This chapter documents the grub shell grub. Note that the grub
shell is an emulator; it doesn't run under the native environment, so it
sometimes does something wrong. Therefore, you shouldn't trust it too
much. If there is anything wrong with it, don't hesitate to try the
native GRUB environment, especially when it guesses a wrong map between
BIOS drives and OS devices.
You can use the command grub for installing GRUB under your
operating systems and for a testbed when you add a new feature into GRUB
or when fix a bug. grub is almost the same as the Stage 2,
and, in fact, it shares the source code with the Stage 2 and you can use
the same commands (see section 13. The list of available commands) in grub. It is emulated by
replacing BIOS calls with UNIX system calls and libc functions.
The command grub accepts the following options:
Print a summary of the command-line options and exit.
Print the version number of GRUB and exit.
Print some verbose messages for debugging purpose.
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 15.3 The map between BIOS drives and OS devices), the grub shell just
ignores this option.
Read the configuration file file instead of
`/boot/grub/menu.lst'. The format is the same as the normal GRUB
syntax. See 11. Filesystem syntax and semantics, for more information.
Set the stage2 boot_drive to drive. This argument should be
an integer (decimal, octal or hexadecimal).
Set the stage2 install_partition to par. This argument
should be an integer (decimal, octal or hexadecimal).
Do not use the configuration file even if it can be read.
Do not use the curses interface even if it is available.
This option has the same meaning as `--no-config-file --no-curses'.
Disable writing to any disk.
Wait until a debugger will attach. This option is useful when you want
to debug the startup code.
The installation procedure is the same as under the native Stage
2. See section 3. Installation, for more information. The command
grub-specific information is described here.
What you should be careful about is buffer cache. 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:
If you can unmount drives to which GRUB may write any amount of data,
unmount them before running grub.
If a drive cannot be unmounted but can be mounted with the read-only
flag, mount it in read-only mode. That should be secure.
If a drive must be mounted with the read-write flag, make sure that any
activity is not being done on it during running the command
Reboot your operating system as soon as possible. Probably that is not
required if you follow these rules above, but reboot is the most secure
In addition, enter the command quit when you finish the
installation. That is very important because 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:
# Use /usr/sbin/grub if you are on an older system.
/sbin/grub --batch <<EOT 1>/dev/null 2>/dev/null
When you specify the option `--device-map' (see section 15.1 Introduction into the grub shell),
the grub shell creates the device map file automatically unless it
already exists. The file name `/boot/grub/device.map' is preferred.
If the device map file exists, the grub shell reads it to map BIOS
drives to OS devices. This file consists of lines like this:
device is a drive, which syntax is the same as the one in GRUB
(see section 11.1 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 `#'.