
GRUB can load Multiboot-compliant kernels in a consistent way, but for some free operating systems you need to use some OS-specific magic.
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.
Multiboot (see 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.
However, DOS and Windows have some deficiencies, so you might have to use more complicated instructions. See DOS/Windows, for more information.
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.
Here, we describe some caveats on several operating systems.
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.
search --file --set /boot/gnumach.gz or similar may help you
(see search).
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)'
It is relatively easy to boot GNU/Linux from GRUB, because it somewhat resembles to boot a Multiboot-compliant OS.
search --file --set /vmlinuz or similar may help you
(see search).
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.
grub> initrd /initrd
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.
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
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.
The program grub-mkconfig (see Invoking grub-mkconfig) generates grub.cfg files suitable for most cases. It is suitable for use when upgrading a distribution, and will discover available kernels and attempt to generate menu entries for them.
The file /etc/default/grub controls the operation of grub-mkconfig. It is sourced by a shell script, and so must be valid POSIX shell input; normally, it will just be a sequence of ‘KEY=value’ lines, but if the value contains spaces or other special characters then it must be quoted. For example:
GRUB_TERMINAL_INPUT="console serial"
Valid keys in /etc/default/grub are as follows:
If you set this to ‘saved’, then the default menu entry will be that saved by ‘GRUB_SAVEDEFAULT’, grub-set-default, or grub-reboot.
The default is ‘0’.
Valid terminal input names depend on the platform, but may include ‘console’ (PC BIOS and EFI consoles), ‘serial’ (serial terminal), ‘ofconsole’ (Open Firmware console), ‘at_keyboard’ (PC AT keyboard, mainly useful with Coreboot), or ‘usb_keyboard’ (USB keyboard using the HID Boot Protocol, for cases where the firmware does not handle this).
The default is to use the platform's native terminal input.
Valid terminal output names depend on the platform, but may include ‘console’ (PC BIOS and EFI consoles), ‘serial’ (serial terminal), ‘gfxterm’ (graphics-mode output), ‘ofconsole’ (Open Firmware console), or ‘vga_text’ (VGA text output, mainly useful with Coreboot).
The default is to use the platform's native terminal output.
After grub-install has been run, the available video drivers are
listed in /boot/grub/video.lst.
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.
For more detailed customisation of grub-mkconfig's output, you may edit the scripts in /etc/grub.d directly. /etc/grub.d/40_custom is particularly useful for adding entire custom menu entries; simply type the menu entries you want to add at the end of that file, making sure to leave at least the first two lines intact.
grub.cfg is written in GRUB's built-in scripting language, which has a syntax quite similar to that of GNU Bash and other Bourne shell derivatives.
A word is a sequence of characters considered as a single unit by GRUB. Words are separated by metacharacters, which are the following plus space, tab, and newline:
{ } | & $ ; < >
Quoting may be used to include metacharacters in words; see below.
Reserved words have a special meaning to GRUB. The following words are
recognised as reserved when unquoted and either the first word of a simple
command or the third word of a for command:
! [[ ]] { }
case do done elif else esac fi for function
if in menuentry select then time until while
Not all of these reserved words have a useful purpose yet; some are reserved for future expansion.
Quoting is used to remove the special meaning of certain characters or words. It can be used to treat metacharacters as part of a word, to prevent reserved words from being recognised as such, and to prevent variable expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows, with the exception of newline.
Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of ‘$’ and ‘\’. The ‘$’ character retains its special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: ‘$’, ‘"’, ‘\’, or newline. A backslash-newline pair is treated as a line continuation (that is, it is removed from the input stream and effectively ignored). A double quote may be quoted within double quotes by preceding it with a backslash.
The ‘$’ 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.
Positional variable names consist of one or more digits. These are reserved for future expansion.
The special variable name ‘?’ expands to the exit status of the most recently executed command.
A word beginning with ‘#’ causes that word and all remaining characters on that line to be ignored.
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.
A compound command is one of the 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 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 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.
$?. When executed, the
exit status of a function is the exit status of the last command executed in
the body.
GRUB supports embedding a configuration file directly into the core image, so that it is loaded before entering normal mode. This is useful, for example, when it is not straightforward to find the real configuration file, or when you need to debug problems with loading that file. grub-install uses this feature when it is not using BIOS disk functions or when installing to a different disk from the one containing /boot/grub, in which case it needs to use the search command (see search) to find /boot/grub.
To embed a configuration file, use the -c option to grub-mkimage. The file is copied into the core image, so it may reside anywhere on the file system, and may be removed after running grub-mkimage.
After the embedded configuration file (if any) is executed, GRUB will load
the ‘normal’ module, which will then read the real configuration file
from $prefix/grub.cfg. By this point, the root variable will
also have been set to the root device name. For example, prefix
might be set to ‘(hd0,1)/boot/grub’, and root might be set to
‘hd0,1’. Thus, in most cases, the embedded configuration file only
needs to set the prefix and root variables, and then drop
through to GRUB's normal processing. A typical example of this might look
like this:
search.fs_uuid 01234567-89ab-cdef-0123-456789abcdef root
set prefix=($root)/boot/grub
(The ‘search_fs_uuid’ module must be included in the core image for this example to work.)
In more complex cases, it may be useful to read other configuration files directly from the embedded configuration file. This allows such things as reading files not called grub.cfg, or reading files from a directory other than that where GRUB's loadable modules are installed. To do this, include the ‘configfile’ and ‘normal’ modules in the core image, and embed a configuration file that uses the configfile command to load another file. The following example of this also requires the echo, search_label, and test modules to be included in the core image:
search.fs_label grub root
if [ -e /boot/grub/example/test1.cfg ]; then
set prefix=($root)/boot/grub
configfile /boot/grub/example/test1.cfg
else
if [ -e /boot/grub/example/test2.cfg ]; then
set prefix=($root)/boot/grub
configfile /boot/grub/example/test2.cfg
else
echo "Could not find an example configuration file!"
fi
fi
The embedded configuration file may not contain menu entries directly, but may only read them from elsewhere using configfile.
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
Colors can be specified in several ways:
The fonts GRUB uses “PFF2 font format” bitmap fonts. Fonts are specified with full font names. Currently there is no provision for a preference list of fonts, or deriving one font from another. Fonts are loaded with the “loadfont” command in GRUB. To see the list of loaded fonts, execute the “lsfonts” command. If there are too many fonts to fit on screen, do “set pager=1” before executing “lsfonts”.
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 is controlled through the “show_text” property, which can be set to either “true” or “false” to control whether text is displayed. When GRUB is counting down to automatic boot, the text informs the user of the number of seconds remaining.
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.
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.
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.
One of the most important features for customizing the layout is the use of *styled boxes*. A styled box is composed of 9 rectangular (and potentially empty) regions, which are used to seamlessly draw the styled box on screen:
| Northwest (nw) | North (n) | Northeast (ne)
|
| West (w) | Center (c) | East (e)
|
| Southwest (sw) | South (s) | Southeast (se)
|
To support any size of box on screen, the center slice and the slices for the top, bottom, and sides are all scaled to the correct size for the component on screen, using the following rules:
As an example of how an image might be sliced up, consider the styled box used for a terminal view.
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:
The theme file is a plain text file. Lines that begin with “#“ are ignored and considered comments. (Note: This may not be the case if the previous line ended where a value was expected.)
The theme file contains two types of statements:
Global properties are specified with the simple format:
In this example, name3 is assigned a color value.
| title-text | Specifies the text to display at the top center of the screen as a title.
|
| title-font | Defines the font used for the title message at the top of the screen.
|
| title-color | Defines the color of the title message.
|
| message-font | Defines the font used for messages, such as when GRUB is unable to automatically boot an entry.
|
| message-color | Defines the color of the message text.
|
| message-bg-color | Defines the background color of the message text area.
|
| desktop-image | Specifies the image to use as the background. It will be scaled to fit the screen size.
|
| desktop-color | Specifies the color for the background if *desktop-image* is not specified.
|
| terminal-box | Specifies the file name pattern for the styled box slices used for the command line terminal window. For example, “terminal-box: terminal_*.png” will use the images “terminal_c.png“ as the center area, “terminal_n.png“ as the north (top) edge, “terminal_nw.png“ as the northwest (upper left) corner, and so on. If the image for any slice is not found, it will simply be left empty.
|
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:
The following is a list of the components and the properties they support.
Properties:
| text | The text to display.
|
| 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“.
|
Properties:
| file | The full path to the image file to load.
|
Properties:
| 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.
|
| show_text | Boolean value indicating whether or not text should be displayed on the progress bar. If set to *false*, then no text will be displayed on the bar. If set to any other value, text will be displayed on the bar.
|
| bar_style | The styled box specification for the frame of the progress bar. Example: “progress_frame_*.png“
|
| 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“.
|
| text | The text to display on the progress bar. If the progress bar's ID is set to “__timeout__“, then GRUB will updated this property with an informative message as the timeout approaches.
|
| value | The progress bar current value. Normally not set manually.
|
| start | The progress bar start value. Normally not set manually.
|
| end | The progress bar end value. Normally not set manually.
|
Properties:
| 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“.
|
| value | The progress indicator current value. Normally not set manually.
|
| start | The progress indicator start value. Normally not set manually.
|
| end | The progress indicator end value. Normally not set manually.
|
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.)
|
| 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“
|
| max_items_shown | The maximum number of items to show on the menu. If there are more than
*max_items_shown* items in the menu, the list will scroll to make all
items accessible.
|
The following properties are supported by all components:
| x | Value in pixels
|
| p% | Percentage
|
| p%+x | mixture of both
|
| “__timeout__“ | Any component with this ID will have its *text*, *start*, *end*, *value*, and *visible* properties set by GRUB when it is counting down to an automatic boot of the default menu entry.
|
The following instructions only work on PC BIOS systems where the Preboot eXecution Environment (PXE) is available.
To generate a PXE boot image, run:
grub-mkimage --format=i386-pc --output=core.img --prefix='(pxe)/boot/grub' pxe pxecmd
cat /boot/grub/pxeboot.img core.img >grub.pxe
Copy grub.pxe, /boot/grub/*.mod, and /boot/grub/*.lst to the PXE (TFTP) server, ensuring that *.mod and *.lst are accessible via the /boot/grub/ path from the TFTP server root. Set the DHCP server configuration to offer grub.pxe as the boot file (the ‘filename’ option in ISC dhcpd).
After GRUB has started, files on the TFTP server will be accessible via the ‘(pxe)’ device.
The server and gateway IP address can be controlled by changing the ‘(pxe)’ device name to ‘(pxe:server-ip)’ or ‘(pxe:server-ip:gateway-ip)’. Note that this should be changed both in the prefix and in any references to the device name in the configuration file.
GRUB provides several environment variables which may be used to inspect or change the behaviour of the PXE device:
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 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.
Some laptop vendors provide an additional power-on button which boots another OS. GRUB supports such buttons with the ‘GRUB_TIMEOUT_BUTTON’, ‘GRUB_DEFAULT_BUTTON’, ‘GRUB_HIDDEN_TIMEOUT_BUTTON’ and ‘GRUB_BUTTON_CMOS_ADDRESS’ variables in default/grub (see Simple configuration). ‘GRUB_TIMEOUT_BUTTON’, ‘GRUB_DEFAULT_BUTTON’ and ‘GRUB_HIDDEN_TIMEOUT_BUTTON’ are used instead of the corresponding variables without the ‘_BUTTON’ suffix when powered on using the special button. ‘GRUB_BUTTON_CMOS_ADDRESS’ is vendor-specific and partially model-specific. Values known to the GRUB team are:
To take full advantage of this function, install GRUB into the MBR (see Installing GRUB using grub-install).
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.
The sole function of boot.img is to read the first sector of the core
image from a local disk and jump to it. Because of the size restriction,
boot.img cannot understand any file system structure, so
grub-setup hardcodes the location of the first sector of the core
image into boot.img when installing GRUB.
On PC systems using the traditional MBR partition table format, the core image is usually installed in the "MBR gap" between the master boot record and the first partition, or sometimes it is installed in a file system and read directly from that. The latter is not recommended because GRUB needs to encode the location of all the core image sectors in diskboot.img, and if the file system ever moves the core image around (as it is entitled to do) then GRUB must be reinstalled; it also means that GRUB will not be able to reliably find the core image if it resides on a different disk than the one to which boot.img was installed.
On PC systems using the more recent GUID Partition Table (GPT) format, the core image should be installed to a BIOS Boot Partition. This may be created by GNU Parted using a command such as the following:
# parted /dev/disk set partition-number bios_grub on
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.
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.
GRUB Legacy could run with only Stage 1 and Stage 2 in some limited
configurations, while GRUB 2 requires core.img and cannot work
without it.
GRUB 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).
The device syntax is like this:
(device[,part-num][,bsd-subpart-letter])
‘[]’ means the parameter is optional. device should be either ‘fd’ or ‘hd’ followed by a digit, like ‘fd0’. But you can also set device to a hexadecimal or a decimal number which is a BIOS drive number, so the following are equivalent:
(hd0)
(0x80)
(128)
part-num represents the partition number of device, starting from one for primary partitions and from five for extended partitions, and bsd-subpart-letter represents the BSD disklabel subpartition, such as ‘a’ or ‘e’.
A shortcut for specifying BSD subpartitions is
(device,bsd-subpart-letter), in this case, GRUB
searches for the first PC partition containing a BSD disklabel, then
finds the subpartition bsd-subpart-letter. Here is an example:
(hd0,a)
The syntax ‘(hd0)’ represents using the entire disk (or the MBR when installing GRUB), while the syntax ‘(hd0,1)’ represents using the first partition of the disk (or the boot sector of the partition when installing GRUB).
If you enabled the network support, the special drive ‘(pxe)’ is also available. Before using the network drive, you must initialize the network. See Network, for more information.
If you boot GRUB from a CD-ROM, ‘(cd)’ is available. See Making a GRUB bootable CD-ROM, for details.
There are two ways to specify files, by absolute file name and by block list.
An absolute file name resembles a Unix absolute file name, using
‘/’ for the directory separator (not ‘\’ as in DOS). One
example is ‘(hd0,1)/boot/grub/grub.cfg’. This means the file
/boot/grub/grub.cfg in the first partition of the first hard
disk. If you omit the device name in an absolute file name, GRUB uses
GRUB's root device implicitly. So if you set the root device to,
say, ‘(hd1,1)’ by the command ‘set root=(hd1,1)’ (see set),
then /boot/kernel is the same as (hd1,1)/boot/kernel.
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)’.
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.
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 entered5. 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:
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.
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.
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>.
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.
The semantics used in parsing the configuration file are the following:
These commands can only be used in the menu:
This defines a GRUB menu entry named title. When this entry is selected from the menu, GRUB will set the chosen environment variable to title, execute the list of commands given within braces, and if the last command in the list returned successfully and a kernel was loaded it will execute the boot command.
The --class option may be used any number of times to group menu entries into classes. Menu themes may display different classes using different styles.
The --users option grants specific users access to specific menu entries. See Security.
The --hotkey option associates a hotkey with a menu entry. key may be a single letter, or one of the aliases ‘backspace’, ‘tab’, or ‘delete’.
Commands usable anywhere in the menu and in the command-line.
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).
This command is only available if GRUB is compiled with serial support. See also Serial terminal.
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.
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.
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.
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).
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.
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.
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).
Display the contents of the file file. This command may be useful to remind you of your OS's root partition:
grub> cat /etc/fstabIf 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.
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.
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.
Load file as a configuration file. If file defines any menu entries, then show a menu containing them immediately.
Check for CPU features. This command is only available on x86 systems.
With the -l option, return true if the CPU supports long mode (64-bit).
If invoked without options, this command currently behaves as if it had been invoked with -l. This may change in the future.
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.
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 ${root} as a drive specification.
With the -s option, perform the reverse mapping as well, swapping the two drives.
With the -l option, list the current mappings.
With the -r option, reset all mappings to the default values.
For example:
drivemap -s (hd0) (hd1)
Display the requested text and, unless the -n option is used, a trailing new line. If there is more than one string, they are separated by spaces in the output. As usual in GRUB commands, variables may be substituted using ‘${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.
Export the environment variable envvar. Exported variables are visible to subsidiary configuration files loaded using configfile.
Translate string into the current language.
The current language code is stored in the ‘lang’ variable in GRUB's environment. Translation files in MO format are read from ‘locale_dir’, usually /boot/grub/locale.
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.
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.
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.
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.
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.
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.
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.
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.
List devices or files.
With no arguments, print all devices known to GRUB.
If the argument is a device name enclosed in parentheses (see Device syntax), then list all files at the root directory of that device.
If the argument is a directory given as an absolute file name (see File name syntax), then list the contents of that directory.
Make various modifications to partition table entries.
Each command is either a boolean option, in which case it must be followed with ‘+’ or ‘-’ (with no intervening space) to enable or disable that option, or else it takes a value in the form ‘command=value’.
Currently, parttool is only useful on DOS partition tables (also known as Master Boot Record, or MBR). On these partition tables, the following commands are available:
- ‘boot’ (boolean)
- When enabled, this makes the selected partition be the active (bootable) partition on its disk, clearing the active flag on all other partitions. This command is limited to primary partitions.
- ‘type’ (value)
- Change the type of an existing partition. The value must be a number in the range 0-0xFF (prefix with ‘0x’ to enter it in hexadecimal).
- ‘hidden’ (boolean)
- When enabled, this hides the selected partition by setting the hidden bit in its partition type code; when disabled, unhides the selected partition by clearing this bit. This is useful only when booting DOS or Wwindows and multiple primary FAT partitions exist in one disk. See also DOS/Windows.
Define a user named user with password clear-password. See Security.
Define a user named user with password hash hashed-password. Use grub-mkpasswd-pbkdf2 (see Invoking grub-mkpasswd-pbkdf2) to generate password hashes. See Security.
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.
Unload the PXE environment (see Network).
This command is only available on PC BIOS systems.
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.
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.
Set the environment variable envvar to value. If invoked with no arguments, print all environment variables with their values.
This command is not yet implemented for GRUB 2, although it is planned.
By default, the boot loader interface is accessible to anyone with physical access to the console: anyone can select and edit any menu entry, and anyone can get direct access to a GRUB shell prompt. For most systems, this is reasonable since anyone with direct physical access has a variety of other ways to gain full access, and requiring authentication at the boot loader level would only serve to make it difficult to recover broken systems.
However, in some environments, such as kiosks, it may be appropriate to lock down the boot loader to require authentication before performing certain operations.
The ‘password’ (see password) and ‘password_pbkdf2’ (see password_pbkdf2) commands can be used to define users, each of which has an associated password. ‘password’ sets the password in plain text, requiring grub.cfg to be secure; ‘password_pbkdf2’ sets the password hashed using the Password-Based Key Derivation Function (RFC 2898), requiring the use of grub-mkpasswd-pbkdf2 (see Invoking grub-mkpasswd-pbkdf2) to generate password hashes.
In order to enable authentication support, the ‘superusers’ environment variable must be set to a list of usernames, separated by any of spaces, commas, semicolons, pipes, or ampersands. Superusers are permitted to use the GRUB command line, edit menu entries, and execute any menu entry. If ‘superusers’ is set, then use of the command line is automatically restricted to superusers.
Other users may be given access to specific menu entries by giving a list of usernames (as above) using the --users option to the ‘menuentry’ command (see menuentry). If the --users option is not used for a menu entry, then that entry is unrestricted.
Putting this together, a typical grub.cfg fragment might look like this:
set superusers="root"
password_pbkdf2 root grub.pbkdf2.sha512.10000.biglongstring
password user1 insecure
menuentry "May be run by any user" {
set root=(hd0,1)
linux /vmlinuz
}
menuentry "Superusers only" --users "" {
set root=(hd0,1)
linux /vmlinuz single
}
menuentry "May be run by user1 or a superuser" --users user1 {
set root=(hd0,2)
chainloader +1
}
The grub-mkconfig program does not yet have built-in support for generating configuration files with authentication. You can use /etc/grub.d/40_custom to add simple superuser authentication, by adding set superusers= and password or password_pbkdf2 commands.
X86 support is summarised in following table. “Yes” means that kernel works on the given platform, “crashes” means an early kernel crash which we hove will be fixed by concerned kernel developpers. “no” means GRUB doesn't load given kernel on a given platform. “headless” means that the kernel works but lacks console drivers (you can still use serial or network console). In case of “no” and “crashes” the reason is given in footnote.
| BIOS | Coreboot
| |
| BIOS chainloading | yes | no (1)
|
| NTLDR | yes | no (1)
|
| FreeBSD bootloader | yes | crashes (1)
|
| 32-bit kFreeBSD | yes | crashes (2,6)
|
| 64-bit kFreeBSD | yes | crashes (2,6)
|
| 32-bit kNetBSD | yes | crashes (1)
|
| 64-bit kNetBSD | yes | crashes (2)
|
| 32-bit kOpenBSD | yes | yes
|
| 64-bit kOpenBSD | yes | yes
|
| Multiboot | yes | yes
|
| Multiboot2 | yes | yes
|
| 32-bit Linux (legacy protocol) | yes | no (1)
|
| 64-bit Linux (legacy protocol) | yes | no (1)
|
| 32-bit Linux (modern protocol) | yes | yes
|
| 64-bit Linux (modern protocol) | yes | yes
|
| 32-bit XNU | yes | ?
|
| 64-bit XNU | yes | ?
|
| 32-bit EFI chainloader | no (3) | no (3)
|
| 64-bit EFI chainloader | no (3) | no (3)
|
| Appleloader | no (3) | no (3)
|
| Multiboot | Qemu
| |
| BIOS chainloading | no (1) | no (1)
|
| NTLDR | no (1) | no (1)
|
| FreeBSD bootloader | crashes (1) | crashes (1)
|
| 32-bit kFreeBSD | crashes (6) | crashes (6)
|
| 64-bit kFreeBSD | crashes (6) | crashes (6)
|
| 32-bit kNetBSD | crashes (1) | crashes (1)
|
| 64-bit kNetBSD | yes | yes
|
| 32-bit kOpenBSD | yes | yes
|
| 64-bit kOpenBSD | yes | yes
|
| Multiboot | yes | yes
|
| Multiboot2 | yes | yes
|
| 32-bit Linux (legacy protocol) | no (1) | no (1)
|
| 64-bit Linux (legacy protocol) | no (1) | no (1)
|
| 32-bit Linux (modern protocol) | yes | yes
|
| 64-bit Linux (modern protocol) | yes | yes
|
| 32-bit XNU | ? | ?
|
| 64-bit XNU | ? | ?
|
| 32-bit EFI chainloader | no (3) | no (3)
|
| 64-bit EFI chainloader | no (3) | no (3)
|
| Appleloader | no (3) | no (3)
|
| 32-bit EFI | 64-bit EFI
| |
| BIOS chainloading | no (1) | no (1)
|
| NTLDR | no (1) | no (1)
|
| FreeBSD bootloader | crashes (1) | crashes (1)
|
| 32-bit kFreeBSD | headless | headless
|
| 64-bit kFreeBSD | headless | headless
|
| 32-bit kNetBSD | crashes (1) | crashes (1)
|
| 64-bit kNetBSD | yes | yes
|
| 32-bit kOpenBSD | headless | headless
|
| 64-bit kOpenBSD | headless | headless
|
| Multiboot | yes | yes
|
| Multiboot2 | yes | yes
|
| 32-bit Linux (legacy protocol) | no (1) | no (1)
|
| 64-bit Linux (legacy protocol) | no (1) | no (1)
|
| 32-bit Linux (modern protocol) | yes | yes
|
| 64-bit Linux (modern protocol) | yes | yes
|
| 32-bit XNU | yes | yes
|
| 64-bit XNU | yes (5) | yes
|
| 32-bit EFI chainloader | yes | no (4)
|
| 64-bit EFI chainloader | no (4) | yes
|
| Appleloader | yes | yes
|
| IEEE1275
| |
| BIOS chainloading | no (1)
|
| NTLDR | no (1)
|
| FreeBSD bootloader | crashes (1)
|
| 32-bit kFreeBSD | crashes (6)
|
| 64-bit kFreeBSD | crashes (6)
|
| 32-bit kNetBSD | crashes (1)
|
| 64-bit kNetBSD | ?
|
| 32-bit kOpenBSD | ?
|
| 64-bit kOpenBSD | ?
|
| Multiboot | ?
|
| Multiboot2 | ?
|
| 32-bit Linux (legacy protocol) | no (1)
|
| 64-bit Linux (legacy protocol) | no (1)
|
| 32-bit Linux (modern protocol) | ?
|
| 64-bit Linux (modern protocol) | ?
|
| 32-bit XNU | ?
|
| 64-bit XNU | ?
|
| 32-bit EFI chainloader | no (3)
|
| 64-bit EFI chainloader | no (3)
|
| Appleloader | no (3)
|
PowerPC and Sparc ports support only Linux. MIPS port supports Linux and multiboot2.
As you have seen in previous chapter the support matrix is pretty big and some of the configurations are only rarely used. To ensure the quality bootchecks are available for all x86 targets except EFI chainloader, Appleloader and XNU. All x86 platforms have bootcheck facility except ieee1275. Multiboot, multiboot2, BIOS chainloader, ntldr and freebsd-bootloader boot targets are tested only with a fake kernel images. Only Linux is tested among the payloads using Linux protocols.
Following variables must be defined:
| GRUB_PAYLOADS_DIR | directory containing the required kernels
|
| GRUB_CBFSTOOL | cbfstoll from Coreboot package (for coreboot platform only)
|
| GRUB_COREBOOT_ROM | empty Coreboot ROM
|
| GRUB_QEMU_OPTS | additional options to be supplied to QEMU
|
Required files are:
| kfreebsd_env.i386 | 32-bit kFreeBSD device hints
|
| kfreebsd.i386 | 32-bit FreeBSD kernel image
|
| kfreebsd.x86_64, kfreebsd_env.x86_64 | same from 64-bit kFreeBSD
|
| knetbsd.i386 | 32-bit NetBSD kernel image
|
| knetbsd.miniroot.i386 | 32-bit kNetBSD miniroot.kmod.
|
| knetbsd.x86_64, knetbsd.miniroot.x86_64 | same from 64-bit kNetBSD
|
| kopenbsd.i386 | 32-bit OpenBSD kernel bsd.rd image
|
| kopenbsd.x86_64 | same from 64-bit kOpenBSD
|
| linux.i386 | 32-bit Linux
|
| linux.x86_64 | 64-bit Linux
|
GRUB's normal start-up procedure involves setting the ‘prefix’ environment variable to a value set in the core image by grub-install, setting the ‘root’ variable to match, loading the ‘normal’ module from the prefix, and running the ‘normal’ command. This command is responsible for reading /boot/grub/grub.cfg, running the menu, and doing all the useful things GRUB is supposed to do.
If, instead, you only get a rescue shell, this usually means that GRUB failed to load the ‘normal’ module for some reason. It may be possible to work around this temporarily: for instance, if the reason for the failure is that ‘prefix’ is wrong (perhaps it refers to the wrong device, or perhaps the path to /boot/grub was not correctly made relative to the device), then you can correct this and enter normal mode manually:
# Inspect the current prefix (and other preset variables):
set
# Set to the correct value, which might be something like this:
set prefix=(hd0,1)/grub
set root=(hd0,1)
insmod normal
normal
However, any problem that leaves you in the rescue shell probably means that GRUB was not correctly installed. It may be more useful to try to reinstall it properly using grub-install device (see Invoking grub-install). When doing this, there are a few things to remember:
The program grub-install installs GRUB on your drive using grub-mkimage and (on some platforms) grub-setup. You must specify the device name on which you want to install GRUB, like this:
grub-install install_device
The device name install_device is an OS device name or a GRUB device name.
grub-install accepts the following options:
grub-install --root-directory=/boot hd0
The program grub-mkconfig generates a configuration file for GRUB (see Simple configuration).
grub-mkconfig -o /boot/grub/grub.cfg
grub-mkconfig accepts the following options:
The program grub-mkpasswd-pbkdf2 generates password hashes for GRUB (see Security).
grub-mkpasswd-pbkdf2
grub-mkpasswd-pbkdf2 accepts the following options:
Caution: GRUB requires binutils-2.9.1.0.23 or later because the GNU assembler has been changed so that it can produce real 16bits machine code between 2.9.1 and 2.9.1.0.x. See http://sources.redhat.com/binutils/, to obtain information on how to get the latest version.
GRUB is available from the GNU alpha archive site ftp://alpha.gnu.org/gnu/grub or any of its mirrors. The file will be named grub-version.tar.gz. The current version is 1.98, so the file you should grab is:
ftp://alpha.gnu.org/gnu/grub/grub-1.98.tar.gz
To unbundle GRUB use the instruction:
zcat grub-1.98.tar.gz | tar xvf -
which will create a directory called grub-1.98 with all the sources. You can look at the file INSTALL for detailed instructions on how to build and install GRUB, but you should be able to just do:
cd grub-1.98
./configure
make install
Also, the latest version is available using Bazaar. See http://www.gnu.org/software/grub/grub-download.en.html for more information.
These are the guideline for how to report bugs. Take a look at this list below before you submit bugs:
The information on your hardware is also essential. These are especially important: the geometries and the partition tables of your hard disk drives and your BIOS.
When you attach a patch, make the patch in unified diff format, and write ChangeLog entries. But, even when you make a patch, don't forget to explain the problem, so that we can understand what your patch is for.
If you follow the guideline above, submit a report to the Bug Tracking System. Alternatively, you can submit a report via electronic mail to bug-grub@gnu.org, but we strongly recommend that you use the Bug Tracking System, because e-mail can be passed over easily.
Once we get your report, we will try to fix the bugs.
We started the next generation of GRUB, GRUB 2. GRUB 2 includes internationalization, dynamic module loading, real memory management, multiple architecture support, a scripting language, and many other nice features. If you are interested in the development of GRUB 2, take a look at the homepage.
GRUB is maintained using the Bazaar revision control system. To fetch the primary development branch:
bzr get http://bzr.savannah.gnu.org/r/grub/trunk/grub
The GRUB developers maintain several other branches with work in progress. Of these, the most interesting is the experimental branch, which is a staging area for new code which we expect to eventually merge into trunk but which is not yet ready:
bzr get http://bzr.savannah.gnu.org/r/grub/branches/experimental
Once you have used bzr get to fetch an initial copy of a branch, you can use bzr pull to keep it up to date. If you have modified your local version, you may need to resolve conflicts when pulling.
Here is a brief map of the GRUB code base.
GRUB uses Autoconf, but not (yet) Automake. The top-level build rules are in configure.ac, Makefile.in, and conf/*.rmk. Each conf/*.rmk file represents a particular target configuration, and is processed into GNU Make rules by genmk.rb (which you only need to look at if you are extending the build system). If you are adding a new module which follows an existing pattern, such as a new command or a new filesystem implementation, it is usually easiest to grep conf/*.rmk for an existing example of that pattern to find out where it should be added.
Low-level boot code, such as the MBR implementation on PC BIOS systems, is in the boot/ directory.
The GRUB kernel is in kern/. This contains core facilities such as the device, disk, and file frameworks, environment variable handling, list processing, and so on. The kernel should contain enough to get up to a rescue prompt. Header files for kernel facilities, among others, are in include/.
Terminal implementations are in term/.
Disk access code is spread across disk/ (for accessing the disk devices themselves), partmap/ (for interpreting partition table data), and fs/ (for accessing filesystems). Note that, with the odd specialised exception, GRUB only contains code to read from filesystems and tries to avoid containing any code to write to filesystems; this lets us confidently assure users that GRUB cannot be responsible for filesystem corruption.
PCI and USB bus handling is in bus/.
Video handling code is in video/. The graphical menu system uses this heavily, but is in a separate directory, gfxmenu/.
Most commands are implemented by files in commands/, with the following exceptions:
There are a few other special-purpose exceptions; grep for them if they matter to you.
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acpi: acpibadram: badramblocklist: blocklistboot: bootcat: catchainloader: chainloadercmp: cmpconfigfile: configfilecpuid: cpuidcrc: crcdate: datedrivemap: drivemapecho: echoexport: exportgettext: gettextgptsync: gptsynchalt: halthelp: helpinitrd: initrdinitrd16: initrd16insmod: insmodkeystatus: keystatuslinux: linuxlinux16: linux16ls: lsmenuentry: menuentryparttool: parttoolpassword: passwordpassword_pbkdf2: password_pbkdf2play: playpxe_unload: pxe_unloadreboot: rebootsearch: searchsendkey: sendkeyserial: serialset: setterminal_input: terminal_inputterminal_output: terminal_outputterminfo: terminfounset: unset[1] chain-load is the mechanism for loading unsupported operating systems by loading another boot loader. It is typically used for loading DOS or Windows.
[2] There are a few pathological cases where loading a very badly organized ELF kernel might take longer, but in practice this never happen.
[3] The LInux LOader, a boot loader that everybody uses, but nobody likes.
[4] El Torito is a specification for bootable CD using BIOS functions.
[5] However, this behavior will be changed in the future version, in a user-invisible way.