Untitled Document

Introduction

This part documents the user-visible aspect of GRUB. If you are looking for the information on the internals, see the Programmer Reference Manual (@xref{Hacking}).

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 (see section `Motivation' in The Multiboot Specification), because they were determined not to add to the large number of mutually-incompatible PC boot methods.

Erich then began modifying the FreeBSD boot loader so that it would understand Multiboot. He soon realized that it would be a lot easier to write his own boot loader from scratch than to keep working on the FreeBSD boot loader, and so GRUB was born.

Erich added many features to GRUB, but other priorities prevented him from keeping up with the demands of its quickly-expanding user base. In 1999, Gordon Matzigkeit and OKUJI Yoshinori 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.

GRUB features

The primary requirement for GRUB is that it be compliant with the Multiboot Specification, which is described in section `Motivation' in The Multiboot Specification.

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

Except for specific compatibility modes (chain-loading and the Linux piggyback format), all kernels will be started in much the same state as in the Multiboot Specification. Only kernels loaded at 1 megabyte or above are presently supported. Any attempt to load below that boundary will simply result in immediate failure and an error message reporting the problem.

In addition to the requirements above, GRUB has the following features (note that the Multiboot Specification doesn't require all the features that GRUB supports):

Multiple Executable Formats
Supports many of the a.out variants plus ELF. Symbol tables are also loaded.
Support Non-Multiboot Kernels
Supports 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.
Load Multiples Modules
GRUB fully supports the Multiboot feature of loading multiple modules.
Configuration File
Supports a human-readable text configuration file with preset boot commands. The list of commands (see section The list of available commands) are a superset of those supported on the command line. An example configuration file is provided in @xref{Configuration}.
Menu Interface
A menu interface listing the 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.
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 (see section The list of available commands) are a subset of those supported for configuration files. Editing commands closely resembles the Bash command line (see section `Command Line Editing' in Bash Features), with TAB-completion of commands, devices, partitions, and files in a directory depending on context.
Multiple Filesystem Types
Supports multiple filesystem types transparently, plus a useful explicit blocklist notation. The currently supported filesystem types are BSD FFS, DOS FAT16 and FAT32, Minix fs, Linux ext2fs, and ReiserFS. See section Filesystem syntax and semantics, for more information.
Decompression Support
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 file size and loading time, a particularly major benefit for floppies.(1) 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.
Access Data on Any Installed Device
Supports reading data from any or all floppy or hard disk(s) recognized by the BIOS, independent of the setting of the root device.
Independent of Drive Geometry Translation
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.
Detect All Installed RAM
GRUB can generally find all the installed RAM on a PC-compatible machine. It uses an advanced BIOS query technique for finding all memory regions (@xref{Memory detection}). As described on the Multiboot Specification (see section `Motivation' in The Multiboot Specification), not all kernels make use of this information, but GRUB provides it for those who do.
Support Logical Block Address Mode
In traditional disk calls (called CHS mode), there is a geometry translation problem, that is, the BIOS cannot access over 1024 cylinders, so the accessible space is limited to at least 508 MB and to at most 8GB. GRUB can't universally solve this problem, as there is no standard interface used in all machines. However, several newer machines have the new interface, Logical Block Address (LBA) mode. GRUB automatically detects if LBA mode is available and uses it if available. In LBA mode, GRUB can access the entire disk.
Network support
GRUB is a disk-based boot loader but also has network support. You can load OS images from a network by using the TFTP protocol.

Future directions might include an internal programming language for supporting richer sets of boot options with control statements (which would make GRUB its own kind of kernel). Support for non-PC hardware architectures is also planned.(2)

The role of a boot loader

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

Some people like to acknowledge both the operating system and kernel when they talk about their computers, so they might say they use "GNU/Linux" or "GNU/Hurd". Other people seem to think that the kernel is the most important part of the system, so they like to call their GNU operating systems "Linux systems."

I, personally, believe that this is a grave injustice, because the boot loader is the most important software of all. I used to refer to the above systems as either "LILO"(3) or "GRUB" systems.

Unfortunately, nobody ever understood what I was talking about; now I just use the word "GNU" as a pseudonym for GRUB.

So, if you ever hear people talking about their alleged "GNU" systems, remember that they are actually paying homage to the best boot loader around... GRUB!

We, the GRUB maintainers, do not (usually) encourage Gordon's level of fanaticism, but it helps to remember that boot loaders deserve recognition. We hope that you enjoy using GNU GRUB as much as we did writing it.

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} (see section The list of available commands).

How to specify devices

The device syntax is like this: 

(bios-device[,part-num][,bsd-subpart-letter])

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

(hd0)
(0x80)
(128)

part-num represents the partition number of bios-device, starting from zero for primary partitions and from four for extended partitions, and bsd-subpart-letter represents the BSD disklabel subpartition, such as `a' or `e'.

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

(hd0,a)

The syntax like `(hd0)' represents using the entire disk (or the MBR when installing GRUB), while the syntax like `(hd0,0)' represents using the partition of the disk (or the boot sector of the partition when installing GRUB).

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

How to specify files

There are two ways to specify files, by absolute filename and by blocklist.

An absolute filename resembles a Unix absolute filename, using `/' for the directory separator (not `\' as in DOS). One example is `(hd0,0)/boot/grub/menu.lst'. This means the file `/boot/grub/menu.lst' in the first partition of the first hard disk. If you omit the device name in an absolute filename, GRUB uses GRUB's root device implicitly. So if you set the root device to, say, `(hd1,0)' by the command @command{root}, then /boot/kernel is the same as (hd1,0)/boot/kernel.

How to specify blocklists

A blocklist is used for specifying a file that doesn't appear in the filesystem, like a chainloader. The syntax is [offset]+length[,[offset]+length].... Here is an example: 

0+100,200+1,300+300

This represents that GRUB should read blocks 0 through 99, block 200, and blocks 300 through 599. If you omit an offset, then GRUB assumes the offset is zero.

Like the filename syntax (see section How to specify files), if a blocklist does not contain a device name, then GRUB uses GRUB's root device. So (hd0,1)+1 is the same as +1 when the root device is `(hd0,1)'.

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 command line menu option, or if the configuration file was not found, then GRUB drops to the command line interface.

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 (4). The commands (see section The list of available 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:

C-f
PC right key
Move forward one character.
C-b
PC left key
Move back one character.
C-a
HOME
Move to the start of the line.
C-e
END
Move the the end of the line.
C-d
DEL
Delete the character underneath the cursor.
C-h
BS
Delete the character to the left of the cursor.
C-k
Kill the text from the current cursor position to the end of the line.
C-u
Kill backward from the cursor to the beginning of the line.
C-y
Yank the killed text back into the buffer at the cursor.
C-p
PC up key
Move up through the history list.
C-n
PC down key
Move down through the history list.

When typing commands interactively, if the cursor is within or before the first word in the command-line, pressing the TAB key (or C-i) will display a listing of the available commands, and if the cursor is after the first word, the TAB will provide a completion listing of disks, partitions, and filenames depending on the context.

Note that you cannot use the completion functionality in the TFTP filesystem. This is because TFTP doesn't support filename listing for the security.

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 boot entries to the user to choose from. Use the arrow keys to select the entry of choice, then press RET to run it. An optional timeout is available to boot the default entry (the first one if not set), which is aborted by pressing any key.

Commands are available to enter a bare command line by pressing c (which operates exactly like the non-config-file version of GRUB, but allows one to return to the menu if desired by pressing ESC) or to edit any of the boot entries by pressing e.

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 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 at a special version of the GRUB command line to edit that line. When the user hits RET, GRUB replaces the line in question in the boot entry with the changes (unless it was aborted via ESC, in which case the changes are thrown away).

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

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

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 either used in the menu or in the menu entries.

The list of commands for the menu only

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

These commands can only be used in the menu:

Command: default num
Set the default entry to the entry number num. Numbering starts from 0, and entry number 0 is the default if the command is not used.

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

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 ESC before the timeout expires.

Command: password passwd [new-config-file]
Disable all interactive editing control (menu entry editor and command line) and entries protected by the command @command{lock}. If the password passwd is entered, it loads the new-config-file as a new config file and restarts the GRUB Stage 2, if new-config-file is specified. Otherwise, GRUB will just unlock the privileged instructions.

Command: timeout sec
Set a timeout, in sec seconds, before automatically booting the default entry (normally the first entry defined).

Command: title name ...
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.

The list of command-line and menu commands

Commands usable both in the menu and in the command line.

Command: bootp
Initialize a network device via the BOOTP protocol. This command is only available if GRUB is compiled with netboot support.

Command: color normal [highlight]
Change the menu colors. The color normal is used for most lines in the menu, and the color highlight is used to highlight the line where the cursor points. If you omit highlight, then the inverted color of normal is used for the highlighted line. The format of a color is foreground/background. foreground and background are symbolic color names. A symbolic color name must be one of these:

But only the first eight names can be used for background. You can prefix blink- to foreground if you want a blinking foreground color.

This command can be used in the configuration file and on the command line, so you may write something like this in your configuration file: 

# Set default colors.
color light-gray/blue black/light-gray

# Change the colors.
title OS-BS like
color magenta/blue black/magenta

Command: device drive file
In the grub shell, specify the file file as the actual drive for a BIOS drive drive. You can use this command to create a disk image, and/or to fix the drives guessed by GRUB when GRUB fails to determine them correctly, like this: 
grub> device (fd0) /floppy-image
grub> device (hd0) /dev/sd0

This command can be used only in the grub shell (see section Invoking the grub shell).

Command: dhcp
Initialize a network device via the DHCP protocol. Currently, this command is just an alias for @command{bootp}, since the two protocols are very similar. This command is only available if GRUB is compiled with netboot support.

Command: hide partition
Hide partition by setting the hidden bit in its partition type code. This is useful only when booting DOS or Windows and multiple primary FAT partitions exist in one disk.

Command: part_new part type from to
Create a new primary partition. part is a partition specification in GRUB syntax (@xref{Naming convention}); type is the partition type and must be a number in the range 0-0xff; from and to are the starting and ending sectors, expressed as an absolute sector number.

Command: part_type part type
Change the type of an existing partition. part is a partition specification in GRUB syntax (@xref{Naming convention}); type is the new partition type and must be a number in the range 0-0xff.

Command: rarp
Initialize a network device via the RARP protocol. This command is only available if GRUB is compiled with netboot support.

Command: serial [@option{--unit=}unit] [@option{--port=}port] [@option{--speed=}speed] [@option{--word=}word] [@option{--parity=}parity] [@option{--stop=}stop] [@option{--device=}dev]
Initialize a serial device. unit is a number in the range 0-3 specifying which serial port to use; default is 0, that corresponds the port often called COM1. port is the I/O port where the UART is to be found; if specified it takes precedence over unit. speed is the transmission speed; default is 9600. word and stop are the number of data bits and stop bits. Data bits must be in the range 5-8 and stop bits are 1 or 2. Default is 8 data bits and one stop bit. parity is one of @option{no}, @option{odd}, @option{even} and defaults to @option{no}. The option @option{--device} can only be used in the GRUB shell and is used to specify the tty device to be used in the host Operating System.

The serial port is not used as a communication channel unless the terminal command is used.

This command is only available if GRUB is compiled with serial support.

Command: setkey to_key from_key
Change the keyboard map. The key from_key is mapped to the key to_key. Note that this command does not exchange the keys. If you want to exchange the keys, run this command again with the arguments exchanged, like this: 
grub> setkey capslock control
grub> setkey control capslock

A key must be an alphabet, a digit, or one of these symbols: `escape', `exclam', `at', `numbersign', `dollar', `percent', `caret', `ampersand', `asterisk', `parenleft', `parenright', `minus', `underscore', `equal', `plus', `backspace', `tab', `bracketleft', `braceleft', `bracketright', `braceright', `enter', `control', `semicolon', `colon', `quote', `doublequote', `backquote', `tilde', `shift', `backslash', `bar', `comma', `less', `period', `greater', `slash', `question', `alt', `space', `capslock', `FX' (`X' is a digit), and `delete'. This table describes to which character each of the symbols corresponds:

`exclam'
`!'
`at'
`@'
`numbersign'
`#'
`dollar'
`$'
`percent'
`%'
`caret'
`^'
`ampersand'
`&'
`asterisk'
`*'
`parenleft'
`('
`parenright'
`)'
`minus'
`-'
`underscore'
`_'
`equal'
`='
`plus'
`+'
`bracketleft'
`['
`braceleft'
`{'
`bracketright'
`]'
`braceright'
`}'
`semicolon'
`;'
`colon'
`:'
`quote'
`''
`doublequote'
`"'
`backquote'
``'
`tilde'
`~'
`backslash'
`\'
`bar'
`|'
`comma'
`,'
`less'
`<'
`period'
`.'
`greater'
`>'
`slash'
`/'
`question'
`?'
`space'
` '

Command: terminal [@option{--dumb}] [@option{--timeout=}secs] [console] [serial]"
Select a terminal for user interaction. The terminal is assumed to be vt100 compatible unless @option{--dumb} is specified. If both @option{console} and @option{serial} are specified, then GRUB will use the one where a key is entered first. If neither are specified, the current setting is reported. Default is @option{console}. This command is only available if GRUB is compiled with serial support.

Command: tftpserver ipaddr
Override a TFTP server address returned by a BOOTP/DHCP/RARP server. The argument ipaddr must be in dotted decimal format, like `192.168.0.15'. This command is only available if GRUB is compiled with netboot support.

Command: unhide partition
Unhide partition by clearing the hidden bit in its partition type code. This is useful only when booting DOS or Windows and multiple primary partitions exist in one disk.

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}.

Command: blocklist file
Print the blocklist notation of the file file (see section How to specify blocklists).

Command: boot
Boot the OS/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).

Command: cat file
Display the contents of the file file. This command may be useful to remind you of your OS's root partition: 
grub> cat /etc/fstab

Command: chainloader [@option{--force}] file
Load file as a chain-loader. Like any other file loaded by the filesystem code, it can use the blocklist notation to grab the first sector of the current partition with `+1'. If you specify the option @option{--force}, then load file forcibly, whether it has a correct signature or not. This is required when you want to load a defective boot loader, such as SCO Unixware 7.1.

Command: cmp file1 file2
Compare the file file1 with the file file2. If they differ in size, print the sizes like this: 
Differ in size: 0x1234 [foo], 0x4321 [bar]

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

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

If they are completely identical, nothing will be printed.

Command: configfile file
Load file as a configuration file.

Command: debug
Toggle debug mode (by default it is off). When debug mode is on, some extra messages are printed to show disk activity. This global debug flag is mainly useful for GRUB developers when testing new code.

Command: displaymem
Display what GRUB thinks the system address space map of the machine is, including all regions of physical RAM installed. GRUB's upper/lower memory display uses the standard BIOS interface for the available memory in the first megabyte, or lower memory, and a synthesized number from various BIOS interfaces of the memory starting at 1MB and going up to the first chipset hole for upper memory (the standard PC upper memory interface is limited to reporting a maximum of 64MB).

Command: embed stage1_5 device
Embed the Stage 1.5 stage1_5 in the sectors after the MBR if device is a drive, or in the boot loader area if device is a FFS partition or a ReiserFS partition.(5) Print the number of sectors which stage1_5 occupies, if successful.

Command: find filename
Search for the filename filename in all of partitions and print the list of the devices which contain the file. The filename filename should be an absolute filename like /boot/grub/stage1.

Command: fstest
Toggle filesystem test mode. Filesystem test mode, when turned on, prints out data corresponding to all the device reads and what values are being sent to the low-level routines. The format is `<partition-offset-sector, byte-offset, byte-length>' for high-level reads inside a partition, and `[disk-offset-sector]' for low-level sector requests from the disk. Filesystem test mode is turned off by any use of the @command{install} or @command{testload} commands.

Command: geometry drive [cylinder head sector [total_sector]]
Print the information for the drive drive. In the GRUB shell, you can set the geometry of the drive arbitrarily. The number of the cylinders, the one of the heads, the one of the sectors and the one of the total sectors are set to CYLINDER, HEAD, SECTOR and TOTAL_SECTOR, respectively. If you omit TOTAL_SECTOR, then it will be calculated based on the C/H/S values automatically.

Command: halt @option{--no-apm}
The commands 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.

Command: help [pattern ...]
Display helpful information about builtin commands. If you do not specify pattern, this command shows short descriptions of all available commands. If you specify any patterns, it displays longer information about each of the commands which match those patterns.

Command: impsprobe
Probe the Intel Multiprocessor Specification 1.1 or 1.4 configuration table and boot the various CPUs which are found into a tight loop. This command can be used only in the Stage 2.

Command: initrd file ...
Load an initial ramdisk for a Linux format boot image and set the appropriate parameters in the Linux setup area in memory.

Command: install [@option{--force-lba}] stage1_file [@option{d}] dest_dev stage2_file [addr] [@option{p}] [config_file] [real_config_file]
This command is fairly complex, and you should not use this command unless you are familiar with GRUB. In short, it will perform a full install presuming the Stage 2 or Stage 1.5(6) is in its final install location.

In slightly more detail, it will load stage1_file, validate that it is a GRUB Stage 1 of the right version number, install a blocklist for loading stage2_file as a Stage 2. If the option @option{d} is present, the Stage 1 will always look for the actual disk stage2_file was installed on, rather than using the booting drive. The Stage 2 will be loaded at address addr, which must be `0x8000' for a true Stage 2, and `0x2000' for a Stage 1.5. If addr is not present, GRUB will determine the address automatically. It then writes the completed Stage 1 to the first block of the device dest_dev. If the options @option{p} or config_file are present, then it reads the first block of stage2, modifies it with the values of the partition stage2_file was found on (for @option{p}) or places the string config_file into the area telling the stage2 where to look for a configuration file at boot time. Likewise, if real_config_file is present and stage2_file is a Stage 1.5, then the Stage 2 config_file is patched with the configuration filename real_config_file. This command preserves the DOS BPB (and for hard disks, the partition table) of the sector the Stage 1 is to be installed into.

Caution: Several buggy BIOSes don't pass a booting drive properly when booting from a hard disk drive. Therefore, you will have to specify the option @option{d}, whether your Stage2 resides at the booting drive or not, if you have such a BIOS unfortunately. We know these are defective in that:

Fujitsu LifeBook 400 BIOS version 31J0103A
HP Vectra XU 6/200 BIOS version GG.06.11

Caution2: A number of BIOSes don't return a correct LBA support bitmap even if they do have the support. So GRUB provides a solution to ignore the wrong bitmap, that is, the option @option{--force-lba}. Don't use this option if you know that your BIOS doesn't have LBA support.

Command: ioprobe drive
Probe I/O ports used for the drive drive. This command will list the I/O ports on the screen. For technical information, @xref{I/O ports detection}.

Command: kernel [@option{--type=type}] file ...
Attempt to load the primary boot image (Multiboot a.out or ELF, Linux zImage or bzImage, FreeBSD a.out, NetBSD a.out, etc.) from file. The rest of the line is passed verbatim as the kernel command line. Any modules must be reloaded after using this command.

This command also accepts the option @option{--type} so that you can specify the kernel type of file explicitly. The argument type must be one of these: `netbsd', `freebsd', `openbsd', `linux', `biglinux', and `multiboot'. However, you need to specify it only if you want to load a NetBSD ELF kernel, because GRUB can automatically determine a kernel type in the other cases, quite safely.

Command: lock
Prevent normal users from executing arbitrary menu entries. You must use the command @command{password} if you really want this command to be useful.

This command is used in a menu, as shown in this this example: 

title This entry is too dangerous to be executed by normal users
lock
root (hd0,a)
kernel /no-security-os

Command: makeactive
Set the active partition on the root disk to GRUB's root device. This command is limited to primary PC partitions on a hard disk.

Command: map to_drive from_drive
Map the drive from_drive to the drive to_drive. This is necessary when you chain-load some operating systems, such as DOS, if such an OS resides at a non-first drive. Here is an example: 
grub> map (hd0) (hd1)
grub> map (hd1) (hd0)

The example exchanges the order between the first hard disk and the second hard disk.

Command: module file ...
Load a boot module file for a Multiboot format boot image (no interpretation of the file contents are made, so that user of this command must know what the kernel in question expects). The rest of the line is passed as the module command line, like the @command{kernel} command. You must load a Multiboot kernel image before loading any module.

Command: modulenounzip file ...
The same as @command{module}, except that automatic decompression is disabled.

Command: pause message ...
Print the message, then wait until a key is pressed. Note that placing ^G (ASCII code 7) in the message will cause the speaker to emit the standard beep sound, which is useful when prompting the user to change floppies.

Command: quit
Exit from the GRUB shell @command{grub} (see section Invoking the grub shell). This command can be used only in the GRUB shell.

Command: reboot
Reboot the computer.

Command: read addr
Read a 32-bit value from memory at address addr and display it in hex format.

Command: root device [hdbias]
Set the current root device to the device device, then attempt to mount it to get the partition size (for passing the partition descriptor in ES:ESI, used by some chain-loaded boot loaders), the BSD drive-type (for booting BSD kernels using their native boot format), and correctly determine the PC partition where a BSD sub-partition is located. The optional hdbias parameter is a number to tell a BSD kernel how many BIOS drive numbers are on controllers before the current one. For example, if there is an IDE disk and a SCSI disk, and your FreeBSD root partition is on the SCSI disk, then use a `1' for hdbias.

Command: rootnoverify device [hdbias]
Similar to @command{root}, but don't attempt to mount the partition. This is useful for when an OS is outside of the area of the disk that GRUB can read, but setting the correct root device is still desired. Note that the items mentioned in @command{root} above which derived from attempting the mount will not work correctly.

Command: savedefault
Save the current menu entry as default entry.

Command: setup [@option{--force-lba}] install_device [image_device]
Set up the installation of GRUB automatically. This command uses the more flexible command @command{install} in the backend and installs GRUB into the device install_device. If image_device is specified, then find the GRUB images in the device image_device, otherwise use the current root device, which can be set by the command @command{root}. If install_device is a hard disk, then embed a Stage 1.5 in the disk if possible.

The option @option{--force-lba} is just passed to @command{install} if specified. See the description on @command{install}, for more information.

Command: testload file
Read the entire contents of file in several different ways and compares them, to test the filesystem code. The output is somewhat cryptic , but if no errors are reported and the final `i=X, filepos=Y' reading has X and Y equal, then it is definitely consistent, and very likely works correctly subject to a consistent offset error. If this test succeeds, then a good next step is to try loading a kernel.

Command: uppermem kbytes
Force GRUB to assume that only kbytes kilobytes of upper memory are installed. Any system address range maps are discarded.

Caution: This should be used with great caution, and should only be necessary on some old machines. GRUB's BIOS probe can pick up all RAM on all new machines the author has ever heard of. It can also be used for debugging purposes to lie to an OS.

Error messages reported by GRUB

This chapter describes error messages reported by GRUB when you encounter trouble. See section Invoking the grub shell, if your problem is specific to the grub shell.

Errors reported by the Stage 1

The general way that the Stage 1 handles errors is to print an error string and then halt. Pressing CTRL-ALT-DEL will reboot.

The following is a comprehensive list of error messages for the Stage 1:

Hard Disk Error
The stage2 or stage1.5 is being read from a hard disk, and the attempt to determine the size and geometry of the hard disk failed.
Floppy Error
The stage2 or stage1.5 is being read from a floppy disk, and the attempt to determine the size and geometry of the floppy disk failed. It's listed as a separate error since the probe sequence is different than for hard disks.
Read Error
A disk read error happened while trying to read the stage2 or stage1.5.
Geom Error
The location of the stage2 or stage1.5 is not in the portion of the disk supported directly by the BIOS read calls. This could occur because the BIOS translated geometry has been changed by the user or the disk is moved to another machine or controller after installation, or GRUB was not installed using itself (if it was, the Stage 2 version of this error would have been seen during that process and it would not have completed the install).

Errors reported by the Stage 1.5

The general way that the Stage 1.5 handles errors is to print an error number in the form Error num and then halt. Pressing CTRL-ALT-DEL will reboot.

The error numbers correspond to the errors reported by Stage 2. See section Errors reported by the Stage 2.

Errors reported by the Stage 2

The general way that the Stage 2 handles errors is to abort the operation in question, print an error string, then (if possible) either continue based on the fact that an error occurred or wait for the user to deal with the error.

The following is a comprehensive list of error messages for the Stage 2 (error numbers for the Stage 1.5 are listed before the colon in each description):

1 : Filename must be either an absolute filename or blocklist
This error is returned if a filename is requested which doesn't fit the syntax/rules listed in the section Filesystem syntax and semantics.
2 : Bad file or directory type
This error is returned if a file requested is not a regular file, but something like a symbolic link, directory, or FIFO.
3 : Bad or corrupt data while decompressing file
This error is returned the run-length decompression code gets an internal error. This is usually from a corrupt file.
4 : Bad or incompatible header in compressed file
This error is returned if the file header for a supposedly compressed file is bad.
5 : Partition table invalid or corrupt
This error s returned if the sanity checks on the integrity of the partition table fail. This is a bad sign.
6 : Mismatched or corrupt version of stage1/stage2
This error is returned if the install command is pointed to incompatible or corrupt versions of the stage1 or stage2. It can't detect corruption in general, but this is a sanity check on the version numbers, which should be correct.
7 : Loading below 1MB is not supported
This error is returned if the lowest address in a kernel is below the 1MB boundary. The Linux zImage format is a special case and can be handled since it has a fixed loading address and maximum size.
8 : Kernel must be loaded before booting
This error is returned if GRUB is told to execute the boot sequence without having a kernel to start.
9 : Unknown boot failure
This error is returned if the boot attempt did not succeed for reasons which are unknown.
10 : Unsupported Multiboot features requested
This error is returned when the Multiboot features word in the Multiboot header requires a feature that is not recognized. The point of this is that the kernel requires special handling which GRUB is likely unable to provide.
11 : Unrecognized device string
This error is returned if a device string was expected, and the string encountered didn't fit the syntax/rules listed in the section Filesystem syntax and semantics.
12 : Invalid device requested
This error is returned if a device string is recognizable but does not fall under the other device errors.
13 : Invalid or unsupported executable format
This error is returned if the kernel image being loaded is not recognized as Multiboot or one of the supported native formats (Linux zImage or bzImage, FreeBSD, or NetBSD).
14 : Filesystem compatibility error, cannot read whole file
Some of the filesystem reading code in GRUB has limits on the length of the files it can read. This error is returned when the user runs into such a limit.
15 : File not found
This error is returned if the specified filename cannot be found, but everything else (like the disk/partition info) is OK.
16 : Inconsistent filesystem structure
This error is returned by the filesystem code to denote an internal error caused by the sanity checks of the filesystem structure on disk not matching what it expects. This is usually caused by a corrupt filesystem or bugs in the code handling it in GRUB.
17 : Cannot mount selected partition
This error is returned if the partition requested exists, but the filesystem type cannot be recognized by GRUB.
18 : Selected cylinder exceeds maximum supported by BIOS
This error is returned when a read is attempted at a linear block address beyond the end of the BIOS translated area. This generally happens if your disk is larger than the BIOS can handle (512MB for (E)IDE disks on older machines or larger than 8GB in general).
19 : Linux kernel must be loaded before initrd
This error is returned if the initrd command is used before loading a Linux kernel. Similar to the above error, it only makes sense in that case anyway.
20 : Multiboot kernel must be loaded before modules
This error is returned if the module load command is used before loading a Multiboot kernel. It only makes sense in this case anyway, as GRUB has no idea how to communicate the presence of location of such modules to a non-Multiboot-aware kernel.
21 : Selected disk does not exist
This error is returned if the device part of a device- or full filename refers to a disk or BIOS device that is not present or not recognized by the BIOS in the system.
22 : No such partition
This error is returned if a partition is requested in the device part of a device- or full filename which isn't on the selected disk.
23 : Error while parsing number
This error is returned if GRUB was expecting to read a number and encountered bad data.
24 : Attempt to access block outside partition
This error is returned if a linear block address is outside of the disk partition. This generally happens because of a corrupt filesystem on the disk or a bug in the code handling it in GRUB (it's a great debugging tool).
25 : Disk read error
This error is returned if there is a disk read error when trying to probe or read data from a particular disk.
26 : Too many symbolic links
This error is returned if the link count is beyond the maximum (currently 5), possibly the symbolic links are looped.
27 : Unrecognized command
This error is returned if an unrecognized command is entered into the command line or in a boot sequence section of a configuration file and that entry is selected.
28 : Selected item cannot fit into memory
This error is returned if a kernel, module, or raw file load command is either trying to load its data such that it won't fit into memory or it is simply too big.
29 : Disk write error
This error is returned if there is a disk write error when trying to write to a particular disk. This would generally only occur during an install of set active partition command.
30 : Invalid argument
This error is returned if an argument specified to a command is invalid.
31 : File is not sector aligned
This error may occur only when you access a ReiserFS partition by block-lists (e.g. the command @command{install}). In this case, you should mount the partition with the `-o notail' option.
32 : Must be authenticated
This error is returned if you try to run a locked entry. You should enter a correct password before running such an entry.

Invoking the grub shell

This chapter documents the grub shell @command{grub}. Note that the grub shell is an emulator; it doesn't run under the native environment, so it sometimes does something wrong. Therefore, you shouldn't trust it too much. If there is anything wrong with it, don't hesitate to try the native GRUB environment, especially when it guesses a wrong map between BIOS drives and OS devices.

Introduction into the grub shell

You can use the command @command{grub} for installing GRUB under your operating systems and for a testbed when you add a new feature into GRUB or when fix a bug. @command{grub} is almost the same as the Stage 2, and, in fact, it shares the source code with the Stage 2 and you can use the same commands (see section The list of available commands) in @command{grub}. It is emulated by replacing BIOS calls with UNIX system calls and libc functions.

The command @command{grub} accepts the following options:

@option{--help}
Print a summary of the command line options and exit.
@option{--version}
Print the version number of GRUB and exit.
@option{--verbose}
Print some verbose messages for debugging purpose.
@option{--device-map=file}
Use the device map file file. The format is described in section The map between BIOS drives and OS devices.
@option{--no-floppy}
Do not probe any floppy drive. This option has no effect if the option @option{--device-map} is specified (see section The map between BIOS drives and OS devices).
@option{--probe-second-floppy}
Probe the second floppy drive. If this option is not specified, the grub shell does not probe it, as that sometimes takes a long time. If you specify the device map file (see section The map between BIOS drives and OS devices), the grub shell just ignores this option.
@option{--config-file=file}
Read the configuration file file instead of `/boot/grub/menu.lst'. The format is the same as the normal GRUB syntax. See section Filesystem syntax and semantics, for more information.
@option{--boot-drive=drive}
Set the stage2 boot_drive to drive. This argument should be an integer (decimal, octal or hexadecimal).
@option{--install-partition=par}
Set the stage2 install_partition to par. This argument should be an integer (decimal, octal or hexadecimal).
@option{--no-config-file}
Do not use the configuration file even if it can be read.
@option{--no-curses}
Do not use the curses interface even if it is available.
@option{--batch}
This option has the same meaning as `--no-config-file --no-curses'.
@option{--read-only}
Disable writing to any disk.
@option{--hold}
Wait until a debugger will attach. This option is useful when you want to debug the startup code.

How to install GRUB via @command{grub}

The installation procedure is the same as under the native Stage 2. @xref{Installation}, for more information. The command @command{grub}-specific information is described here.

What you should be careful about is buffer cache. @command{grub} makes use of raw devices instead of filesystems that your operating systems serve, so there exists a potential problem that some cache inconsistency may corrupt your filesystems. What we recommend is:

In addition, enter the command @command{quit} when you finish the installation. That is very important because @command{quit} makes the buffer cache consistent. Do not push C-c.

If you want to install GRUB non-interactively, specify `--batch' option in the command line. This is a simple example: 

#!/bin/sh

# Use /usr/sbin/grub if you are on an older system.
/sbin/grub --batch <<EOT 1>/dev/null 2>/dev/null
root (hd0,0)
setup (hd0)
quit
EOT

The map between BIOS drives and OS devices

When you specify the option @option{--device-map} (see section Introduction into the grub shell), the grub shell creates the device map file automatically unless it already exists. The filename `/boot/grub/device.map' is preferred.

If the device map file exists, the grub shell reads it to map BIOS drives to OS devices. This file consists of lines like this: 

device file

device is a drive, which syntax is the same as the one in GRUB (see section How to specify devices), and file is an OS's file, which is normally a device file.

The reason why the grub shell gives you the device map file is that it cannot guess the map between BIOS drives and OS devices correctly in some environments. For example, if you exchange the boot sequence between IDE and SCSI in your BIOS, it mistakes the order.

Thus, edit the file if the grub shell makes a mistake. You can put any comments in the file if needed, as the grub shell assumes that a line is just a comment if the first character is `#'.

Invoking grub-install

The program @command{grub-install} installs GRUB on your drive by the grub shell (see section Invoking the grub shell). You must specify the device name on which you want to install GRUB, like this: 

grub-install install_device

The device name install_device is an OS device name or a GRUB device name.

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

@option{--help}
Print a summary of the command line options and exit.
@option{--version}
Print the version number of GRUB and exit.
@option{--force-lba}
Force GRUB to use LBA mode even for a buggy BIOS. Use this option only if your BIOS doesn't work in LBA mode even though it supports LBA mode.
@option{--root-directory=dir}
Install GRUB images under the directory dir instead of the root directory. This option is useful when you want to install GRUB into a separate partition or a removable disk. Here is an example when you have a separate boot partition which is mounted on `/boot': 
grub-install --root-directory=/boot '(hd0)'
@option{--grub-shell=file}
Use file as the grub shell. You can append arbitrary options to file after the filename, like this: 
grub-install --grub-shell="grub --read-only" /dev/fd0

Invoking mbchk

The program @command{mbchk} checks for the format of a Multiboot kernel. We recommend using this program before booting your own kernel by GRUB.

@command{mbchk} accepts the following options:

@option{--help}
Print a summary of the command line options and exit.
@option{--version}
Print the version number of GRUB and exit.
@option{--quiet}
Suppress all normal output.

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