grub/docs/grub.texi

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@settitle GNU GRUB Manual @value{VERSION}
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@copying
This manual is for GNU GRUB (version @value{VERSION},
@value{UPDATED}).

Copyright @copyright{} 1999,2000,2001,2002,2004,2006,2008,2009,2010 Free Software Foundation, Inc.

@quotation
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.
@end quotation
@end copying

@dircategory Kernel
@direntry
* GRUB: (grub).                 The GRand Unified Bootloader
* grub-install: (grub)Invoking grub-install.    Install GRUB on your drive
* grub-mkconfig: (grub)Invoking grub-mkconfig.  Generate GRUB configuration
* grub-mkpasswd-pbkdf2: (grub)Invoking grub-mkpasswd-pbkdf2.
@end direntry

@setchapternewpage odd

@titlepage
@sp 10
@title the GNU GRUB manual
@subtitle The GRand Unified Bootloader, version @value{VERSION}, @value{UPDATED}.
@author Gordon Matzigkeit
@author Yoshinori K. Okuji
@c The following two commands start the copyright page.
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@vskip 0pt plus 1filll
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@c Output the table of contents at the beginning.
@contents

@finalout
@headings double

@ifnottex
@node Top
@top GNU GRUB manual

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

This edition documents version @value{VERSION}.

@insertcopying
@end ifnottex

@menu
* Introduction::                Capturing the spirit of GRUB
* Naming convention::           Names of your drives in GRUB
* Installation::                Installing GRUB on your drive
* Booting::                     How to boot different operating systems
* Configuration::               Writing your own configuration file
* Network::                     Downloading OS images from a network
* Serial terminal::             Using GRUB via a serial line
* Vendor power-on keys::        Changing GRUB behaviour on vendor power-on keys
* Images::                      GRUB image files
* Filesystem::                  Filesystem syntax and semantics
* Interface::                   The menu and the command-line
* Commands::                    The list of available builtin commands
* Security::                    Authentication and authorisation
* Troubleshooting::             Error messages produced by GRUB
* Invoking grub-install::       How to use the GRUB installer
* Invoking grub-mkconfig::      Generate a GRUB configuration file
* Invoking grub-mkpasswd-pbkdf2::
                                Generate GRUB password hashes
* Obtaining and Building GRUB:: How to obtain and build GRUB
* Reporting bugs::              Where you should send a bug report
* Future::                      Some future plans on GRUB
* Internals::                   Hacking GRUB
* Copying This Manual::         Copying This Manual
* Index::
@end menu


@node Introduction
@chapter Introduction to GRUB

@menu
* Overview::                    What exactly GRUB is and how to use it
* History::                     From maggot to house fly
* Features::                    GRUB features
* Changes from GRUB Legacy::    Differences from previous versions
* Role of a boot loader::       The role of a boot loader
@end menu


@node Overview
@section Overview

Briefly, a @dfn{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 @dfn{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-loading@footnote{@dfn{chain-load} is the mechanism for loading
unsupported operating systems by loading another boot loader. It is
typically used for loading DOS or Windows.}. 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
(@pxref{Command-line interface}), or a menu interface (@pxref{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
(@pxref{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 (@pxref{Naming convention}) to GRUB, how to
install GRUB on your drive (@pxref{Installation}), and how to boot your
OSes (@pxref{Booting}), step by step.


@node History
@section History of GRUB

GRUB originated in 1995 when Erich Boleyn was trying to boot the GNU
Hurd with the University of Utah's Mach 4 microkernel (now known as GNU
Mach).  Erich and Brian Ford designed the Multiboot Specification
(@pxref{Top, Multiboot Specification, Motivation, multiboot, 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. @xref{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.


@node Changes from GRUB Legacy
@section Differences from previous versions

GRUB 2 is a rewrite of GRUB (@pxref{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.

@itemize @bullet
@item
The configuration file has a new name (@file{grub.cfg} rather than
@file{menu.lst} or @file{grub.conf}), new syntax (@pxref{Configuration}) and
many new commands (@pxref{Commands}).  Configuration cannot be copied over
directly, although most GRUB Legacy users should not find the syntax too
surprising.

@item
@file{grub.cfg} is typically automatically generated by
@command{grub-mkconfig} (@pxref{Simple configuration}).  This makes it
easier to handle versioned kernel upgrades.

@item
Partition numbers in GRUB device names now start at 1, not 0 (@pxref{Naming
convention}).

@item
The configuration file is now written in something closer to a full
scripting language: variables, conditionals, and loops are available.

@item
A small amount of persistent storage is available across reboots, using the
@command{save_env} and @command{load_env} commands in GRUB and the
@command{grub-editenv} utility.

@item
GRUB 2 has more reliable ways to find its own files and those of target
kernels on multiple-disk systems, and has commands (@pxref{search}) to find
devices using file system labels or Universally Unique Identifiers (UUIDs).

@item
GRUB 2 is available for several other types of system in addition to the PC
BIOS systems supported by GRUB Legacy: PC EFI, PC coreboot, PowerPC, SPARC,
and MIPS Lemote Yeeloong are all supported.

@item
Many more file systems are supported, including but not limited to ext4,
HFS+, and NTFS.

@item
GRUB 2 can read files directly from LVM and RAID devices.

@item
A graphical terminal and a graphical menu system are available.

@item
GRUB 2's interface can be translated, including menu entry names.

@item
The image files (@pxref{Images}) that make up GRUB have been reorganised;
Stage 1, Stage 1.5, and Stage 2 are no more.

@item
GRUB 2 puts many facilities in dynamically loaded modules, allowing the core
image to be smaller, and allowing the core image to be built in more
flexible ways.
@end itemize


@node Features
@section GRUB features

The primary requirement for GRUB is that it be compliant with the
@dfn{Multiboot Specification}, which is described in @ref{Top, Multiboot
Specification, Motivation, multiboot, The Multiboot Specification}.

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

@itemize @bullet{}
@item
Basic functions must be straightforward for end-users.

@item
Rich functionality to support kernel experts and designers.

@item
Backward compatibility for booting FreeBSD, NetBSD, OpenBSD, and
Linux. Proprietary kernels (such as DOS, Windows NT, and OS/2) are
supported via a chain-loading function.
@end itemize

Except for specific compatibility modes (chain-loading and the Linux
@dfn{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):

@table @asis
@item Recognize multiple executable formats
Support many of the @dfn{a.out} variants plus @dfn{ELF}. Symbol
tables are also loaded.

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

@item Load multiples modules
Fully support the Multiboot feature of loading multiple modules.

@item Load a configuration file
Support a human-readable text configuration file with preset boot
commands. You can also load another configuration file dynamically and
embed a preset configuration file in a GRUB image file. The list of
commands (@pxref{Commands}) are a superset of those supported on the
command-line. An example configuration file is provided in
@ref{Configuration}.

@item Provide a menu interface
A menu interface listing preset boot commands, with a programmable
timeout, is available. There is no fixed limit on the number of boot
entries, and the current implementation has space for several hundred.

@item Have a flexible command-line interface
A fairly flexible command-line interface, accessible from the menu,
is available to edit any preset commands, or write a new boot command
set from scratch. If no configuration file is present, GRUB drops to
the command-line.

The list of commands (@pxref{Commands}) are a subset of those supported
for configuration files. Editing commands closely resembles the Bash
command-line (@pxref{Command Line Editing, Bash, Command Line Editing,
features, Bash Features}), with @key{TAB}-completion of commands,
devices, partitions, and files in a directory depending on context.

@item Support multiple filesystem types
Support multiple filesystem types transparently, plus a useful explicit
blocklist notation. The currently supported filesystem types are @dfn{Amiga
Fast FileSystem (AFFS)}, @dfn{AtheOS fs}, @dfn{BeFS}, @dfn{cpio}, @dfn{Linux
ext2/ext3/ext4}, @dfn{DOS FAT12/FAT16/FAT32}, @dfn{HFS}, @dfn{HFS+},
@dfn{ISO9660}, @dfn{JFS}, @dfn{Minix fs}, @dfn{nilfs2}, @dfn{NTFS},
@dfn{ReiserFS}, @dfn{Amiga Smart FileSystem (SFS)}, @dfn{tar}, @dfn{UDF},
@dfn{BSD UFS/UFS2}, and @dfn{XFS}. @xref{Filesystem}, for more information.

@item Support automatic decompression
Can decompress files which were compressed by @command{gzip}. This
function is both automatic and transparent to the user (i.e. all
functions operate upon the uncompressed contents of the specified
files). This greatly reduces a file size and loading time, a
particularly great benefit for floppies.@footnote{There are a few
pathological cases where loading a very badly organized ELF kernel might
take longer, but in practice this never happen.}

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.

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

@item Be independent of drive geometry translations
Unlike many other boot loaders, GRUB makes the particular drive
translation irrelevant. A drive installed and running with one
translation may be converted to another translation without any adverse
effects or changes in GRUB's configuration.

@item Detect all installed @sc{ram}
GRUB can generally find all the installed @sc{ram} on a PC-compatible
machine. It uses an advanced BIOS query technique for finding all
memory regions. As described on the Multiboot Specification (@pxref{Top,
Multiboot Specification, Motivation, multiboot, The Multiboot
Specification}), not all kernels make use of this information, but GRUB
provides it for those who do.

@item Support Logical Block Address mode
In traditional disk calls (called @dfn{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 (@dfn{LBA}) mode. GRUB
automatically detects if LBA mode is available and uses it if
available. In LBA mode, GRUB can access the entire disk.

@item Support network booting
GRUB is basically a disk-based boot loader but also has network
support. You can load OS images from a network by using the @dfn{TFTP}
protocol.

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


@node Role of a boot loader
@section The role of a boot loader

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

@quotation
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
@emph{boot loader} is the most important software of all. I used to
refer to the above systems as either ``LILO''@footnote{The LInux LOader,
a boot loader that everybody uses, but nobody likes.} 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@dots{} GRUB!
@end quotation

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.


@node Naming convention
@chapter Naming convention

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

Look at the following examples and explanations:

@example
(fd0)
@end example

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

@example
(hd0,msdos2)
@end example

Here, @samp{hd} means it is a hard disk drive. The first integer
@samp{0} indicates the drive number, that is, the first hard disk,
the string @samp{msdos} indicates the partition scheme, while
the second integer, @samp{2}, indicates the partition number (or the
@sc{pc} slice number in the BSD terminology). The partition numbers are
counted from @emph{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.

@example
(hd0,msdos5)
@end example

This specifies the first @dfn{extended partition} of the first hard disk
drive. Note that the partition numbers for extended partitions are
counted from @samp{5}, regardless of the actual number of primary
partitions on your hard disk.

@example
(hd1,msdos1,bsd1)
@end example

This means the BSD @samp{a} partition on first @sc{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 @samp{set
root=(fd0)} or @samp{parttool (hd0,msdos3) hidden-}. To help you find out
which number specifies a partition you want, the GRUB command-line
(@pxref{Command-line interface}) options have argument
completion. This means that, for example, you only need to type

@example
set root=(
@end example

followed by a @key{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 @emph{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:

@example
(hd0,msdos1)/vmlinuz
@end example

This specifies the file named @samp{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.


@node Installation
@chapter Installation

In order to install GRUB as your boot loader, you need to first
install the GRUB system and utilities under your UNIX-like operating
system (@pxref{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 @command{grub-install} (@pxref{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
@file{/usr/lib/grub/i386-pc}. Hereafter, the directory where GRUB images are
initially placed (normally @file{/usr/lib/grub/i386-pc}) will be
called the @dfn{image directory}, and the directory where the boot
loader needs to find them (usually @file{/boot/grub}) will be called
the @dfn{boot directory}.

@menu
* Installing GRUB using grub-install::
* Making a GRUB bootable CD-ROM::
* Device map::
@end menu


@node Installing GRUB using grub-install
@section Installing GRUB using grub-install

@strong{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 @dfn{guesses}
the mapping. This will succeed in most cases, but not
always. Therefore, GRUB provides you with a map file called the
@dfn{device map}, which you must fix if it is wrong. @xref{Device
map}, for more details.

If you still do want to install GRUB under a UNIX-like OS (such
as @sc{gnu}), invoke the program @command{grub-install} (@pxref{Invoking
grub-install}) as the superuser (@dfn{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 @samp{/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:

@example
# @kbd{grub-install /dev/hda}
@end example

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

@example
# @kbd{grub-install /dev/hd0}
@end example

If it is the first BIOS drive, this is the same as well:

@example
# @kbd{grub-install '(hd0)'}
@end example

Or you can omit the parentheses:

@example
# @kbd{grub-install hd0}
@end example

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
@option{--root-directory}. The typical usage is that you create a GRUB
boot floppy with a filesystem. Here is an example:

@example
@group
# @kbd{mke2fs /dev/fd0}
# @kbd{mount -t ext2 /dev/fd0 /mnt}
# @kbd{grub-install --root-directory=/mnt fd0}
# @kbd{umount /mnt}
@end group
@end example

Another example is when you have a separate boot partition
which is mounted at @file{/boot}. Since GRUB is a boot loader, it
doesn't know anything about mountpoints at all. Thus, you need to run
@command{grub-install} like this:

@example
# @kbd{grub-install --root-directory=/boot /dev/hda}
@end example

By the way, as noted above, it is quite difficult to guess BIOS drives
correctly under a UNIX-like OS. Thus, @command{grub-install} will prompt
you to check if it could really guess the correct mappings, after the
installation. The format is defined in @ref{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 @command{grub-install} is actually just a shell script and the
real task is done by @command{grub-mkimage} and @command{grub-setup}.
Therefore, you may run those commands directly to install GRUB, without
using @command{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.


@node Making a GRUB bootable CD-ROM
@section Making a GRUB bootable CD-ROM

GRUB supports the @dfn{no emulation mode} in the El Torito
specification@footnote{El Torito is a specification for bootable CD
using BIOS functions.}. 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
@file{stage2_eltorito}. The only GRUB files you need to have in your
bootable CD-ROM are this @file{stage2_eltorito} and optionally a config file
@file{grub.cfg}. You don't need to use @file{stage1} or @file{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,
@samp{iso}:

@example
$ @kbd{mkdir iso}
@end example

Make a directory for GRUB:

@example
$ @kbd{mkdir -p iso/boot/grub}
@end example

Copy the file @file{stage2_eltorito}:

@example
$ @kbd{cp /usr/lib/grub/i386-pc/stage2_eltorito iso/boot/grub}
@end example

If desired, make the config file @file{grub.cfg} under @file{iso/boot/grub}
(@pxref{Configuration}), and copy any files and directories for the disc to the
directory @file{iso/}.

Finally, make a ISO9660 image file like this:

@example
$ @kbd{mkisofs -R -b boot/grub/stage2_eltorito -no-emul-boot \
    -boot-load-size 4 -boot-info-table -o grub.iso iso}
@end example

This produces a file named @file{grub.iso}, which then can be burned
into a CD (or a DVD).  @kbd{mkisofs} has already set up the disc to boot
from the @kbd{boot/grub/stage2_eltorito} file, so there is no need to
setup GRUB on the disc.  (Note that the @kbd{-boot-load-size 4} bit is
required for compatibility with the BIOS on many older machines.)

You can use the device @samp{(cd)} to access a CD-ROM in your
config file. This is not required; GRUB automatically sets the root device
to @samp{(cd)} when booted from a CD-ROM. It is only necessary to refer to
@samp{(cd)} if you want to access other drives as well.


@node Device map
@section The map between BIOS drives and OS devices

The @command{grub-mkdevicemap} program can be used to create the @dfn{device
map file}.  It is often run automatically by tools such as
@command{grub-install} if the device map file does not already exist.  The
file name @file{/boot/grub/device.map} is preferred.

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

@example
@var{device} @var{file}
@end example

@var{device} is a drive specified in the GRUB syntax (@pxref{Device
syntax}), and @var{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 (@file{/dev/hd*} versus @file{/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 @file{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 @command{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 @samp{#}.


@node Booting
@chapter Booting

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

@menu
* General boot methods::        How to boot OSes with GRUB generally
* OS-specific notes::           Notes on some operating systems
@end menu


@node General boot methods
@section How to boot operating systems

GRUB has two distinct boot methods. One of the two is to load an
operating system directly, and the other is to chain-load another boot
loader which then will load an operating system actually. Generally
speaking, the former is more desirable, because you don't need to
install or maintain other boot loaders and GRUB is flexible enough to
load an operating system from an arbitrary disk/partition. However,
the latter is sometimes required, since GRUB doesn't support all the
existing operating systems natively.

@menu
* Loading an operating system directly::
* Chain-loading::
@end menu


@node Loading an operating system directly
@subsection How to boot an OS directly with GRUB

Multiboot (@pxref{Top, Multiboot Specification, Motivation, multiboot,
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 (@pxref{Chain-loading}).

FIXME: this section is incomplete.

@enumerate
@item
Run the command @command{boot} (@pxref{boot}).
@end enumerate

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


@node Chain-loading
@subsection Chain-loading an OS

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

The @command{chainloader} command (@pxref{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:

@verbatim
menuentry "Windows" {
	insmod chain
	insmod ntfs
	set root=(hd0,1)
	chainloader +1
}
@end verbatim
@c FIXME: document UUIDs.

On systems with multiple hard disks, an additional workaround may be
required.  @xref{DOS/Windows}.

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


@node OS-specific notes
@section Some caveats on OS-specific issues

Here, we describe some caveats on several operating systems.

@menu
* GNU/Hurd::
* GNU/Linux::
* DOS/Windows::
@end menu


@node GNU/Hurd
@subsection GNU/Hurd

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

FIXME: this section is incomplete.

@enumerate
@item
Run the command @command{boot} (@pxref{boot}).
@end enumerate


@node GNU/Linux
@subsection GNU/Linux

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

@enumerate
@item
Set GRUB's root device to the same drive as GNU/Linux's.  The command
@code{search --file --set /vmlinuz} or similar may help you
(@pxref{search}).

@item
Load the kernel using the command @command{linux} (@pxref{linux}):

@example
grub> @kbd{linux /vmlinuz root=/dev/sda1}
@end example

If you need to specify some kernel parameters, just append them to the
command.  For example, to set @option{acpi} to @samp{off}, do this:

@example
grub> @kbd{linux /vmlinuz root=/dev/sda1 acpi=off}
@end example

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

@item
If you use an initrd, execute the command @command{initrd} (@pxref{initrd})
after @command{linux}:

@example
grub> @kbd{initrd /initrd}
@end example

@item
Finally, run the command @command{boot} (@pxref{boot}).
@end enumerate

@strong{Caution:} If you use an initrd and specify the @samp{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 @command{uppermem} @emph{before} loading the
kernel. @xref{uppermem}, for more information.


@node DOS/Windows
@subsection DOS/Windows

GRUB cannot boot DOS or Windows directly, so you must chain-load them
(@pxref{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 @command{drivemap} (@pxref{drivemap}), like this:

@example
drivemap -s (hd0) (hd1)
@end example

This performs a @dfn{virtual} swap between your first and second hard
drive.

@strong{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 @dfn{hides} a DOS (or Windows) partition (@pxref{parttool}), DOS (or
Windows) will ignore the partition. If GRUB @dfn{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:

@example
@group
parttool (hd0,1) hidden-
parttool (hd0,2) hidden+
set root=(hd0,1)
chainloader +1
parttool @verb{'${root}'} boot+
boot
@end group
@end example


@node Configuration
@chapter Writing your own configuration file

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

@menu
* Simple configuration::        Recommended for most users
* Shell-like scripting::        For power users and developers
* Embedded configuration::      Embedding a configuration file into GRUB
* Themes::                      Graphical menu themes
@end menu


@node Simple configuration
@section Simple configuration handling

The program @command{grub-mkconfig} (@pxref{Invoking grub-mkconfig})
generates @file{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 @file{/etc/default/grub} controls the operation of
@command{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
@samp{KEY=value} lines, but if the value contains spaces or other special
characters then it must be quoted.  For example:

@example
GRUB_TERMINAL_INPUT="console serial"
@end example

Valid keys in @file{/etc/default/grub} are as follows:

@table @samp
@item GRUB_DEFAULT
The default menu entry.  This may be a number, in which case it identifies
the Nth entry in the generated menu counted from zero, or the full name of a
menu entry, or the special string @samp{saved}.  Using the full name may be
useful if you want to set a menu entry as the default even though there may
be a variable number of entries before it.

If you set this to @samp{saved}, then the default menu entry will be that
saved by @samp{GRUB_SAVEDEFAULT}, @command{grub-set-default}, or
@command{grub-reboot}.

The default is @samp{0}.

@item GRUB_SAVEDEFAULT
If this option is set to @samp{true}, then, when an entry is selected, save
it as a new default entry for use by future runs of GRUB.  This is only
useful if @samp{GRUB_DEFAULT=saved}; it is a separate option because
@samp{GRUB_DEFAULT=saved} is useful without this option, in conjunction with
@command{grub-set-default} or @command{grub-reboot}.  Unset by default.

@item GRUB_TIMEOUT
Boot the default entry this many seconds after the menu is displayed, unless
a key is pressed.  The default is @samp{5}.  Set to @samp{0} to boot
immediately without displaying the menu, or to @samp{-1} to wait
indefinitely.

@item GRUB_HIDDEN_TIMEOUT
Wait this many seconds for a key to be pressed before displaying the menu.
If no key is pressed during that time, boot immediately.  Unset by default.

@item GRUB_HIDDEN_TIMEOUT_QUIET
In conjunction with @samp{GRUB_HIDDEN_TIMEOUT}, set this to @samp{true} to
suppress the verbose countdown while waiting for a key to be pressed before
displaying the menu.  Unset by default.

@item GRUB_DEFAULT_BUTTON
@itemx GRUB_TIMEOUT_BUTTON
@itemx GRUB_HIDDEN_TIMEOUT_BUTTON
@itemx GRUB_BUTTON_CMOS_ADDRESS
Variants of the corresponding variables without the @samp{_BUTTON} suffix,
used to support vendor-specific power buttons.  @xref{Vendor power-on keys}.

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

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

Valid terminal input names depend on the platform, but may include
@samp{console} (PC BIOS and EFI consoles), @samp{serial} (serial terminal),
@samp{ofconsole} (Open Firmware console), @samp{at_keyboard} (PC AT
keyboard, mainly useful with Coreboot), or @samp{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.

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

Valid terminal output names depend on the platform, but may include
@samp{console} (PC BIOS and EFI consoles), @samp{serial} (serial terminal),
@samp{gfxterm} (graphics-mode output), @samp{ofconsole} (Open Firmware
console), or @samp{vga_text} (VGA text output, mainly useful with Coreboot).

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

@item GRUB_TERMINAL
If this option is set, it overrides both @samp{GRUB_TERMINAL_INPUT} and
@samp{GRUB_TERMINAL_OUTPUT} to the same value.

@item GRUB_SERIAL_COMMAND
A command to configure the serial port when using the serial console.
@xref{serial}.  Defaults to @samp{serial}.

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

@item GRUB_CMDLINE_LINUX_DEFAULT
Unless @samp{GRUB_DISABLE_LINUX_RECOVERY} is set to @samp{true}, two menu
entries will be generated for each Linux kernel: one default entry and one
entry for recovery mode.  This option lists command-line arguments to add
only to the default menu entry, after those listed in
@samp{GRUB_CMDLINE_LINUX}.

@item GRUB_CMDLINE_NETBSD
@itemx GRUB_CMDLINE_NETBSD_DEFAULT
As @samp{GRUB_CMDLINE_LINUX} and @samp{GRUB_CMDLINE_LINUX_DEFAULT}, but for
NetBSD.

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

@item GRUB_DISABLE_LINUX_RECOVERY
If this option is set to @samp{true}, disable the generation of recovery
mode menu entries for Linux.

@item GRUB_DISABLE_NETBSD_RECOVERY
If this option is set to @samp{true}, disable the generation of recovery
mode menu entries for NetBSD.

@item GRUB_VIDEO_BACKEND
If graphical video support is required, either because the @samp{gfxterm}
graphical terminal is in use or because @samp{GRUB_GFXPAYLOAD_LINUX} is set,
then @command{grub-mkconfig} will normally load all available GRUB video
drivers and use the one most appropriate for your hardware.  If you need to
override this for some reason, then you can set this option.

After @command{grub-install} has been run, the available video drivers are
listed in @file{/boot/grub/video.lst}.

@item GRUB_GFXMODE
Set the resolution used on the @samp{gfxterm} graphical terminal.  Note that
you can only use modes which your graphics card supports via VESA BIOS
Extensions (VBE), so for example native LCD panel resolutions may not be
available.  The default is @samp{640x480}.

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

@item GRUB_THEME
Set a theme for use with the @samp{gfxterm} graphical terminal.
@xref{Themes}.

@item GRUB_GFXPAYLOAD_LINUX
Set to @samp{text} to force the Linux kernel to boot in normal text mode,
@samp{keep} to preserve the graphics mode set using @samp{GRUB_GFXMODE},
@samp{@var{width}x@var{height}}[@samp{x@var{depth}}] to set a particular
graphics mode, or a sequence of these separated by commas or semicolons to
try several modes in sequence.

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, simply unset this option and GRUB will
tell Linux to boot in normal text mode.

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

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

@item GRUB_BADRAM
If this option is set, GRUB will issue a @ref{badram} command to filter
out specified regions of RAM.

@end table

For more detailed customisation of @command{grub-mkconfig}'s output, you may
edit the scripts in @file{/etc/grub.d} directly.
@file{/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.


@node Shell-like scripting
@section Writing full configuration files directly


@node Embedded configuration
@section Embedding a configuration file into GRUB

GRUB supports embedding a configuration file directly into the core image,
so that it is loaded before entering normal mode.  This is useful, for
example, when it is not straightforward to find the real configuration file,
or when you need to debug problems with loading that file.
@command{grub-install} uses this feature when it is not using BIOS disk
functions or when installing to a different disk from the one containing
@file{/boot/grub}, in which case it needs to use the @command{search}
command (@pxref{search}) to find @file{/boot/grub}.

To embed a configuration file, use the @option{-c} option to
@command{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
@command{grub-mkimage}.

After the embedded configuration file (if any) is executed, GRUB will load
the @samp{normal} module, which will then read the real configuration file
from @file{$prefix/grub.cfg}.  By this point, the @code{root} variable will
also have been set to the root device name.  For example, @code{prefix}
might be set to @samp{(hd0,1)/boot/grub}, and @code{root} might be set to
@samp{hd0,1}.  Thus, in most cases, the embedded configuration file only
needs to set the @code{prefix} and @code{root} variables, and then drop
through to GRUB's normal processing.  A typical example of this might look
like this:

@example
@group
search.fs_uuid 01234567-89ab-cdef-0123-456789abcdef root
set prefix=($root)/boot/grub
@end group
@end example

(The @samp{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 @file{grub.cfg}, or reading files from a directory
other than that where GRUB's loadable modules are installed.  To do this,
include the @samp{configfile} and @samp{normal} modules in the core image,
and embed a configuration file that uses the @command{configfile} command to
load another file.  The following example of this also requires the
@command{echo}, @command{search_label}, and @command{test} modules to be
included in the core image:

@example
@group
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
@end group
@end example

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


@node Themes
@section Graphical menu themes


@node Network
@chapter Booting GRUB from the network

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

To generate a PXE boot image, run:

@example
@group
grub-mkimage --format=i386-pc --output=core.img --prefix='(pxe)/boot/grub' pxe pxecmd
cat /boot/grub/pxeboot.img core.img >grub.pxe
@end group
@end example

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

After GRUB has started, files on the TFTP server will be accessible via the
@samp{(pxe)} device.

The server and gateway IP address can be controlled by changing the
@samp{(pxe)} device name to @samp{(pxe:@var{server-ip})} or
@samp{(pxe:@var{server-ip}:@var{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:

@table @samp
@item net_pxe_ip
The IP address of this machine.  Read-only.

@item net_pxe_mac
The network interface's MAC address.  Read-only.

@item net_pxe_hostname
The client host name provided by DHCP.  Read-only.

@item net_pxe_domain
The client domain name provided by DHCP.  Read-only.

@item net_pxe_rootpath
The path to the client's root disk provided by DHCP.  Read-only.

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

@item net_pxe_boot_file
The boot file name provided by DHCP.  Read-only.

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

@item net_pxe_blksize
The PXE transfer block size.  Read-write, defaults to 512.

@item pxe_default_server
The default PXE server.  Read-write, although setting this is only useful
before opening a PXE device.

@item pxe_default_gateway
The default gateway to use when contacting the PXE server.  Read-write,
although setting this is only useful before opening a PXE device.
@end table


@node Serial terminal
@chapter Using GRUB via a serial line

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

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

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

@example
@group
grub> @kbd{serial --unit=0 --speed=9600}
grub> @kbd{terminal_input serial}
grub> @kbd{terminal_output serial}
@end group
@end example

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

The commands @command{terminal_input} (@pxref{terminal_input}) and
@command{terminal_output} (@pxref{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 @code{console} to the command,
as @samp{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 @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.


@node Vendor power-on keys
@chapter Using GRUB with vendor power-on keys

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

@table @key
@item Dell XPS M1530
85:3
@item Asus EeePC 1005PE 
84:1 (unconfirmed)
@end table

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


@node Images
@chapter GRUB image files

@c FIXME: parts of this section are specific to PC BIOS right now.

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.

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

The sole function of @file{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,
@file{boot.img} cannot understand any file system structure, so
@command{grub-setup} hardcodes the location of the first sector of the core
image into @file{boot.img} when installing GRUB.

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

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

@item pxeboot.img
This image is used as the start of the core image when booting from the
network using PXE.  @xref{Network}.

@item lnxboot.img
This image may be placed at the start of the core image in order to make
GRUB look enough like a Linux kernel that it can be booted by LILO using an
@samp{image=} section.

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

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

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 @file{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 @file{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:

@example
# @kbd{parted /dev/@var{disk} set @var{partition-number} bios_grub on}
@end example

@strong{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.

@item *.mod
Everything else in GRUB resides in dynamically loadable modules.  These are
often loaded automatically, or built into the core image if they are
essential, but may also be loaded manually using the @command{insmod}
command (@pxref{insmod}).
@end table

@heading For GRUB Legacy users

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

@table @file
@item stage1
Stage 1 from GRUB Legacy was very similar to @file{boot.img} in GRUB 2, and
they serve the same function.

@item *_stage1_5
In GRUB Legacy, Stage 1.5's function was to include enough filesystem code
to allow the much larger Stage 2 to be read from an ordinary filesystem.  In
this respect, its function was similar to @file{core.img} in GRUB 2.
However, @file{core.img} is much more capable than Stage 1.5 was; since it
offers a rescue shell, it is sometimes possible to recover manually in the
event that it is unable to load any other modules, for example if partition
numbers have changed.  @file{core.img} is built in a more flexible way,
allowing GRUB 2 to support reading modules from advanced disk types such as
LVM and RAID.

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

@item stage2
GRUB 2 has no single Stage 2 image.  Instead, it loads modules from
@file{/boot/grub} at run-time.

@item stage2_eltorito
In GRUB 2, images for booting from CD-ROM drives are now constructed using
@file{cdboot.img} and @file{core.img}, making sure that the core image
contains the @samp{iso9660} module.  It is usually best to use the
@command{grub-mkrescue} program for this.

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

@item pxegrub
In GRUB 2, images for PXE network booting are now constructed using
@file{pxeboot.img} and @file{core.img}, making sure that the core image
contains the @samp{pxe} and @samp{pxecmd} modules.  @xref{Network}.
@end table


@node Filesystem
@chapter Filesystem syntax and semantics

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

@menu
* Device syntax::               How to specify devices
* File name syntax::            How to specify files
* Block list syntax::           How to specify block lists
@end menu


@node Device syntax
@section How to specify devices

The device syntax is like this:

@example
@code{(@var{device}[,@var{part-num}][,@var{bsd-subpart-letter}])}
@end example

@samp{[]} means the parameter is optional. @var{device} should be
either @samp{fd} or @samp{hd} followed by a digit, like @samp{fd0}.
But you can also set @var{device} to a hexadecimal or a decimal number
which is a BIOS drive number, so the following are equivalent:

@example
(hd0)
(0x80)
(128)
@end example

@var{part-num} represents the partition number of @var{device}, starting
from one for primary partitions and from five for extended partitions,
and @var{bsd-subpart-letter} represents the BSD disklabel subpartition,
such as @samp{a} or @samp{e}.

A shortcut for specifying BSD subpartitions is
@code{(@var{device},@var{bsd-subpart-letter})}, in this case, GRUB
searches for the first PC partition containing a BSD disklabel, then
finds the subpartition @var{bsd-subpart-letter}. Here is an example:

@example
(hd0,a)
@end example

The syntax @samp{(hd0)} represents using the entire disk (or the
MBR when installing GRUB), while the syntax @samp{(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 @samp{(pxe)} is
also available. Before using the network drive, you must initialize the
network. @xref{Network}, for more information.

If you boot GRUB from a CD-ROM, @samp{(cd)} is available. @xref{Making
a GRUB bootable CD-ROM}, for details.


@node File name syntax
@section How to specify files

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

An absolute file name resembles a Unix absolute file name, using
@samp{/} for the directory separator (not @samp{\} as in DOS). One
example is @samp{(hd0,1)/boot/grub/grub.cfg}. This means the file
@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 @dfn{root device} implicitly. So if you set the root device to,
say, @samp{(hd1,1)} by the command @samp{set root=(hd1,1)} (@pxref{set}),
then @code{/boot/kernel} is the same as @code{(hd1,1)/boot/kernel}.


@node Block list syntax
@section How to specify block lists

A block list is used for specifying a file that doesn't appear in the
filesystem, like a chainloader. The syntax is
@code{[@var{offset}]+@var{length}[,[@var{offset}]+@var{length}]@dots{}}.
Here is an example:

@example
@code{0+100,200+1,300+300}
@end 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 (@pxref{File name syntax}), if a blocklist
does not contain a device name, then GRUB uses GRUB's @dfn{root
device}. So @code{(hd0,2)+1} is the same as @code{+1} when the root
device is @samp{(hd0,2)}.


@node Interface
@chapter GRUB's user interface

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

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

@menu
* Command-line interface::      The flexible command-line interface
* Menu interface::              The simple menu interface
* Menu entry editor::           Editing a menu entry
@end menu


@node Command-line interface
@section The flexible command-line interface

The command-line interface provides a prompt and after it an editable
text area much like a command-line in Unix or DOS. Each command is
immediately executed after it is entered@footnote{However, this
behavior will be changed in the future version, in a user-invisible
way.}. The commands (@pxref{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:

@table @key
@item C-f
@itemx PC right key
Move forward one character.

@item C-b
@itemx PC left key
Move back one character.

@item C-a
@itemx HOME
Move to the start of the line.

@item C-e
@itemx END
Move the the end of the line.

@item C-d
@itemx DEL
Delete the character underneath the cursor.

@item C-h
@itemx BS
Delete the character to the left of the cursor.

@item C-k
Kill the text from the current cursor position to the end of the line.

@item C-u
Kill backward from the cursor to the beginning of the line.

@item C-y
Yank the killed text back into the buffer at the cursor.

@item C-p
@itemx PC up key
Move up through the history list.

@item C-n
@itemx PC down key
Move down through the history list.
@end table

When typing commands interactively, if the cursor is within or before
the first word in the command-line, pressing the @key{TAB} key (or
@key{C-i}) will display a listing of the available commands, and if the
cursor is after the first word, the @kbd{@key{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 @command{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.


@node Menu interface
@section The simple menu interface

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

Basically, the menu interface provides a list of @dfn{boot entries} to
the user to choose from. Use the arrow keys to select the entry of
choice, then press @key{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 @key{c}
(which operates exactly like the non-config-file version of GRUB, but
allows one to return to the menu if desired by pressing @key{ESC}) or to
edit any of the @dfn{boot entries} by pressing @key{e}.

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


@node Menu entry editor
@section Editing a menu entry

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

If an @key{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 @key{RET} at the end of a line.  To boot the edited entry, press
@key{Ctrl-x}.

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


@node Commands
@chapter The list of available commands

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

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

In rescue mode, only the @command{insmod} (@pxref{insmod}), @command{ls}
(@pxref{ls}), @command{set} (@pxref{set}), and @command{unset}
(@pxref{unset}) commands are normally available.

@menu
* Menu-specific commands::
* General commands::
* Command-line and menu entry commands::
@end menu


@node Menu-specific commands
@section The list of commands for the menu only

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

@itemize @bullet
@item
The menu-specific commands have to be used before any others.

@item
The files @emph{must} be in plain-text format.

@item
@samp{#} at the beginning of a line in a configuration file means it is
only a comment.

@item
Options are separated by spaces.

@item
All numbers can be either decimal or hexadecimal. A hexadecimal number
must be preceded by @samp{0x}, and is case-insensitive.

@item
Extra options or text at the end of the line are ignored unless otherwise
specified.

@item
Unrecognized commands are added to the current entry, except before entries
start, where they are ignored.
@end itemize

These commands can only be used in the menu:

@menu
* menuentry::                   Start a menu entry
@end menu


@node menuentry
@subsection menuentry

@deffn Command title name @dots{}
Start a new boot entry, and set its name to the contents of the rest of
the line, starting with the first non-space character.
@end deffn


@node General commands
@section The list of general commands

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

@menu
* serial::                      Set up a serial device
* terminal_input::              Manage input terminals
* terminal_output::             Manage output terminals
* terminfo::                    Define terminal type
@end menu


@node serial
@subsection serial

@deffn Command serial [@option{--unit=unit}] [@option{--port=port}] [@option{--speed=speed}] [@option{--word=word}] [@option{--parity=parity}] [@option{--stop=stop}]
Initialize a serial device. @var{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. @var{port} is the I/O port where the UART
is to be found; if specified it takes precedence over @var{unit}.
@var{speed} is the transmission speed; default is 9600. @var{word} and
@var{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. @var{parity} is one of @samp{no}, @samp{odd},
@samp{even} and defaults to @samp{no}.

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

This command is only available if GRUB is compiled with serial
support. See also @ref{Serial terminal}.
@end deffn


@node terminal_input
@subsection terminal_input

@deffn Command terminal_input [@option{--append}|@option{--remove}] @
 [terminal1] [terminal2] @dots{}
List or select an input terminal.

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

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

With @option{--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.
@end deffn


@node terminal_output
@subsection terminal_output

@deffn Command terminal_output [@option{--append}|@option{--remove}] @
 [terminal1] [terminal2] @dots{}
List or select an output terminal.

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

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

With @option{--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.
@end deffn


@node terminfo
@subsection terminfo

@deffn Command terminfo [-a|-u|-v] [term]
Define the capabilities of your terminal by giving the name of an entry in
the terminfo database, which should correspond roughly to a @samp{TERM}
environment variable in Unix.

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

The @option{-a} (@option{--ascii}), @option{-u} (@option{--utf8}), and
@option{-v} (@option{--visual-utf8}) options control how non-ASCII text is
displayed.  @option{-a} specifies an ASCII-only terminal; @option{-u}
specifies logically-ordered UTF-8; and @option{-v} specifies
visually-ordered UTF-8.

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


@node Command-line and menu entry commands
@section The list of command-line and menu entry commands

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

@menu
* acpi::                        Load ACPI tables
* badram::                      Filter out bad regions of RAM
* blocklist::                   Print a block list
* boot::                        Start up your operating system
* cat::                         Show the contents of a file
* chainloader::                 Chain-load another boot loader
* cmp::                         Compare two files
* configfile::                  Load a configuration file
* cpuid::                       Check for CPU features
* crc::                         Calculate CRC32 checksums
* date::                        Display or set current date and time
* drivemap::                    Map a drive to another
* echo::                        Display a line of text
* export::                      Export an environment variable
* gettext::                     Translate a string
* gptsync::                     Fill an MBR based on GPT entries
* halt::                        Shut down your computer
* help::                        Show help messages
* initrd::                      Load a Linux initrd
* initrd16::                    Load a Linux initrd (16-bit mode)
* insmod::                      Insert a module
* keystatus::                   Check key modifier status
* linux::                       Load a Linux kernel
* linux16::                     Load a Linux kernel (16-bit mode)
* ls::                          List devices or files
* parttool::                    Modify partition table entries
* password::                    Set a clear-text password
* password_pbkdf2::             Set a hashed password
* play::                        Play a tune
* pxe_unload::                  Unload the PXE environment
* reboot::                      Reboot your computer
* search::                      Search devices by file, label, or UUID
* set::                         Set an environment variable
* unset::                       Unset an environment variable
* uppermem::                    Set the upper memory size
@end menu


@node acpi
@subsection acpi

@deffn Command acpi [@option{-1}|@option{-2}] @
 [@option{--exclude=table1,@dots{}}|@option{--load-only=table1,@dots{}}] @
 [@option{--oemid=id}] [@option{--oemtable=table}] @
 [@option{--oemtablerev=rev}] [@option{--oemtablecreator=creator}] @
 [@option{--oemtablecreatorrev=rev}] [@option{--no-ebda}] @
 filename @dots{}
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
@option{--no-ebda} option is used, the new tables will be known only to
GRUB, but may be used by GRUB's EFI emulation.
@end deffn

@node badram
@subsection badram

@deffn Command badram addr,mask[,addr,mask...]
Filter out bad RAM.
@end deffn

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 @uref{http://www.memtest.org/,
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.

@node blocklist
@subsection blocklist

@deffn Command blocklist file
Print a block list (@pxref{Block list syntax}) for @var{file}.
@end deffn


@node boot
@subsection boot

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


@node cat
@subsection cat

@deffn Command cat [@option{--dos}] file
Display the contents of the file @var{file}. This command may be useful
to remind you of your OS's root partition:

@example
grub> @kbd{cat /etc/fstab}
@end example

If the @option{--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 (@samp{<d>}) to make it easier to see
when boot problems are caused by a file formatted using DOS-style line
endings.
@end deffn


@node chainloader
@subsection chainloader

@deffn Command chainloader [@option{--force}] file
Load @var{file} as a chain-loader. Like any other file loaded by the
filesystem code, it can use the blocklist notation (@pxref{Block list
syntax}) to grab the first sector of the current partition with @samp{+1}.
If you specify the option @option{--force}, then load @var{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.
@end deffn


@node cmp
@subsection cmp

@deffn Command cmp file1 file2
Compare the file @var{file1} with the file @var{file2}. If they differ
in size, print the sizes like this:

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

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

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

If they are completely identical, nothing will be printed.
@end deffn


@node configfile
@subsection configfile

@deffn Command configfile file
Load @var{file} as a configuration file.  If @var{file} defines any menu
entries, then show a menu containing them immediately.
@end deffn


@node cpuid
@subsection cpuid

@deffn Command cpuid [-l]
Check for CPU features.  This command is only available on x86 systems.

With the @option{-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 @option{-l}.  This may change in the future.
@end deffn


@node crc
@subsection crc

@deffn Command crc file
Display the CRC32 checksum of @var{file}.
@end deffn


@node date
@subsection date

@deffn Command date [[year-]month-day] [hour:minute[:second]]
With no arguments, print the current date and time.

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


@node drivemap
@subsection drivemap

@deffn Command drivemap @option{-l}|@option{-r}|[@option{-s}] @
 from_drive to_drive
Without options, map the drive @var{from_drive} to the drive @var{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 @verb{'${root}'} as a
drive specification.

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

With the @option{-l} option, list the current mappings.

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

For example:

@example
drivemap -s (hd0) (hd1)
@end example
@end deffn


@node echo
@subsection echo

@deffn Command echo [@option{-n}] [@option{-e}] string @dots{}
Display the requested text and, unless the @option{-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 @samp{$@{var@}}.

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

@table @code
@item \\
backslash

@item \a
alert (BEL)

@item \c
suppress trailing new line

@item \f
form feed

@item \n
new line

@item \r
carriage return

@item \t
horizontal tab

@item \v
vertical tab
@end table

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


@node export
@subsection export

@deffn Command export envvar
Export the environment variable @var{envvar}. Exported variables are visible
to subsidiary configuration files loaded using @command{configfile}.
@end deffn


@node gettext
@subsection gettext

@deffn Command gettext string
Translate @var{string} into the current language.

The current language code is stored in the @samp{lang} variable in GRUB's
environment.  Translation files in MO format are read from
@samp{locale_dir}, usually @file{/boot/grub/locale}.
@end deffn


@node gptsync
@subsection gptsync

@deffn Command gptsync device [partition[+/-[type]]] @dots{}
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 @var{partition}
entries on @var{device}.  Up to three partitions may be used.

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


@node halt
@subsection halt

@deffn Command halt @option{--no-apm}
The command halts the computer. If the @option{--no-apm} option
is specified, no APM BIOS call is performed. Otherwise, the computer
is shut down using APM.
@end deffn


@node help
@subsection help

@deffn Command help [pattern @dots{}]
Display helpful information about builtin commands. If you do not
specify @var{pattern}, this command shows short descriptions of all
available commands.

If you specify any @var{patterns}, it displays longer information
about each of the commands whose names begin with those @var{patterns}.
@end deffn


@node initrd
@subsection initrd

@deffn Command initrd file
Load an initial ramdisk for a Linux kernel image, and set the appropriate
parameters in the Linux setup area in memory.  This may only be used after
the @command{linux} command (@pxref{linux}) has been run.  See also
@ref{GNU/Linux}.
@end deffn


@node initrd16
@subsection initrd16

@deffn Command initrd16 file
Load an initial ramdisk for a Linux kernel image to be booted in 16-bit
mode, and set the appropriate parameters in the Linux setup area in memory.
This may only be used after the @command{linux16} command (@pxref{linux16})
has been run.  See also @ref{GNU/Linux}.

This command is only available on x86 systems.
@end deffn


@node insmod
@subsection insmod

@deffn Command insmod module
Insert the dynamic GRUB module called @var{module}.
@end deffn


@node keystatus
@subsection keystatus

@deffn Command keystatus [@option{--shift}] [@option{--ctrl}] [@option{--alt}]
Return true if the Shift, Control, or Alt modifier keys are held down, as
requested by options. This is useful in scripting, to allow some user
control over behaviour without having to wait for a keypress.

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


@node linux
@subsection linux

@deffn Command linux file @dots{}
Load a Linux kernel image from @var{file}.  The rest of the line is passed
verbatim as the @dfn{kernel command-line}.  Any initrd must be reloaded
after using this command (@pxref{initrd}).

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


@node linux16
@subsection linux16

@deffn Command linux16 file @dots{}
Load a Linux kernel image from @var{file} in 16-bit mode.  The rest of the
line is passed verbatim as the @dfn{kernel command-line}.  Any initrd must
be reloaded after using this command (@pxref{initrd16}).

The kernel will be booted using the traditional 16-bit boot protocol.  As
well as bypassing problems with @samp{vga=} described in @ref{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.
@end deffn


@node ls
@subsection ls

@deffn Command ls [arg]
List devices or files.

With no arguments, print all devices known to GRUB.

If the argument is a device name enclosed in parentheses (@pxref{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 (@pxref{File
name syntax}), then list the contents of that directory.
@end deffn


@node parttool
@subsection parttool

@deffn Command parttool partition commands
Make various modifications to partition table entries.

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

Currently, @command{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:

@table @asis
@item @samp{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 @emph{primary} partitions.

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

@item @samp{hidden} (boolean)
When enabled, this hides the selected partition by setting the @dfn{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
@ref{DOS/Windows}.
@end table
@end deffn


@node password
@subsection password

@deffn Command password user clear-password
Define a user named @var{user} with password @var{clear-password}.
@xref{Security}.
@end deffn


@node password_pbkdf2
@subsection password_pbkdf2

@deffn Command password_pbkdf2 user hashed-password
Define a user named @var{user} with password hash @var{hashed-password}.
Use @command{grub-mkpasswd-pbkdf2} (@pxref{Invoking grub-mkpasswd-pbkdf2})
to generate password hashes.  @xref{Security}.
@end deffn


@node play
@subsection play

@deffn Command play file | tempo [pitch1 duration1] [pitch2 duration2] ...
Plays a tune

If the argument is a file name (@pxref{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.
@end deffn


@node pxe_unload
@subsection pxe_unload

@deffn Command pxe_unload
Unload the PXE environment (@pxref{Network}).

This command is only available on PC BIOS systems.
@end deffn


@node reboot
@subsection reboot

@deffn Command reboot
Reboot the computer.
@end deffn


@node search
@subsection search

@deffn Command search @
 [@option{--file}|@option{--label}|@option{--fs-uuid}] @
 [@option{--set} [var]] [@option{--no-floppy}] name
Search devices by file (@option{-f}, @option{--file}), filesystem label
(@option{-l}, @option{--label}), or filesystem UUID (@option{-u},
@option{--fs-uuid}).

If the @option{--set} option is used, the first device found is set as the
value of environment variable @var{var}.  The default variable is
@samp{root}.

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

The @samp{search.file}, @samp{search.fs_label}, and @samp{search.fs_uuid}
commands are aliases for @samp{search --file}, @samp{search --label}, and
@samp{search --fs-uuid} respectively.
@end deffn


@node set
@subsection set

@deffn Command set [envvar=value]
Set the environment variable @var{envvar} to @var{value}. If invoked with no
arguments, print all environment variables with their values.
@end deffn


@node unset
@subsection unset

@deffn Command unset envvar
Unset the environment variable @var{envvar}.
@end deffn


@node uppermem
@subsection uppermem

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


@node Security
@chapter Authentication and authorisation

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

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

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

In order to enable authentication support, the @samp{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
@samp{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 @option{--users} option to the
@samp{menuentry} command (@pxref{menuentry}).  If the @option{--users}
option is not used for a menu entry, then that entry is unrestricted.

Putting this together, a typical @file{grub.cfg} fragment might look like
this:

@example
@group
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
@}
@end group
@end example

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


@node Troubleshooting
@chapter Error messages produced by GRUB

@menu
* GRUB only offers a rescue shell::
@end menu


@node GRUB only offers a rescue shell
@section GRUB only offers a rescue shell

GRUB's normal start-up procedure involves setting the @samp{prefix}
environment variable to a value set in the core image by
@command{grub-install}, setting the @samp{root} variable to match, loading
the @samp{normal} module from the prefix, and running the @samp{normal}
command.  This command is responsible for reading
@file{/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 @samp{normal} module for some reason.  It may be possible
to work around this temporarily: for instance, if the reason for the failure
is that @samp{prefix} is wrong (perhaps it refers to the wrong device, or
perhaps the path to @file{/boot/grub} was not correctly made relative to the
device), then you can correct this and enter normal mode manually:

@example
@group
# 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
@end group
@end example

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 @kbd{grub-install @var{device}} (@pxref{Invoking
grub-install}).  When doing this, there are a few things to remember:

@itemize @bullet{}
@item
Drive ordering in your operating system may not be the same as the boot
drive ordering used by your firmware.  Do not assume that your first hard
drive (e.g. @samp{/dev/sda}) is the one that your firmware will boot from.
@file{device.map} (@pxref{Device map}) can be used to override this, but it
is usually better to use UUIDs or file system labels and avoid depending on
drive ordering entirely.

@item
At least on BIOS systems, if you tell @command{grub-install} to install GRUB
to a partition but GRUB has already been installed in the master boot
record, then the GRUB installation in the partition will be ignored.

@item
If possible, it is generally best to avoid installing GRUB to a partition
(unless it is a special partition for the use of GRUB alone, such as the
BIOS Boot Partition used on GPT).  Doing this means that GRUB may stop being
able to read its core image due to a file system moving blocks around, such
as while defragmenting, running checks, or even during normal operation.
Installing to the whole disk device is normally more robust.

@item
Check that GRUB actually knows how to read from the device and file system
containing @file{/boot/grub}.  It will not be able to read from encrypted
devices, nor from file systems for which support has not yet been added to
GRUB.
@end itemize


@node Invoking grub-install
@chapter Invoking grub-install

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

@example
grub-install @var{install_device}
@end example

The device name @var{install_device} is an OS device name or a GRUB
device name.

@command{grub-install} accepts the following options:

@table @option
@item --help
Print a summary of the command-line options and exit.

@item --version
Print the version number of GRUB and exit.

@item --root-directory=@var{dir}
Install GRUB images under the directory @var{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 @dfn{boot} partition which is mounted on
@file{/boot}:

@example
@kbd{grub-install --root-directory=/boot hd0}
@end example

@item --recheck
Recheck the device map, even if @file{/boot/grub/device.map} already
exists. You should use this option whenever you add/remove a disk
into/from your computer.
@end table


@node Invoking grub-mkconfig
@chapter Invoking grub-mkconfig

The program @command{grub-mkconfig} generates a configuration file for GRUB
(@pxref{Simple configuration}).

@example
grub-mkconfig -o /boot/grub/grub.cfg
@end example

@command{grub-mkconfig} accepts the following options:

@table @option
@item --help
Print a summary of the command-line options and exit.

@item --version
Print the version number of GRUB and exit.

@item -o @var{file}
@itemx --output=@var{file}
Send the generated configuration file to @var{file}.  The default is to send
it to standard output.
@end table


@node Invoking grub-mkpasswd-pbkdf2
@chapter Invoking grub-mkpasswd-pbkdf2

The program @command{grub-mkpasswd-pbkdf2} generates password hashes for
GRUB (@pxref{Security}).

@example
grub-mkpasswd-pbkdf2
@end example

@command{grub-mkpasswd-pbkdf2} accepts the following options:

@table @option
@item -c @var{number}
@itemx --iteration-count=@var{number}
Number of iterations of the underlying pseudo-random function.  Defaults to
10000.

@item -l @var{number}
@itemx --buflen=@var{number}
Length of the generated hash.  Defaults to 64.

@item -s @var{number}
@itemx --salt=@var{number}
Length of the salt.  Defaults to 64.
@end table


@node Obtaining and Building GRUB
@appendix How to obtain and build GRUB

@quotation
@strong{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
@uref{http://sources.redhat.com/binutils/}, to obtain information on
how to get the latest version.
@end quotation

GRUB is available from the GNU alpha archive site
@uref{ftp://alpha.gnu.org/gnu/grub} or any of its mirrors. The file
will be named grub-version.tar.gz. The current version is
@value{VERSION}, so the file you should grab is:

@uref{ftp://alpha.gnu.org/gnu/grub/grub-@value{VERSION}.tar.gz}

To unbundle GRUB use the instruction:

@example
@kbd{zcat grub-@value{VERSION}.tar.gz | tar xvf -}
@end example

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

@example
@group
@kbd{cd grub-@value{VERSION}}
@kbd{./configure}
@kbd{make install}
@end group
@end example

Also, the latest version is available using Bazaar. See
@uref{http://www.gnu.org/software/grub/grub-download.en.html} for more
information.

@node Reporting bugs
@appendix Reporting bugs

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

@enumerate
@item
Before getting unsettled, read this manual through and through. Also,
see the @uref{http://www.gnu.org/software/grub/grub-faq.html, GNU GRUB FAQ}.

@item
Always mention the information on your GRUB. The version number and the
configuration are quite important. If you build it yourself, write the
options specified to the configure script and your operating system,
including the versions of gcc and binutils.

@item
If you have trouble with the installation, inform us of how you
installed GRUB. Don't omit error messages, if any. Just @samp{GRUB hangs
up when it boots} is not enough.

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

@item
If GRUB cannot boot your operating system, write down
@emph{everything} you see on the screen. Don't paraphrase them, like
@samp{The foo OS crashes with GRUB, even though it can boot with the
bar boot loader just fine}. Mention the commands you executed, the
messages printed by them, and information on your operating system
including the version number.

@item
Explain what you wanted to do. It is very useful to know your purpose
and your wish, and how GRUB didn't satisfy you.

@item
If you can investigate the problem yourself, please do. That will give
you and us much more information on the problem. Attaching a patch is
even better.

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

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

If you follow the guideline above, submit a report to the
@uref{http://savannah.gnu.org/bugs/?group=grub, Bug Tracking System}.
Alternatively, you can submit a report via electronic mail to
@email{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.


@node Future
@appendix Where GRUB will go

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 @uref{http://www.gnu.org/software/grub/grub.html, the
homepage}.


@node Internals
@appendix Hacking GRUB

@menu
* Getting the source code::
* Finding your way around::
@end menu


@node Getting the source code
@section Getting the source code

GRUB is maintained using the @uref{http://bazaar-vcs.org/, Bazaar revision
control system}.  To fetch the primary development branch:

@example
bzr get http://bzr.savannah.gnu.org/r/grub/trunk/grub
@end example

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:

@example
bzr get http://bzr.savannah.gnu.org/r/grub/branches/experimental
@end example

Once you have used @kbd{bzr get} to fetch an initial copy of a branch, you
can use @kbd{bzr pull} to keep it up to date.  If you have modified your
local version, you may need to resolve conflicts when pulling.


@node Finding your way around
@section Finding your way around

Here is a brief map of the GRUB code base.

GRUB uses Autoconf, but not (yet) Automake.  The top-level build rules are
in @file{configure.ac}, @file{Makefile.in}, and @file{conf/*.rmk}.  Each
@file{conf/*.rmk} file represents a particular target configuration, and is
processed into GNU Make rules by @file{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 @file{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 @file{boot/} directory.

The GRUB kernel is in @file{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
@file{include/}.

Terminal implementations are in @file{term/}.

Disk access code is spread across @file{disk/} (for accessing the disk
devices themselves), @file{partmap/} (for interpreting partition table
data), and @file{fs/} (for accessing filesystems).  Note that, with the odd
specialised exception, GRUB only contains code to @emph{read} from
filesystems and tries to avoid containing any code to @emph{write} to
filesystems; this lets us confidently assure users that GRUB cannot be
responsible for filesystem corruption.

PCI and USB bus handling is in @file{bus/}.

Video handling code is in @file{video/}.  The graphical menu system uses
this heavily, but is in a separate directory, @file{gfxmenu/}.

Most commands are implemented by files in @file{commands/}, with the
following exceptions:

@itemize
@item
A few core commands live in @file{kern/corecmd.c}.

@item
Commands related to normal mode live under @file{normal/}.

@item
Commands that load and boot kernels live under @file{loader/}.

@item
The @samp{loopback} command is really a disk device, and so lives in
@file{disk/loopback.c}.

@item
The @samp{gettext} command lives under @file{gettext/}.

@item
The @samp{loadfont} and @samp{lsfonts} commands live under @file{font/}.

@item
The @samp{serial}, @samp{terminfo}, and @samp{background_image} commands
live under @file{term/}.

@item
The @samp{efiemu_*} commands live under @file{efiemu/}.
@end itemize

There are a few other special-purpose exceptions; grep for them if they
matter to you.



@node Copying This Manual
@appendix Copying This Manual

@menu
* GNU Free Documentation License::  License for copying this manual.
@end menu

@include fdl.texi


@node Index
@unnumbered Index

@c Currently, we use only the Concept Index.
@printindex cp


@bye

Some notes:

  This is an attempt to make a manual for GRUB 2. The contents are
  copied from the GRUB manual in GRUB Legacy, so they are not always
  appropriate yet for GRUB 2.