This is the documentation of GNU GRUB, the GRand Unified Bootloader, a flexible and powerful boot loader program for PCs.
This edition documents version 0.97.
Briefly, a boot loader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to an operating system kernel software (such as Linux or GNU Mach). The kernel, in turn, initializes the rest of the operating system (e.g. a GNU system).
GNU GRUB is a very powerful boot loader, which can load a wide variety of free operating systems, as well as proprietary operating systems with chain-loading1. GRUB is designed to address the complexity of booting a personal computer; both the program and this manual are tightly bound to that computer platform, although porting to other platforms may be addressed in the future.
One of the important features in GRUB is flexibility; GRUB understands filesystems and kernel executable formats, so you can load an arbitrary operating system the way you like, without recording the physical position of your kernel on the disk. Thus you can load the kernel just by specifying its file name and the drive and partition where the kernel resides.
When booting with GRUB, you can use either a command-line interface (see The flexible command-line interface), or a menu interface (see The simple menu interface). Using the command-line interface, you type the drive specification and file name of the kernel manually. In the menu interface, you just select an OS using the arrow keys. The menu is based on a configuration file which you prepare beforehand (see 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.
Besides the GRUB boot loader itself, there is a grub shell
grub (see Invoking the grub shell) which can be run when
you are in your operating system. It emulates the boot loader and can
be used for installing the boot loader.
GRUB originated in 1995 when Erich Boleyn was trying to boot the GNU Hurd with the University of Utah’s Mach 4 microkernel (now known as GNU Mach). Erich and Brian Ford designed the Multiboot Specification (see Motivation in The Multiboot Specification), because they were determined not to add to the large number of mutually-incompatible PC boot methods.
Erich then began modifying the FreeBSD boot loader so that it would understand Multiboot. He soon realized that it would be a lot easier to write his own boot loader from scratch than to keep working on the FreeBSD boot loader, and so GRUB was born.
Erich added many features to GRUB, but other priorities prevented him from keeping up with the demands of its quickly-expanding user base. In 1999, Gordon Matzigkeit and Yoshinori K. Okuji adopted GRUB as an official GNU package, and opened its development by making the latest sources available via anonymous CVS. See How to obtain and build GRUB, for more information.
The primary requirement for GRUB is that it be compliant with the Multiboot Specification, which is described in Motivation in The Multiboot Specification.
The other goals, listed in approximate order of importance, are:
Except for specific compatibility modes (chain-loading and the Linux piggyback format), all kernels will be started in much the same state as in the Multiboot Specification. Only kernels loaded at 1 megabyte or above are presently supported. Any attempt to load below that boundary will simply result in immediate failure and an error message reporting the problem.
In addition to the requirements above, GRUB has the following features (note that the Multiboot Specification doesn’t require all the features that GRUB supports):
Support many of the a.out variants plus ELF. Symbol tables are also loaded.
Support many of the various free 32-bit kernels that lack Multiboot compliance (primarily FreeBSD, NetBSD, OpenBSD, and Linux). Chain-loading of other boot loaders is also supported.
Fully support the Multiboot feature of loading multiple modules.
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 The list of available commands) are a superset of those supported on the command-line. An example configuration file is provided in Configuration.
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.
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 The list of available commands) are a subset of those supported for configuration files. Editing commands closely resembles the Bash command-line (see Command Line Editing in Bash Features), with TAB-completion of commands, devices, partitions, and files in a directory depending on context.
Support multiple filesystem types transparently, plus a useful explicit blocklist notation. The currently supported filesystem types are BSD FFS, DOS FAT16 and FAT32, Minix fs, Linux ext2fs, ReiserFS, JFS, XFS, and VSTa fs. See Filesystem syntax and semantics, for more information.
Can decompress files which were compressed by
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.2
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.
Support reading data from any or all floppies or hard disk(s) recognized by the BIOS, independent of the setting of the root device.
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.
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.
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.
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.
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.
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:
First of all, GRUB requires that the device name be enclosed with ‘(’ and ‘)’. The ‘fd’ part means that it is a floppy disk. The number ‘0’ is the drive number, which is counted from zero. This expression means that GRUB will use the whole floppy disk.
Here, ‘hd’ means it is a hard disk drive. The first integer ‘0’ indicates the drive number, that is, the first hard disk, while the second integer, ‘1’, indicates the partition number (or the PC slice number in the BSD terminology). Once again, please note that the partition numbers are counted from zero, not from one. This expression means the second partition of the first hard disk drive. In this case, GRUB uses one partition of the disk, instead of the whole disk.
This specifies the first extended partition of the first hard disk drive. Note that the partition numbers for extended partitions are counted from ‘4’, regardless of the actual number of primary partitions on your hard disk.
This means the BSD ‘a’ partition of the second hard disk. If you need to specify which PC slice number should be used, use something like this: ‘(hd1,0,a)’. If the PC slice number is omitted, GRUB searches for the first PC slice which has a BSD ‘a’ partition.
Of course, to actually access the disks or partitions with GRUB, you need to use the device specification in a command, like ‘root (fd0)’ or ‘unhide (hd0,2)’. To help you find out which number specifies a partition you want, the GRUB command-line (see The flexible command-line interface) options have argument completion. This means that, for example, you only need to type
followed by a TAB, and GRUB will display the list of drives, partitions, or file names. So it should be quite easy to determine the name of your target partition, even with minimal knowledge of the syntax.
Note that GRUB does not distinguish IDE from SCSI - it simply counts the drive numbers from zero, regardless of their type. Normally, any IDE drive number is less than any SCSI drive number, although that is not true if you change the boot sequence by swapping IDE and SCSI drives in your BIOS.
Now the question is, how to specify a file? Again, consider an example:
This specifies the file named ‘vmlinuz’, found on the first partition of the first hard disk drive. Note that the argument completion works with file names, too.
That was easy, admit it. Now read the next chapter, to find out how to actually install GRUB on your drive.
In order to install GRUB as your boot loader, you need to first install the GRUB system and utilities under your UNIX-like operating system (see How to obtain and build GRUB). You can do this either from the source tarball, or as a package for your OS.
After you have done that, you need to install the boot loader on a
drive (floppy or hard disk). 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. If you do not use grub-install, then
you need to copy the files stage1, stage2, and
*stage1_5 to the directory /boot/grub, and run the
grub-set-default (see Invoking grub-set-default) if you
intend to use ‘default saved’ (see default) in your
configuration file. 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.
To create a GRUB boot floppy, you need to take the files stage1 and stage2 from the image directory, and write them to the first and the second block of the floppy disk, respectively.
Caution: This procedure will destroy any data currently stored on the floppy.
On a UNIX-like operating system, that is done with the following commands:
# cd /usr/lib/grub/i386-pc # dd if=stage1 of=/dev/fd0 bs=512 count=1 1+0 records in 1+0 records out # dd if=stage2 of=/dev/fd0 bs=512 seek=1 153+1 records in 153+1 records out #
The device file name may be different. Consult the manual for your OS.
Caution: Installing GRUB’s stage1 in this manner will erase the normal boot-sector used by an OS.
GRUB can currently boot GNU Mach, Linux, FreeBSD, NetBSD, and OpenBSD directly, so using it on a boot sector (the first sector of a partition) should be okay. But generally, it would be a good idea to back up the first sector of the partition on which you are installing GRUB’s stage1. This isn’t as important if you are installing GRUB on the first sector of a hard disk, since it’s easy to reinitialize it (e.g. by running ‘FDISK /MBR’ from DOS).
If you decide to install GRUB in the native environment, which is definitely desirable, you’ll need to create a GRUB boot disk, and reboot your computer with it. Otherwise, see Installing GRUB using grub-install.
grub> root (hd0,0)
If you are not sure which partition actually holds this directory, use the
find (see find), like this:
grub> find /boot/grub/stage1
This will search for the file name /boot/grub/stage1 and show the devices which contain the file.
Once you’ve set the root device correctly, run the command
setup (see setup):
grub> setup (hd0)
This command will install the GRUB boot loader on the Master Boot Record (MBR) of the first drive. If you want to put GRUB into the boot sector of a partition instead of putting it in the MBR, specify the partition into which you want to install GRUB:
grub> setup (hd0,0)
If you install GRUB into a partition or a drive other than the first one, you must chain-load GRUB from another boot loader. Refer to the manual for the boot loader to know how to chain-load GRUB.
After using the setup command, you will boot into GRUB without the GRUB floppy. See the chapter Booting to find out how to boot your operating systems from GRUB.
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 The map between BIOS drives and OS devices, 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 The map between BIOS drives and OS devices. Please be
quite careful. If the output is wrong, it is unlikely that your
computer will be able to boot with no problem.
grub-install is actually just a shell script and the
real task is done by the grub shell
grub (see Invoking the grub shell). Therefore, you may run
grub 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 specification5. This means that you can use the whole CD-ROM from GRUB and you don’t have to make a floppy or hard disk image file, which can cause compatibility problems.
For booting from a CD-ROM, GRUB uses a special 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 menu.lst. 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 menu.lst 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.
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 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 Load another boot loader to boot unsupported operating systems).
Generally, GRUB can boot any Multiboot-compliant OS in the following steps:
module(see module) or
Linux, FreeBSD, NetBSD and OpenBSD can be booted in a similar
manner. You load a kernel image with the command
then run the command
boot. If the kernel requires some
parameters, just append the parameters to
kernel, after the
file name of the kernel. Also, please refer to Some caveats on OS-specific issues,
for information on your OS-specific issues.
If you want to boot an unsupported operating system (e.g. Windows 95), chain-load a boot loader for the operating system. Normally, the boot loader is embedded in the boot sector of the partition on which the operating system is installed.
grub> rootnoverify (hd0,0)
makeactive6 (see makeactive):
grub> chainloader +1
‘+1’ indicates that GRUB should read one sector from the start of the partition. The complete description about this syntax can be found in How to specify block lists.
However, DOS and Windows have some deficiencies, so you might have to use more complicated instructions. See DOS/Windows, for more information.
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.
find /boot/gnumachor similar can help you (see find).
grub> kernel /boot/gnumach root=hd0s1 grub> module /boot/serverboot
It is relatively easy to boot GNU/Linux from GRUB, because it somewhat resembles to boot a Multiboot-compliant OS.
find /vmlinuzor similar can help you (see find).
grub> kernel /vmlinuz root=/dev/hda1
If you need to specify some kernel parameters, just append them to the command. For example, to set vga to ‘ext’, do this:
grub> kernel /vmlinuz root=/dev/hda1 vga=ext
See the documentation in the Linux source tree for complete information on the available options.
initrd(see initrd) after
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 can load the kernel directly, either in ELF or a.out format. But this is not recommended, since FreeBSD’s bootstrap interface sometimes changes heavily, so GRUB can’t guarantee to pass kernel parameters correctly.
Thus, we’d recommend loading the very flexible loader /boot/loader instead. See this example:
grub> root (hd0,a) grub> kernel /boot/loader grub> boot
GRUB can load NetBSD a.out and ELF directly, follow these steps:
kernel(see kernel). You should append the ugly option --type=netbsd, if you want to load an ELF kernel, like this:
grub> kernel --type=netbsd /netbsd-elf
For now, however, GRUB doesn’t allow you to pass kernel parameters, so it may be better to chain-load it instead. For more information, please see Load another boot loader to boot unsupported operating systems.
The booting instruction is exactly the same as for NetBSD (see NetBSD).
GRUB cannot boot DOS or Windows directly, so you must chain-load them (see Load another boot loader to boot unsupported operating systems). 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
map (see map), like this:
grub> map (hd0) (hd1) grub> map (hd1) (hd0)
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 hide), DOS (or Windows) will ignore the partition. If GRUB unhides a DOS (or Windows) partition (see unhide), 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:
grub> unhide (hd0,0) grub> hide (hd0,1) grub> rootnoverify (hd0,0) grub> chainloader +1 grub> makeactive grub> boot
It is known that the signature in the boot loader for SCO UnixWare is
wrong, so you will have to specify the option --force to
chainloader (see chainloader), like this:
grub> rootnoverify (hd1,0) grub> chainloader --force +1 grub> makeactive grub> boot
When you test a new kernel or a new OS, it is important to make sure that your computer can boot even if the new system is unbootable. This is crucial especially if you maintain servers or remote systems. To accomplish this goal, you need to set up two things:
The former requirement is very specific to each OS, so this documentation does not cover that topic. It is better to consult some backup tools.
So let’s see the GRUB part. There are two possibilities: one of them is quite simple but not very robust, and the other is a bit complex to set up but probably the best solution to make sure that your system can start as long as GRUB itself is bootable.
You can teach GRUB to boot an entry only at next boot time. Suppose that your have an old kernel old_kernel and a new kernel new_kernel. You know that old_kernel can boot your system correctly, and you want to test new_kernel.
To ensure that your system will go back to the old kernel even if the new kernel fails (e.g. it panics), you can specify that GRUB should try the new kernel only once and boot the old kernel after that.
First, modify your configuration file. Here is an example:
default saved # This is important!!! timeout 10 title the old kernel root (hd0,0) kernel /old_kernel savedefault title the new kernel root (hd0,0) kernel /new_kernel savedefault 0 # This is important!!!
Note that this configuration file uses ‘default saved’ (see default) at the head and ‘savedefault 0’ (see savedefault) in the entry for the new kernel. This means that GRUB boots a saved entry by default, and booting the entry for the new kernel saves ‘0’ as the saved entry.
With this configuration file, after all, GRUB always tries to boot the
old kernel after it booted the new one, because ‘0’ is the entry
the old kernel.
The next step is to tell GRUB to boot the new kernel at next boot
time. For this, execute
grub-set-default (see Invoking grub-set-default):
# grub-set-default 1
This command sets the saved entry to ‘1’, that is, to the new kernel.
This method is useful, but still not very robust, because GRUB stops booting, if there is any error in the boot entry, such that the new kernel has an invalid executable format. Thus, it it even better to use the fallback mechanism of GRUB. Look at next subsection for this feature.
GRUB supports a fallback mechanism of booting one or more other entries if a default boot entry fails. You can specify multiple fallback entries if you wish.
Suppose that you have three systems, ‘A’, ‘B’ and ‘C’. ‘A’ is a system which you want to boot by default. ‘B’ is a backup system which is supposed to boot safely. ‘C’ is another backup system which is used in case where ‘B’ is broken.
Then you may want GRUB to boot the first system which is bootable among ‘A’, ‘B’ and ‘C’. A configuration file can be written in this way:
default saved # This is important!!! timeout 10 fallback 1 2 # This is important!!! title A root (hd0,0) kernel /kernel savedefault fallback # This is important!!! title B root (hd1,0) kernel /kernel savedefault fallback # This is important!!! title C root (hd2,0) kernel /kernel savedefault
Note that ‘default saved’ (see default), ‘fallback 1 2’ and ‘savedefault fallback’ are used. GRUB will boot a saved entry by default and save a fallback entry as next boot entry with this configuration.
When GRUB tries to boot ‘A’, GRUB saves ‘1’ as next boot
entry, because the command
fallback specifies that ‘1’
is the first fallback entry. The entry ‘1’ is ‘B’, so GRUB
will try to boot ‘B’ at next boot time.
Likewise, when GRUB tries to boot ‘B’, GRUB saves ‘2’ as
next boot entry, because
fallback specifies ‘2’ as next
fallback entry. This makes sure that GRUB will boot ‘C’ after
It is noteworthy that GRUB uses fallback entries both when GRUB itself fails in booting an entry and when ‘A’ or ‘B’ fails in starting up your system. So this solution ensures that your system is started even if GRUB cannot find your kernel or if your kernel panics.
However, you need to run
grub-set-default (see Invoking grub-set-default) when ‘A’ starts correctly or you fix ‘A’
after it crashes, since GRUB always sets next boot entry to a fallback
entry. You should run this command in a startup script such as
rc.local to boot ‘A’ by default:
# grub-set-default 0
where ‘0’ is the number of the boot entry for the system ‘A’.
If you want to see what is current default entry, you can look at the
file /boot/grub/default (or /grub/default in
some systems). Because this file is plain-text, you can just
cat this file. But it is strongly recommended not to
modify this file directly, because GRUB may fail in saving a default
entry in this file, if you change this file in an unintended
manner. Therefore, you should use
grub-set-default when you
need to change the default entry.
You’ve probably noticed that you need to type several commands to boot your OS. There’s a solution to that - GRUB provides a menu interface (see The simple menu interface) from which you can select an item (using arrow keys) that will do everything to boot an OS.
To enable the menu, you need a configuration file, menu.lst under the boot directory. We’ll analyze an example file.
The file first contains some general settings, the menu interface
related options. You can put these commands (see The list of commands for the menu only) before any of the items (starting with
# # Sample boot menu configuration file #
As you may have guessed, these lines are comments. Lines starting with a hash character (‘#’), and blank lines, are ignored by GRUB.
# By default, boot the first entry. default 0
The first entry (here, counting starts with number zero, not one!) will be the default choice.
# Boot automatically after 30 secs. timeout 30
As the comment says, GRUB will boot automatically in 30 seconds, unless interrupted with a keypress.
# Fallback to the second entry. fallback 1
If, for any reason, the default entry doesn’t work, fall back to the second one (this is rarely used, for obvious reasons).
Note that the complete descriptions of these commands, which are menu interface specific, can be found in The list of commands for the menu only. Other descriptions can be found in The list of available commands.
Now, on to the actual OS definitions. You will see that each entry
begins with a special command,
title (see title), and the
action is described after it. Note that there is no command
boot (see boot) at the end of each item. That is because
GRUB automatically executes
boot if it loads other commands
The argument for the command
title is used to display a short
title/description of the entry in the menu. Since
displays the argument as is, you can write basically anything there.
# For booting GNU/Hurd title GNU/Hurd root (hd0,0) kernel /boot/gnumach.gz root=hd0s1 module /boot/serverboot.gz
This boots GNU/Hurd from the first hard disk.
# For booting GNU/Linux title GNU/Linux kernel (hd1,0)/vmlinuz root=/dev/hdb1
This boots GNU/Linux, but from the second hard disk.
# For booting Mach (getting kernel from floppy) title Utah Mach4 multiboot root (hd0,2) pause Insert the diskette now^G!! kernel (fd0)/boot/kernel root=hd0s3 module (fd0)/boot/bootstrap
This boots Mach with a kernel on a floppy, but the root filesystem at
hd0s3. It also contains a
pause line (see pause), which
will cause GRUB to display a prompt and delay, before actually executing
the rest of the commands and booting.
# For booting FreeBSD title FreeBSD root (hd0,2,a) kernel /boot/loader
This item will boot FreeBSD kernel loaded from the ‘a’ partition of the third PC slice of the first hard disk.
# For booting OS/2 title OS/2 root (hd0,1) makeactive # chainload OS/2 bootloader from the first sector chainloader +1 # This is similar to "chainload", but loads a specific file #chainloader /boot/chain.os2
This will boot OS/2, using a chain-loader (see Load another boot loader to boot unsupported operating systems).
# For booting Windows NT or Windows95 title Windows NT / Windows 95 boot menu root (hd0,0) makeactive chainloader +1 # For loading DOS if Windows NT is installed # chainload /bootsect.dos
The same as the above, but for Windows.
# For installing GRUB into the hard disk title Install GRUB into the hard disk root (hd0,0) setup (hd0)
This will just (re)install GRUB onto the hard disk.
# Change the colors. title Change the colors color light-green/brown blink-red/blue
In the last entry, the command
color is used (see color),
to change the menu colors (try it!). This command is somewhat special,
because it can be used both in the command-line and in the menu. GRUB
has several such commands, see The list of general commands.
We hope that you now understand how to use the basic features of GRUB. To learn more about GRUB, see the following chapters.
Although GRUB is a disk-based boot loader, it does provide network support. To use the network support, you need to enable at least one network driver in the GRUB build process. For more information please see netboot/README.netboot in the source distribution.
GRUB requires a file server and optionally a server that will assign an IP address to the machine on which GRUB is running. For the former, only TFTP is supported at the moment. The latter is either BOOTP, DHCP or a RARP server7. It is not necessary to run both the servers on one computer. How to configure these servers is beyond the scope of this document, so please refer to the manuals specific to those protocols/servers.
If you decided to use a server to assign an IP address, set up the
server and run
bootp (see bootp),
(see dhcp) or
rarp (see rarp) for BOOTP, DHCP or RARP,
respectively. Each command will show an assigned IP address, a netmask,
an IP address for your TFTP server and a gateway. If any of the
addresses is wrong or it causes an error, probably the configuration of
your servers isn’t set up properly.
ifconfig, like this:
grub> ifconfig --address=192.168.110.23 --server=192.168.110.14
You can also use
ifconfig in conjuction with
rarp (e.g. to reassign the server address
manually). See ifconfig, for more details.
Finally, download your OS images from your network. The network can be accessed using the network drive ‘(nd)’. Everything else is very similar to the normal instructions (see Booting).
Here is an example:
grub> bootp Probing... [NE*000] NE2000 base ... Address: 192.168.110.23 Netmask: 255.255.255.0 Server: 192.168.110.14 Gateway: 192.168.110.1 grub> root (nd) grub> kernel /tftproot/gnumach.gz root=sd0s1 grub> module /tftproot/serverboot.gz grub> boot
It is sometimes very useful to boot from a network, especially when you use a machine which has no local disk. In this case, you need to obtain a kind of Net Boot ROM, such as a PXE ROM or a free software package like Etherboot. Such a Boot ROM first boots the machine, sets up the network card installed into the machine, and downloads a second stage boot image from the network. Then, the second image will try to boot an operating system actually from the network.
GRUB provides two second stage images, nbgrub and pxegrub (see GRUB image files). These images are the same as the normal Stage 2, except that they set up a network automatically, and try to load a configuration file from the network, if specified. The usage is very simple: If the machine has a PXE ROM, use pxegrub. If the machine has an NBI loader such as Etherboot, use nbgrub. There is no difference between them except their formats. Since the way to load a second stage image you want to use should be described in the manual on your Net Boot ROM, please refer to the manual, for more information.
However, there is one thing specific to GRUB. Namely, how to specify a configuration file in a BOOTP/DHCP server. For now, GRUB uses the tag ‘150’, to get the name of a configuration file. The following is an example with a BOOTP configuration:
.allhost:hd=/tmp:bf=null:\ :ds=126.96.36.199 188.8.131.52:\ :sm=255.255.254.0:\ :gw=184.108.40.206:\ :sa=220.127.116.11: foo:ht=1:ha=63655d0334a7:ip=18.104.22.168:\ :bf=/nbgrub:\ :tc=.allhost:\ :T150="(nd)/tftpboot/menu.lst.foo":
Note that you should specify the drive name
(nd) in the name of
the configuration file. This is because you might change the root drive
before downloading the configuration from the TFTP server when the
preset menu feature is used (see Embedding a configuration file into GRUB).
See the manual of your BOOTP/DHCP server for more information. The exact syntax should differ a little from the example.
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. First of all, make sure that you haven’t specified the option --disable-serial to the configure script when you built your GRUB images. If you get them in binary form, probably they have serial terminal support already.
Then, initialize your serial terminal after GRUB starts up. Here is an example:
grub> serial --unit=0 --speed=9600 grub> terminal serial
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.
terminal (see terminal) chooses 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.
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.
GRUB supports a preset menu which is to be always loaded before
starting. The preset menu feature is useful, for example, when your
computer has no console but a serial cable. In this case, it is
critical to set up the serial terminal as soon as possible, since you
cannot see any message until the serial terminal begins to work. So it
is good to run the commands
serial (see serial) and
terminal (see terminal) before anything else at the
How the preset menu works is slightly complicated:
To enable the preset menu feature, you must rebuild GRUB specifying a file to the configure script with the option --enable-preset-menu. The file has the same semantics as normal configuration files (see Configuration).
Another point you should take care is that the diskless support
(see Booting from a network) diverts the preset menu. Diskless images embed a
preset menu to execute the command
bootp (see bootp)
automatically, unless you specify your own preset menu to the configure
script. This means that you must put commands to initialize a network in
the preset menu yourself, because diskless images don’t set it up
implicitly, when you use the preset menu explicitly.
Therefore, a typical preset menu used with diskless support would be like this:
# Set up the serial terminal, first of all. serial --unit=0 --speed=19200 terminal --timeout=0 serial # Initialize the network. dhcp
You may be interested in how to prevent ordinary users from doing whatever they like, if you share your computer with other people. So this chapter describes how to improve the security of GRUB.
One thing which could be a security hole is that the user can do too
many things with GRUB, because GRUB allows one to modify its configuration
and run arbitrary commands at run-time. For example, the user can even
read /etc/passwd in the command-line interface by the command
cat (see cat). So it is necessary to disable all the
Thus, GRUB provides a password feature, so that only administrators
can start the interactive operations (i.e. editing menu entries and
entering the command-line interface). To use this feature, you need to
run the command
password in your configuration file
(see password), like this:
password --md5 PASSWORD
If this is specified, GRUB disallows any interactive control, until you press the key p and enter a correct password. The option --md5 tells GRUB that ‘PASSWORD’ is in MD5 format. If it is omitted, GRUB assumes the ‘PASSWORD’ is in clear text.
grub> md5crypt Password: ********** Encrypted: $1$U$JK7xFegdxWH6VuppCUSIb.
Then, cut and paste the encrypted password to your configuration file.
Also, you can specify an optional argument to
password PASSWORD /boot/grub/menu-admin.lst
In this case, GRUB will load /boot/grub/menu-admin.lst as a configuration file when you enter the valid password.
Another thing which may be dangerous is that any user can choose any menu entry. Usually, this wouldn’t be problematic, but you might want to permit only administrators to run some of your menu entries, such as an entry for booting an insecure OS like DOS.
GRUB provides the command
lock (see lock). This command
always fails until you enter the valid password, so you can use it, like
title Boot DOS lock rootnoverify (hd0,1) makeactive chainload +1
You should insert
lock right after
any user can execute commands in an entry until GRUB encounters
You can also use the command
password instead of
lock. In this case the boot process will ask for the password
and stop if it was entered incorrectly. Since the
takes its own PASSWORD argument this is useful if you want
different passwords for different entries.
GRUB consists of several images: two essential stages, optional stages called Stage 1.5, one image for bootable CD-ROM, and two network boot images. Here is a short overview of them. See Hacking GRUB, for more details.
This is an essential image used for booting up GRUB. Usually, this is embedded in an MBR or the boot sector of a partition. Because a PC boot sector is 512 bytes, the size of this image is exactly 512 bytes.
All stage1 must do is to load Stage 2 or Stage 1.5 from a local disk. Because of the size restriction, stage1 encodes the location of Stage 2 (or Stage 1.5) in a block list format, so it never understand any filesystem structure.
This is the core image of GRUB. It does everything but booting up itself. Usually, this is put in a filesystem, but that is not required.
These are called Stage 1.5, because they serve as a bridge between stage1 and stage2, that is to say, Stage 1.5 is loaded by Stage 1 and Stage 1.5 loads Stage 2. The difference between stage1 and *_stage1_5 is that the former doesn’t understand any filesystem while the latter understands one filesystem (e.g. e2fs_stage1_5 understands ext2fs). So you can move the Stage 2 image to another location safely, even after GRUB has been installed.
While Stage 2 cannot generally be embedded in a fixed area as the size is so large, Stage 1.5 can be installed into the area right after an MBR, or the boot loader area of a ReiserFS or a FFS.
This is a boot image for CD-ROMs using the no emulation mode in El Torito specification. This is identical to Stage 2, except that this boots up without Stage 1 and sets up a special drive ‘(cd)’.
This is a network boot image for the Network Image Proposal used by some network boot loaders, such as Etherboot. This is mostly the same as Stage 2, but it also sets up a network and loads a configuration file from the network.
This is another network boot image for the Preboot Execution Environment used by several Netboot ROMs. This is identical to nbgrub, except for the format.
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
The device syntax is like this:
‘’ 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 zero for primary partitions and from four 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:
The syntax ‘(hd0)’ represents using the entire disk (or the MBR when installing GRUB), while the syntax ‘(hd0,0)’ 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, ‘(nd)’, is also available. Before using the network drive, you must initialize the network. See Downloading OS images from a 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 ‘(hd0,0)/boot/grub/menu.lst’. This means the file
/boot/grub/menu.lst 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, ‘(hd1,0)’ by the command
root (see root), then
/boot/kernel is the same as
A block list is used for specifying a file that doesn’t appear in the
filesystem, like a chainloader. The syntax is
Here is an example:
This represents that GRUB should read blocks 0 through 99, block 200, and blocks 300 through 599. If you omit an offset, then GRUB assumes the offset is zero.
Like the file name syntax (see How to specify files), if a blocklist
does not contain a device name, then GRUB uses GRUB’s root
(hd0,1)+1 is the same as
+1 when the root
device is ‘(hd0,1)’.
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 entered8. The commands (see The list of 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:
Move forward one character.
Move back one character.
Move to the start of the line.
Move the the end of the line.
Delete the character underneath the cursor.
Delete the character to the left of the cursor.
Kill the text from the current cursor position to the end of the line.
Kill backward from the cursor to the beginning of the line.
Yank the killed text back into the buffer at the cursor.
Move up through the history list.
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
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 Protecting your computer from cracking), 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.
When a particular line is selected, the editor places the user in a special version of the GRUB command-line to edit that line. When the user hits RET, GRUB replaces the line in question in the boot entry with the changes (unless it was aborted via ESC, in which case the changes are thrown away).
If you want to add a new line to the menu entry, press o if adding a line after the current line or press O if before the current line.
To delete a line, hit the key d. Although GRUB unfortunately does not support undo, you can do almost the same thing by just returning to the main menu.
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.
The semantics used in parsing the configuration file are the following:
These commands can only be used in the menu:
Set the default entry to the entry number num. Numbering starts from 0, and the entry number 0 is the default if the command is not used.
You can specify ‘saved’ instead of a number. In this case, the
default entry is the entry saved with the command
savedefault. See savedefault, for more information.
Go into unattended boot mode: if the default boot entry has any errors,
instead of waiting for the user to do something, immediately start
over using the num entry (same numbering as the
command (see default)). This obviously won’t help if the machine was
rebooted by a kernel that GRUB loaded. You can specify multiple
fallback entry numbers.
Set a timeout, in sec seconds, before automatically booting the default entry (normally the first entry defined).
Commands usable anywhere in the menu and in the command-line.
Initialize a network device via the BOOTP protocol. This command is only available if GRUB is compiled with netboot support. See also Downloading OS images from a network.
If you specify --with-configfile to this command, GRUB will fetch and load a configuration file specified by your BOOTP server with the vendor tag ‘150’.
Change the menu colors. The color normal is used for most
lines in the menu (see The simple menu interface), and the color
highlight is used to highlight the line where the cursor
points. If you omit highlight, then the inverted color of
normal is used for the highlighted line. The format of a color is
foreground/background. foreground and
background are symbolic color names. A symbolic color name must be
one of these:
These below can be specified only for the foreground.
But only the first eight names can be used for background. You can
blink- to foreground if you want a blinking
This command can be used in the configuration file and on the command line, so you may write something like this in your configuration file:
# Set default colors. color light-gray/blue black/light-gray # Change the colors. title OS-BS like color magenta/blue black/magenta
In the grub shell, specify the file file as the actual drive for a BIOS drive drive. You can use this command to create a disk image, and/or to fix the drives guessed by GRUB when GRUB fails to determine them correctly, like this:
grub> device (fd0) /floppy-image grub> device (hd0) /dev/sd0
This command can be used only in the grub shell (see Invoking the grub shell).
Initialize a network device via the DHCP protocol. Currently,
this command is just an alias for
bootp, since the two
protocols are very similar. This command is only available if GRUB is
compiled with netboot support. See also Downloading OS images from a network.
If you specify --with-configfile to this command, GRUB will fetch and load a configuration file specified by your DHCP server with the vendor tag ‘150’.
Hide the partition partition by setting the hidden bit in its partition type code. This is useful only when booting DOS or Windows and multiple primary FAT partitions exist in one disk. See also DOS/Windows.
Configure the IP address, the netmask, the gateway, and the server address of a network device manually. The values must be in dotted decimal format, like ‘192.168.11.178’. The order of the options is not important. This command shows current network configuration, if no option is specified. See also Downloading OS images from a network.
Toggle or set the state of the internal pager. If flag is ‘on’, the internal pager is enabled. If flag is ‘off’, it is disabled. If no argument is given, the state is toggled.
Create a new primary partition. part is a partition specification
in GRUB syntax (see Naming convention); type is the partition
type and must be a number in the range
0-0xff; from is
the starting address and len is the length, both in sector units.
Change the type of an existing partition. part is a partition specification in GRUB syntax (see Naming convention); type is the new partition type and must be a number in the range 0-0xff.
If used in the first section of a menu file, disable all interactive
editing control (menu entry editor and command-line) and entries
protected by the command
lock. If the password passwd is
entered, it loads the new-config-file as a new config file and
restarts the GRUB Stage 2, if new-config-file is
specified. Otherwise, GRUB will just unlock the privileged instructions.
You can also use this command in the script section, in which case it
will ask for the password, before continuing. The option
--md5 tells GRUB that passwd is encrypted with
md5crypt (see md5crypt).
Initialize a network device via the RARP protocol. This command is only available if GRUB is compiled with netboot support. See also Downloading OS images from a network.
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 option --device can only be used in the grub shell and is used to specify the tty device to be used in the host operating system (see Invoking the grub shell).
The serial port is not used as a communication channel unless the
terminal command is used (see terminal).
This command is only available if GRUB is compiled with serial support. See also Using GRUB via a serial line.
Change the keyboard map. The key from_key is mapped to the key to_key. If no argument is specified, reset key mappings. Note that this command does not exchange the keys. If you want to exchange the keys, run this command again with the arguments exchanged, like this:
grub> setkey capslock control grub> setkey control capslock
A key must be an alphabet letter, a digit, or one of these symbols: ‘escape’, ‘exclam’, ‘at’, ‘numbersign’, ‘dollar’, ‘percent’, ‘caret’, ‘ampersand’, ‘asterisk’, ‘parenleft’, ‘parenright’, ‘minus’, ‘underscore’, ‘equal’, ‘plus’, ‘backspace’, ‘tab’, ‘bracketleft’, ‘braceleft’, ‘bracketright’, ‘braceright’, ‘enter’, ‘control’, ‘semicolon’, ‘colon’, ‘quote’, ‘doublequote’, ‘backquote’, ‘tilde’, ‘shift’, ‘backslash’, ‘bar’, ‘comma’, ‘less’, ‘period’, ‘greater’, ‘slash’, ‘question’, ‘alt’, ‘space’, ‘capslock’, ‘FX’ (‘X’ is a digit), and ‘delete’. This table describes to which character each of the symbols corresponds:
Select an image to use as the background image. This should be specified using normal GRUB device naming syntax. The format of the file is a gzipped xpm which is 640x480 with a 14 color palette.
Select a terminal for user interaction. The terminal is assumed to be VT100-compatible unless --dumb is specified. If both console and serial are specified, then GRUB will use the one where a key is entered first or the first when the timeout expires. If neither are specified, the current setting is reported. This command is only available if GRUB is compiled with serial support. See also Using GRUB via a serial line.
This may not make sense for most users, but GRUB supports Hercules console as well. Hercules console is usable like the ordinary console, and the usage is quite similar to that for serial terminals: specify hercules as the argument.
The option --lines defines the number of lines in your terminal, and it is used for the internal pager function. If you don’t specify this option, the number is assumed as 24.
The option --silent suppresses the message to prompt you to hit any key. This might be useful if your system has no terminal device.
The option --no-echo has GRUB not to echo back input characters. This implies the option --no-edit.
The option --no-edit disables the BASH-like editing feature.
Define the capabilities of your terminal. Use this command to define escape sequences, if it is not vt100-compatible. You may use ‘\e’ for ESC and ‘^X’ for a control character.
You can use the utility
grub-terminfo to generate
appropriate arguments to this command. See Invoking grub-terminfo.
If no option is specified, the current settings are printed.
Caution: This command exists only for backward
ifconfig (see ifconfig) instead.
Override a TFTP server address returned by a BOOTP/DHCP/RARP server. The argument ipaddr must be in dotted decimal format, like ‘192.168.0.15’. This command is only available if GRUB is compiled with netboot support. See also Downloading OS images from a network.
Unhide the partition partition by clearing the hidden bit in its partition type code. This is useful only when booting DOS or Windows and multiple primary partitions exist on one disk. See also DOS/Windows.
This chapter describes error messages reported by GRUB when you encounter trouble. See Invoking the grub shell, if your problem is specific to the grub shell.
The general way that the Stage 1 handles errors is to print an error string and then halt. Pressing CTRL-ALT-DEL will reboot.
The following is a comprehensive list of error messages for the Stage 1:
The stage2 or stage1.5 is being read from a hard disk, and the attempt to determine the size and geometry of the hard disk failed.
The stage2 or stage1.5 is being read from a floppy disk, and the attempt to determine the size and geometry of the floppy disk failed. It’s listed as a separate error since the probe sequence is different than for hard disks.
A disk read error happened while trying to read the stage2 or stage1.5.
The location of the stage2 or stage1.5 is not in the portion of the disk supported directly by the BIOS read calls. This could occur because the BIOS translated geometry has been changed by the user or the disk is moved to another machine or controller after installation, or GRUB was not installed using itself (if it was, the Stage 2 version of this error would have been seen during that process and it would not have completed the install).
The general way that the Stage 1.5 handles errors is to print an error
number in the form
Error num and then halt. Pressing
CTRL-ALT-DEL will reboot.
The error numbers correspond to the errors reported by Stage 2. See Errors reported by the Stage 2.
The general way that the Stage 2 handles errors is to abort the operation in question, print an error string, then (if possible) either continue based on the fact that an error occurred or wait for the user to deal with the error.
The following is a comprehensive list of error messages for the Stage 2 (error numbers for the Stage 1.5 are listed before the colon in each description):
This error is returned if a file name is requested which doesn’t fit the syntax/rules listed in the Filesystem syntax and semantics.
This error is returned if a file requested is not a regular file, but something like a symbolic link, directory, or FIFO.
This error is returned if the run-length decompression code gets an internal error. This is usually from a corrupt file.
This error is returned if the file header for a supposedly compressed file is bad.
This error is returned if the sanity checks on the integrity of the partition table fail. This is a bad sign.
This error is returned if the install command points to incompatible or corrupt versions of the stage1 or stage2. It can’t detect corruption in general, but this is a sanity check on the version numbers, which should be correct.
This error is returned if the lowest address in a kernel is below the 1MB boundary. The Linux zImage format is a special case and can be handled since it has a fixed loading address and maximum size.
This error is returned if GRUB is told to execute the boot sequence without having a kernel to start.
This error is returned if the boot attempt did not succeed for reasons which are unknown.
This error is returned when the Multiboot features word in the Multiboot header requires a feature that is not recognized. The point of this is that the kernel requires special handling which GRUB is probably unable to provide.
This error is returned if a device string was expected, and the string encountered didn’t fit the syntax/rules listed in the Filesystem syntax and semantics.
This error is returned if a device string is recognizable but does not fall under the other device errors.
This error is returned if the kernel image being loaded is not recognized as Multiboot or one of the supported native formats (Linux zImage or bzImage, FreeBSD, or NetBSD).
Some of the filesystem reading code in GRUB has limits on the length of the files it can read. This error is returned when the user runs into such a limit.
This error is returned if the specified file name cannot be found, but everything else (like the disk/partition info) is OK.
This error is returned by the filesystem code to denote an internal error caused by the sanity checks of the filesystem structure on disk not matching what it expects. This is usually caused by a corrupt filesystem or bugs in the code handling it in GRUB.
This error is returned if the partition requested exists, but the filesystem type cannot be recognized by GRUB.
This error is returned when a read is attempted at a linear block address beyond the end of the BIOS translated area. This generally happens if your disk is larger than the BIOS can handle (512MB for (E)IDE disks on older machines or larger than 8GB in general).
This error is returned if the initrd command is used before loading a Linux kernel.
This error is returned if the module load command is used before loading a Multiboot kernel. It only makes sense in this case anyway, as GRUB has no idea how to communicate the presence of such modules to a non-Multiboot-aware kernel.
This error is returned if the device part of a device- or full file name refers to a disk or BIOS device that is not present or not recognized by the BIOS in the system.
This error is returned if a partition is requested in the device part of a device- or full file name which isn’t on the selected disk.
This error is returned if GRUB was expecting to read a number and encountered bad data.
This error is returned if a linear block address is outside of the disk partition. This generally happens because of a corrupt filesystem on the disk or a bug in the code handling it in GRUB (it’s a great debugging tool).
This error is returned if there is a disk read error when trying to probe or read data from a particular disk.
This error is returned if the link count is beyond the maximum (currently 5), possibly the symbolic links are looped.
This error is returned if an unrecognized command is entered on the command-line or in a boot sequence section of a configuration file and that entry is selected.
This error is returned if a kernel, module, or raw file load command is either trying to load its data such that it won’t fit into memory or it is simply too big.
This error is returned if there is a disk write error when trying to write to a particular disk. This would generally only occur during an install of set active partition command.
This error is returned if an argument specified to a command is invalid.
This error may occur only when you access a ReiserFS partition by
block-lists (e.g. the command
install). In this case, you
should mount the partition with the ‘-o notail’ option.
This error is returned if you try to run a locked entry. You should enter a correct password before running such an entry.
This error is returned if you try to change your terminal to a serial one before initializing any serial device.
This error is returned if a disk doesn’t have enough spare space. This happens when you try to embed Stage 1.5 into the unused sectors after the MBR, but the first partition starts right after the MBR or they are used by EZ-BIOS.
This chapter documents the grub shell
grub. Note that the grub
shell is an emulator; it doesn’t run under the native environment, so it
sometimes does something wrong. Therefore, you shouldn’t trust it too
much. If there is anything wrong with it, don’t hesitate to try the
native GRUB environment, especially when it guesses a wrong map between
BIOS drives and OS devices.
You can use the command
grub for installing GRUB under your
operating systems and for a testbed when you add a new feature into GRUB
or when fixing a bug.
grub is almost the same as the Stage 2,
and, in fact, it shares the source code with the Stage 2 and you can use
the same commands (see The list of available commands) in
grub. It is emulated by
replacing BIOS calls with UNIX system calls and libc functions.
grub accepts the following options:
Print a summary of the command-line options and exit.
Print the version number of GRUB and exit.
Print some verbose messages for debugging purpose.
Use the device map file file. The format is described in The map between BIOS drives and OS devices.
Do not probe any floppy drive. This option has no effect if the option --device-map is specified (see The map between BIOS drives and OS devices).
Probe the second floppy drive. If this option is not specified, the grub shell does not probe it, as that sometimes takes a long time. If you specify the device map file (see The map between BIOS drives and OS devices), the grub shell just ignores this option.
Read the configuration file file instead of /boot/grub/menu.lst. The format is the same as the normal GRUB syntax. See Filesystem syntax and semantics, for more information.
Set the stage2 boot_drive to drive. This argument should be an integer (decimal, octal or hexadecimal).
Set the stage2 install_partition to par. This argument should be an integer (decimal, octal or hexadecimal).
Do not use the configuration file even if it can be read.
Do not use the screen handling interface by the curses even if it is available.
This option has the same meaning as ‘--no-config-file --no-curses’.
Disable writing to any disk.
Wait until a debugger will attach. This option is useful when you want to debug the startup code.
The installation procedure is the same as under the native Stage
2. See Installation, for more information. The command
grub-specific information is described here.
What you should be careful about is buffer cache.
makes use of raw devices instead of filesystems that your operating
systems serve, so there exists a potential problem that some cache
inconsistency may corrupt your filesystems. What we recommend is:
In addition, enter the command
quit when you finish the
installation. That is very important because
the buffer cache consistent. Do not push C-c.
If you want to install GRUB non-interactively, specify ‘--batch’ option in the command-line. This is a simple example:
#!/bin/sh # Use /usr/sbin/grub if you are on an older system. /sbin/grub --batch <<EOT 1>/dev/null 2>/dev/null root (hd0,0) setup (hd0) quit EOT
When you specify the option --device-map (see Introduction into the grub shell), the grub shell creates the device map file automatically unless it already exists. The file name /boot/grub/device.map is preferred.
If the device map file exists, the grub shell reads it to map BIOS drives to OS devices. This file consists of lines like this:
device is a drive specified in the GRUB syntax (see How to specify devices), and file is an OS file, which is normally a device file.
The reason why the grub shell gives you the device map file is that it cannot guess the map between BIOS drives and OS devices correctly in some environments. For example, if you exchange the boot sequence between IDE and SCSI in your BIOS, it gets the order wrong.
Thus, edit the file if the grub shell makes a mistake. You can put any comments in the file if needed, as the grub shell assumes that a line is just a comment if the first character is ‘#’.
grub-install installs GRUB on your drive using the
grub shell (see Invoking the grub shell). You must specify the
device name on which you want to install GRUB, like this:
The device name install_device is an OS device name or a GRUB device name.
grub-install accepts the following options:
Print a summary of the command-line options and exit.
Print the version number of GRUB and exit.
Force GRUB to use LBA mode even for a buggy BIOS. Use this option only if your BIOS doesn’t work properly in LBA mode even though it supports LBA mode.
Install GRUB images under the directory dir instead of the root directory. This option is useful when you want to install GRUB into a separate partition or a removable disk. Here is an example in which you have a separate boot partition which is mounted on /boot:
grub-install --root-directory=/boot hd0
Use file as the grub shell. You can append arbitrary options to file after the file name, like this:
grub-install --grub-shell="grub --read-only" /dev/fd0
Recheck the device map, even if /boot/grub/device.map already exists. You should use this option whenever you add/remove a disk into/from your computer.
grub-md5-crypt encrypts a password in MD5 format.
This is just a frontend of the grub shell (see Invoking the grub shell). Passwords encrypted by this program can be used with the
password (see password).
grub-md5-crypt accepts the following options:
Print a summary of the command-line options and exit.
Print the version information and exit.
Use file as the grub shell.
grub-terminfo generates a terminfo command from
a terminfo name (see terminfo). The result can be used in the
configuration file, to define escape sequences. Because GRUB assumes
that your terminal is vt100-compatible by default, this would be
useful only if your terminal is uncommon (such as vt52).
grub-terminfo accepts the following options:
Print a summary of the command-line options and exit.
Print the version information and exit.
You must specify one argument to this command. For example:
grub-set-default sets the default boot entry for
GRUB. This automatically creates a file named default under
your GRUB directory (i.e. /boot/grub), if it is not
present. This file is used to determine the default boot entry when
GRUB boots up your system when you use ‘default saved’ in your
configuration file (see default), and to save next default boot
entry when you use ‘savedefault’ in a boot entry
grub-set-default accepts the following options:
Print a summary of the command-line options and exit.
Print the version information and exit.
Use the directory dir instead of the root directory (i.e. /) to define the location of the default file. This is useful when you mount a disk which is used for another system.
You must specify a single argument to
argument is normally the number of a default boot entry. For example,
if you have this configuration file:
default saved timeout 10 title GNU/Hurd root (hd0,0) ... title GNU/Linux root (hd0,1) ...
and if you want to set the next default boot entry to GNU/Linux, you may execute this command:
Because the entry for GNU/Linux is ‘1’. Note that entries are counted from zero. So, if you want to specify GNU/Hurd here, then you should specify ‘0’.
This feature is very useful if you want to test a new kernel or to make your system quite robust. See How to make your system robust, for more hints about how to set up a robust system.
mbchk checks for the format of a Multiboot
kernel. We recommend using this program before booting your own kernel
mbchk accepts the following options:
Print a summary of the command-line options and exit.
Print the version number of GRUB and exit.
Suppress all normal output.
Caution: GRUB requires binutils-22.214.171.124.23 or later because the GNU assembler has been changed so that it can produce real 16bits machine code between 2.9.1 and 126.96.36.199.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 0.97, so the file you should grab is:
To unbundle GRUB use the instruction:
zcat grub-0.97.tar.gz | tar xvf -
which will create a directory called grub-0.97 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-0.97 ./configure make install
Also, the latest version is available from the CVS. See http://savannah.gnu.org/cvs/?group=grub 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 firstname.lastname@example.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. This will include internationalization, dynamic module loading, real memory management, multiple architecture support, a scripting language, and many other nice feature. If you are interested in the development of GRUB 2, take a look at the homepage.
This chapter documents the user-invisible aspect of GRUB.
As a general rule of software development, it is impossible to keep the descriptions of the internals up-to-date, and it is quite hard to document everything. So refer to the source code, whenever you are not satisfied with this documentation. Please assume that this gives just hints to you.
GRUB consists of two distinct components, called stages, which are loaded at different times in the boot process. Because they run mutual-exclusively, sometimes a memory area overlaps with another memory area. And, even in one stage, a single memory area can be used for various purposes, because their usages are mutually exclusive.
Here is the memory map of the various components:
BIOS and real mode interrupts
Partition table passed to another boot loader
Real mode stack
The optional Stage 1.5 is loaded here
Command-line buffer for Multiboot kernels and modules
Stage 1 is loaded here by BIOS or another boot loader
LBA drive parameters
Stage2 is loaded here
Heap, in particular used for the menu
Protected mode stack
Raw device buffer
512-byte scratch area
Buffers for various functions, such as password, command-line, cut and paste, and completion.
Disk swapping code and data
See the file stage2/shared.h, for more information.
Stage 1 and Stage 2 have embedded variables whose locations are well-defined, so that the installation can patch the binary file directly without recompilation of the stages.
In Stage 1, these are defined:
The version number (not GRUB’s, but the installation mechanism’s).
The boot drive. If it is 0xFF, use a drive passed by BIOS.
The flag for if forcing LBA.
The starting address of Stage 2.
The first sector of Stage 2.
The starting segment of Stage 2.
The signature (
See the file stage1/stage1.S, for more information.
In the first sector of Stage 1.5 and Stage 2, the block lists are
lastlist. The address of
lastlist is determined when assembling the file
The trick here is that it is actually read backward, and the first 8-byte block list is not read here, but after the pointer is decremented 8 bytes, then after reading it, it decrements again, reads, and so on, until it is finished. The terminating condition is when the number of sectors to be read in the next block list is zero.
The format of a block list can be seen from the example in the code just
firstlist label. Note that it is always from the
beginning of the disk, but not relative to the partition
In the second sector of Stage 1.5 and Stage 2, these are defined:
The version number (likewise, the installation mechanism’s).
The installed partition.
The saved entry number.
The flag for if forcing LBA.
The version string (GRUB’s).
0x12+ the length of the version string
The name of a configuration file.
See the file stage2/asm.S, for more information.
For any particular partition, it is presumed that only one of the normal filesystems such as FAT, FFS, or ext2fs can be used, so there is a switch table managed by the functions in disk_io.c. The notation is that you can only mount one at a time.
The block list filesystem has a special place in the system. In addition to the normal filesystem (or even without one mounted), you can access disk blocks directly (in the indicated partition) via the block list notation. Using the block list filesystem doesn’t effect any other filesystem mounts.
The variables which can be read by the filesystem backend are:
The current BIOS drive number (numbered from 0, if a floppy, and numbered from 0x80, if a hard disk).
The current partition number.
The current partition type.
The drive part of the root device.
The partition part of the root device.
The current partition starting address, in sectors.
The current partition length, in sectors.
True when the
dir function should print the possible completions
of a file, and false when it should try to actually open a file of that
Filesystem buffer which is 32K in size, to use in any way which the filesystem backend desires.
The variables which need to be written by a filesystem backend are:
The current position in the file, in sectors.
Caution: the value of filepos can be changed out from under the filesystem code in the current implementation. Don’t depend on it being the same for later calls into the backend code!
The length of the file.
The value of disk_read_hook only during reading of data
for the file, not any other fs data, inodes, FAT tables, whatever, then
NULL at all other times (it will be
default). If this isn’t done correctly, then the
install commands won’t work correctly.
The functions expected to be used by the filesystem backend are:
Only read sectors from within a partition. Sector 0 is the first sector in the partition.
If the backend uses the block list code, then
grub_read can be
used, after setting block_file to 1.
If print_possibilities is true, call
each possible file name. Otherwise, the file name completion won’t work.
The functions expected to be defined by the filesystem backend are described at least moderately in the file filesys.h. Their usage is fairly evident from their use in the functions in disk_io.c, look for the use of the fsys_table array.
Caution: The semantics are such that then ‘mount’ing the
filesystem, presume the filesystem buffer
FSYS_BUF is corrupted,
and (re-)load all important contents. When opening and reading a file,
presume that the data from the ‘mount’ is available, and doesn’t
get corrupted by the open/read (i.e. multiple opens and/or reads will be
done with only one mount if in the same filesystem).
GRUB built-in commands are defined in a uniformal interface, whether they are menu-specific or can be used anywhere. The definition of a builtin command consists of two parts: the code itself and the table of the information.
The code must be a function which takes two arguments, a command-line string and flags, and returns an ‘int’ value. The flags argument specifies how the function is called, using a bit mask. The return value must be zero if successful, otherwise non-zero. So it is normally enough to return errnum.
The table of the information is represented by the structure
struct builtin, which contains the name of the command, a pointer
to the function, flags, a short description of the command and a long
description of the command. Since the descriptions are used only for
help messages interactively, you don’t have to define them, if the
command may not be called interactively (such as
The table is finally registered in the table builtin_table, so
enter_cmdline can find the
command. See the files cmdline.c and builtins.c, for more
The disk space can be used in a boot loader is very restricted because a MBR (see The structure of Master Boot Record) is only 512 bytes but it also contains a partition table (see The format of partition tables) and a BPB. So the question is how to make a boot loader code enough small to be fit in a MBR.
However, GRUB is a very large program, so we break GRUB into 2 (or 3) distinct components, Stage 1 and Stage 2 (and optionally Stage 1.5). See The memory map of various components, for more information.
We embed Stage 1 in a MBR or in the boot sector of a partition, and place Stage 2 in a filesystem. The optional Stage 1.5 can be installed in a filesystem, in the boot loader area in a FFS or a ReiserFS, and in the sectors right after a MBR, because Stage 1.5 is enough small and the sectors right after a MBR is normally an unused region. The size of this region is the number of sectors per head minus 1.
Thus, all Stage1 must do is just load Stage2 or Stage1.5. But even if Stage 1 needs not to support the user interface or the filesystem interface, it is impossible to make Stage 1 less than 400 bytes, because GRUB should support both the CHS mode and the LBA mode (see INT 13H disk I/O interrupts).
The solution used by GRUB is that Stage 1 loads only the first sector of Stage 2 (or Stage 1.5) and Stage 2 itself loads the rest. The flow of Stage 1 is:
The flow of Stage 2 (and Stage 1.5) is:
Note that Stage 2 (or Stage 1.5) does not probe the geometry or the accessing mode of the loading drive, since Stage 1 has already probed them.
FIXME: I will write this chapter after implementing the new technique.
FIXME: I doubt if Erich didn’t write this chapter only himself wholly, so I will rewrite this chapter.
FIXME: I’m not sure where some part of the original chapter is derived, so I will rewrite this chapter.
FIXME: Probably the original chapter is derived from "How It Works", so I will rewrite this chapter.
When you write patches for GRUB, please send them to the mailing list email@example.com. Here is the list of items of which you should take care:
chain-load is the mechanism for loading unsupported operating systems by loading another boot loader. It is typically used for loading DOS or Windows.
There are a few pathological cases where loading a very badly organized ELF kernel might take longer, but in practice this never happen.
The LInux LOader, a boot loader that everybody uses, but nobody likes.
Note that GRUB’s root device doesn’t necessarily mean
your OS’s root partition; if you need to specify a root partition for
your OS, add the argument into the command
El Torito is a specification for bootable CD using BIOS functions.
This is not necessary for most of the modern operating systems.
RARP is not advised, since it cannot serve much information
However, this behavior will be changed in the future version, in a user-invisible way.
The latter feature has not been implemented yet.
They’re loaded the same way, so we will refer to the Stage 1.5 as a Stage 2 from now on.