grub.texi

Linux的启动装载(BootLoader)程序

These are called @dfn{Stage 1.5}, because they serve as a bridge
between @file{stage1} and @file{stage2}, that is to say, Stage 1.5 is
loaded by Stage 1 and Stage 1.5 loads Stage 2. The difference between
@file{stage1} and @file{*_stage1_5} is that the former doesn't
understand any filesystem while the latter understands one filesystem
(e.g. @file{e2fs_stage1_5} understands ext2fs). So you can move the
Stage 2 image to another location safely, even after GRUB has been
installed.

While Stage 2 cannot generally be embedded in a fixed area as the size
is so large, Stage 1.5 can be installed into the area right after an MBR,
or the boot loader area of a ReiserFS or a FFS.

@item nbgrub
This is a network boot image for the Network Image Proposal used by some
network boot loaders, such as Etherboot. This is mostly the same as
Stage 2, but it also sets up a network and loads a configuration file
from the network.

@item pxegrub
This is another network boot image for the Preboot Execution Environment
used by several Netboot ROMs. This is identical to @file{nbgrub}, except
for the format.
@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{find}
(@pxref{find}).

@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 zero for primary partitions and from four 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{(hd0,0)}
represents using the first partition of the disk (or the boot sector
of the partition when installing GRUB).

If you enabled the network support, the special drive, @samp{(nd)}, is
also available. Before using the network drive, you must initialize the
network. @xref{Network}, for more information.


@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{(hd0,0)/boot/grub/menu.lst}. This means the file
@file{/boot/grub/menu.lst} in the first partition of the first hard
disk. If you omit the device name in an absolute file name, GRUB uses
GRUB's @dfn{root device} implicitly. So if you set the root device to,
say, @samp{(hd1,0)} by the command @command{root} (@pxref{root}), then
@code{/boot/kernel} is the same as @code{(hd1,0)/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{(hd0,1)+1} is the same as @code{+1} when the root
device is @samp{(hd0,1)}.


@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
* Hidden menu interface::       The hidden menu interface
@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.

When a particular line is selected, the editor places the user in a
special version of the GRUB command-line to edit that line.  When the
user hits @key{RET}, GRUB replaces the line in question in the boot
entry with the changes (unless it was aborted via @key{ESC},
in which case the changes are thrown away).

If you want to add a new line to the menu entry, press @key{o} if adding
a line after the current line or press @key{O} if before the current
line.

To delete a line, hit the key @key{d}. Although GRUB unfortunately
does not support @dfn{undo}, you can do almost the same thing by just
returning to the main menu.


@node Hidden menu interface
@section The hidden menu interface

When your terminal is dumb or you request GRUB to hide the menu
interface explicitly with the command @command{hiddenmenu}
(@pxref{hiddenmenu}), GRUB doesn't show the menu interface (@pxref{Menu
interface}) and automatically boots the default entry, unless
interrupted by pressing @key{ESC}.

When you interrupt the timeout and your terminal is dumb, GRUB falls
back to the command-line interface (@pxref{Command-line interface}).


@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.

@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
* default::                     Set the default entry
* fallback::                    Set the fallback entry
* hiddenmenu::                  Hide the menu interface
* timeout::                     Set the timeout
* title::                       Start a menu entry
@end menu


@node default
@subsection default

@deffn Command default num
Set the default entry to the entry number @var{num}. Numbering starts
from 0, and the entry number 0 is the default if the command is not
used.

You can specify @samp{saved} instead of a number. In this case, the
default entry is the entry saved with the command
@command{savedefault}. @xref{savedefault}, for more information.
@end deffn


@node fallback
@subsection fallback

@deffn Command fallback num
Go into unattended boot mode: if the default boot entry has any errors,
instead of waiting for the user to do something, immediately start
over using the @var{num} entry (same numbering as the @code{default}
command (@pxref{default})). This obviously won't help if the machine was
rebooted by a kernel that GRUB loaded.
@end deffn


@node hiddenmenu
@subsection hiddenmenu

@deffn Command hiddenmenu
Don't display the menu. If the command is used, no menu will be
displayed on the control terminal, and the default entry will be
booted after the timeout expired. The user can still request the
menu to be displayed by pressing @key{ESC} before the timeout
expires. See also @ref{Hidden menu interface}.
@end deffn


@node timeout
@subsection timeout

@deffn Command timeout sec
Set a timeout, in @var{sec} seconds, before automatically booting the
default entry (nor