grub.texi

Linux的启动装载(BootLoader)程序

@emph{zero}. This expression means that GRUB will use the whole floppy
disk.

@example
(hd0,1)
@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, while
the second integer, @samp{1}, indicates the partition number (or the
@sc{pc} slice number in the BSD terminology). Once again, please note
that the partition numbers are counted from @emph{zero}, not from
one. This expression means the second partition of the first hard disk
drive. In this case, GRUB uses one partition of the disk, instead of the
whole disk.

@example
(hd0,4)
@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{4}, regardless of the actual number of primary
partitions on your hard disk.

@example
(hd1,a)
@end example

This means the BSD @samp{a} partition of the second hard disk. If you
need to specify which @sc{pc} slice number should be used, use something
like this: @samp{(hd1,0,a)}. If the @sc{pc} slice number is omitted,
GRUB searches for the first @sc{pc} slice which has a BSD @samp{a}
partition.

Of course, to actually access the disks or partitions with GRUB, you
need to use the device specification in a command, like @samp{root
(fd0)} or @samp{unhide (hd0,2)}. 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
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
(hd0,0)/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/share/grub/i386-pc}. If you do not use grub-install, then
you need to copy the files @file{stage1}, @file{stage2}, and
@file{*stage1_5} to the directory @file{/boot/grub}. Hereafter, the
directory where GRUB images are initially placed (normally
@file{/usr/share/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
* Creating a GRUB boot floppy::
* Installing GRUB natively::
* Installing GRUB using grub-install::
@end menu


@node Creating a GRUB boot floppy
@section Creating a GRUB boot floppy

To create a GRUB boot floppy, you need to take the files @file{stage1}
and @file{stage2} from the image directory, and write them to the first
and the second block of the floppy disk, respectively.

@strong{Caution:} This procedure will destroy any data currently stored
on the floppy.

On a UNIX-like operating system, that is done with the following
commands:

@example
@group
# @kbd{cd /usr/share/grub/i386-pc}
# @kbd{dd if=stage1 of=/dev/fd0 bs=512 count=1}
1+0 records in
1+0 records out
# @kbd{dd if=stage2 of=/dev/fd0 bs=512 seek=1}
153+1 records in
153+1 records out
#
@end group
@end example

The device file name may be different. Consult the manual for your OS.


@node Installing GRUB natively
@section Installing GRUB natively

@strong{Caution:} Installing GRUB's stage1 in this manner will erase the
normal boot-sector used by an OS.

GRUB can currently boot GNU Mach, Linux, FreeBSD, NetBSD, and OpenBSD
directly, so using it on a boot sector (the first sector of a
partition) should be okay. But generally, it would be a good idea to
back up the first sector of the partition on which you are installing
GRUB's stage1. This isn't as important if you are installing GRUB on
the first sector of a hard disk, since it's easy to reinitialize it
(e.g. by running @samp{FDISK /MBR} from DOS).

If you decide to install GRUB in the native environment, which is
definitely desirable, you'll need to create a GRUB boot disk, and
reboot your computer with it. Otherwise, see @ref{Installing GRUB using
grub-install}.

Once started, GRUB will show the command-line interface
(@pxref{Command-line interface}). First, set the GRUB's @dfn{root
device}@footnote{Note that GRUB's root device doesn't necessarily mean
your OS's root partition; if you need to specify a root partition for
your OS, add the argument into the command @command{kernel}.} to the
partition containing the boot directory, like this:

@example
grub> @kbd{root (hd0,0)}
@end example

If you are not sure which partition actually holds this directory, use the
command @command{find} (@pxref{find}), like this:

@example
grub> @kbd{find /boot/grub/stage1}
@end example

This will search for the file name @file{/boot/grub/stage1} and show the
devices which contain the file.

Once you've set the root device correctly, run the command
@command{setup} (@pxref{setup}):

@example
grub> @kbd{setup (hd0)}
@end example

This command will install the GRUB boot loader on the Master Boot
Record (MBR) of the first drive. If you want to put GRUB into the boot
sector of a partition instead of putting it in the MBR, specify the
partition into which you want to install GRUB:

@example
grub> @kbd{setup (hd0,0)}
@end example

If you install GRUB into a partition or a drive other than the first
one, you must chain-load GRUB from another boot loader. Refer to the
manual for the boot loader to know how to chain-load GRUB.

After using the setup command, you will boot into GRUB without the
GRUB floppy. See the chapter @ref{Booting} to find out how to boot
your operating systems from GRUB.


@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 the grub shell @command{grub} (@pxref{Invoking the
grub shell}). Therefore, you may run @command{grub} 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 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}).

Generally, GRUB can boot any Multiboot-compliant OS in the following
steps:

@enumerate
@item
Set GRUB's root device to the drive where the OS images are stored with
the command @command{root} (@pxref{root}).

@item
Load the kernel image with the command @command{kernel} (@pxref{kernel}).

@item
If you need modules, load them with the command @command{module}
(@pxref{module}) or @command{modulenounzip} (@pxref{modulenounzip}).

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

Linux, FreeBSD, NetBSD and OpenBSD can be booted in a similar
manner. You load a kernel image with the command @command{kernel} and
then run the command @command{boot}. If the kernel requires some
parameters, just append the parameters to @command{kernel}, after the
file name of the kernel. Also, please refer to @ref{OS-specific notes},
for information on your OS-specific issues.


@node Chain-loading
@subsection Load another boot loader to boot unsupported operating systems

If you want to boot an unsupported operating system (e.g. Windows 95),
chain-load a boot loader for the operating system. Normally, the boot
loader is embedded in the @dfn{boot sector} of the partition on which
the operating system is installed.

@enumerate
@item
Set GRUB's root device to the partition by the command
@command{rootnoverify} (@pxref{rootnoverify}):

@example
grub> @kbd{rootnoverify (hd0,0)}
@end example

@item
Set the @dfn{active} flag in the partition using the command
@command{makeactive}@footnote{This is not necessary for most of the
modern operating systems.} (@pxref{makeactive}):

@example
grub> @kbd{makeactive}
@end example

@item
Load the boot loader with the command @command{chainloader}
(@pxref{chainloader}):

@example
grub> @kbd{chainloader +1}