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Briefly, a 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 kernel software (such as the Linux or GNU Hurd kernel). 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(1). 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 filename and the drive (and the partition) where the kernel resides. To let GRUB know the drive and the filename, you can either type in them manually via the command-line interface (@xref{Command line}), or use the nice menu interface (@xref{Menu interface}) through which you can easily select which OS it boots. To allow you to customize the menu, GRUB will load a preexisting configuration file (see section Configuration). Note that you can not only enter the command-line interface whenever you like, but also you can edit specific menu entries prior to using them.

In the following chapters, you will learn how to specify a drive or a partition, and a file name (see section Naming convention) to GRUB, how to install GRUB on your drive (see section Installation), and how to boot your OSes (see section Booting), step by step.

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:


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


Here, `hd' means it is a hard disk drive. The first integer `0' indicates the drive number, that is, the first hard disk, while the second integer, `1', indicates the partition number (or the PC slice number in the BSD terminology). Once again, please note that the partition numbers are counted from 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.


This specifies the first extended partition of the first hard disk drive. Note that the partition numbers for extended partitions are counted from `4', regardless of the actual number of primary partitions on your hard disk.


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

Of course, to actually access the disks or partitions with GRUB, you need to use the device specification in a command, like `root (fd0)' or `unhide (hd0,2)'. To help you find out which number is a partition you want, the GRUB command-line (@xref{Command line}) options have argument completion. That means that, for example, you only need to type `root (', followed by a TAB, and GRUB will display the list of drives, partitions, or filenames, 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 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, see this example:


This specifies the file named `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. Do read the next chapter, to find out how to actually install GRUB on your drive.


First, you need to have GRUB itself properly installed on your system, (@xref{Obtaining and Building GRUB}) either from the source tarball, or as a package for your OS.

To use GRUB, you need to install it on your drive. There are two ways of doing that - either using the utility grub-install (@xref{Invoking grub-install}) on a UNIX-like OS, or by using the native Stage 2. These are quite similar, however, the utility might probe a wrong BIOS drive, so better 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 installed in the `/usr/share/grub/i386-pc' directory. You need to copy the files `stage1', `stage2', and `*stage1_5' to the directory `/boot/grub'.

Creating a GRUB boot floppy

To create a GRUB boot floppy, you need to take the files `stage1' and `stage2' from `/boot/grub' directory, and write them to the first and the second block of the floppy disk, respectively.

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:

# cd /boot/grub
# dd if=stage1 of=/dev/fd0 bs=512 count=1
1+0 records in
1+0 records out
# dd if=stage2 of=/dev/fd0 bs=512 seek=1
153+1 records in
153+1 records out

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

Installing GRUB natively

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 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 `FDISK /MBR' from DOS).

If you decide to install GRUB in the native environment, which is definitely desirable, you'll need to create the GRUB boot disk, and reboot your computer with it. Otherwise, see section Installing GRUB using grub-install, for more details.

Once started, GRUB will show the command-line interface (@xref{Command line}). First, set the GRUB's root device(2) to the partition which has your GRUB images, like this:

grub> root (hd0,0)

If you are not sure which partition actually holds these files, use the find command, like this:

grub> find /boot/grub/stage1

This will search for the filename `/boot/grub/stage1' and show the devices which contain the file.

Once you've set the root device correctly, run the command setup:

grub> setup (hd0)

This command will install GRUB on the MBR (@xref{MBR}) in the first drive. If you want to install GRUB into the boot sector of a partition instead of the MBR, specify a partition into which you want to install GRUB:

grub> setup (hd0,0)

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.

Now you can boot GRUB without a GRUB floppy. See the chapter section Booting to find out how to boot your operating systems from GRUB.

Installing GRUB using grub-install

Unfortunately, if you do want to install GRUB under a UNIX-like OS (such as GNU), invoke the program grub-install as the superuser (root).

The usage is basically very easy. You only need to specify one argument to the program, namely, where to install GRUB. The argument can be either of a device file or a GRUB's drive/partition. So, this will install GRUB into the MBR of the first IDE disk under Linux:

# grub-install /dev/hda

Likewise, under Hurd, this has the same effect:

# grub-install /dev/hd0

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

# grub-install '(hd0)'

But all the above examples assume that you use GRUB 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:

# mke2fs /dev/fd0
# mount -t ext2 /dev/fd0 /mnt
# grub-install --root-directory=/mnt '(fd0)'
# umount /mnt

Another example is in case that you have a separate boot partition which is mounted at `/boot'. Since GRUB is a boot loader, it doesn't know anything about mountpoints at all. Thus, you need to run grub-install like this:

# grub-install --root-directory=/boot /dev/hda

By the way, as you may already know, it is quite difficult to guess BIOS drives correctly under a UNIX-like OS. Thus, grub-install will prompt you to check if it could really guess the correct mappings, after the installation. The format is defined in @xref{Device map}. Please be careful enough. If the output is wrong, it is unlikely that your computer can boot with no problem.

Note that grub-install is actually just a shell script and the real task is done by the grub shell grub (@xref{Invoking the grub shell}). Therefore, you may run grub directly to install GRUB, without using 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 dangerous.


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

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 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.

How to boot an OS directly by GRUB

Multiboot (see section `Motivation' in The Multiboot Specification) is the native format supported by GRUB. For the sake of convenience, there are also support for Linux, FreeBSD, NetBSD and OpenBSD. If you want to boot other operating systems, you will have to chain-load them (see section Load another boot loader to boot unsupported operating systems).

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

  1. Set GRUB's root device to the drive where the OS images are stored by the command root.
  2. Load the kernel image by the command kernel.
  3. If you need modules, load them with the command module or modulenounzip.
  4. Run the command boot.

Linux, FreeBSD, NetBSD and OpenBSD can be booted in a similar manner. You can load a kernel image by the command kernel and then run the command boot. If the kernel requires some parameters, just append the parameters to kernel, after the filename of the kernel. Also, please refer to section Some caveats on OS-specific issues, for the information on your OS-specific issues.

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 boot sector of the partition on which the operating system is installed.

First, set GRUB's root device to the partition by the command rootnoverify:

grub> rootnoverify (hd0,0)

Second, set the active flag in the partition by the command makeactive(3):

grub> makeactive

Third, load the boot loader by the command chainloader:

grub> chainloader +1

`+1' indicates that GRUB should read one sector from the start of the partition. The complete description about this syntax can be found in @xref{Filesystem}. If this succeeds, run the command boot.

However, DOS and Windows have a deficiency, so you might have to use more complicated instructions. See section Some caveats on OS-specific issues, for more information.

Some caveats on OS-specific issues

Here, we describe some caveats on several operating systems.


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.

  1. Set GRUB's root device to the same drive as GNU/Hurd's. Probably the command find /boot/gnumach or similar can help you.
  2. Load the kernel and the module, like this:
    grub> kernel /boot/gnumach root=hd0s1
    grub> module /boot/serverboot
  3. Run the command boot.


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

  1. Set GRUB's root device to the same drive as GNU/Linux's. Probably the command find /vmlinuz or similar can help you.
  2. Load the kernel:
    grub> kernel /vmlinuz root=/dev/hda1
    If you need to specify some kernel parameters, just append them to the command. For example, to set @option{vga} to `ext', do this:
    grub> kernel /vmlinuz root=/dev/hda1 vga=ext
    See the documentation in the Linux source tree for the complete information on the available options.
  3. If you use an initrd, execute the command initrd after kernel:
    grub> initrd /initrd
  4. Finally, run the command boot.

Caution: If you use an initrd and specify the `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 uppermem before loading the kernel. @xref{Command-line and menu entry commands}, for more information.


GRUB can load the kernel directly, either in ELF or a.out format. But this is not recommended, since FreeBSD's bootstrap interface sometimes changes heavily, so GRUB can't guarantee to pass kernel parameters correctly.

Thus, we'd recommend loading the very flexible loader `/boot/loader' instead. See this example:

grub> root (hd0,a)
grub> kernel /boot/loader
grub> boot


GRUB can load NetBSD a.out and ELF directly, follow these steps:

  1. Set GRUB's root device with root.
  2. Load the kernel with kernel. You should append the ugly option @option{--type=netbsd}, if you want to load an ELF kernel, like this:
    grub> kernel --type=netbsd /netbsd-elf
  3. Run boot.

For now, however, GRUB doesn't allow you to pass kernel parameters, so it may be better to chain-load it instead, for more information please see section Load another boot loader to boot unsupported operating systems.


The booting instruction is exactly the same as for NetBSD (see section NetBSD).


GRUB cannot boot DOS or Windows directly, so you must chain-load them (see section Load another boot loader to boot unsupported operating systems). 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 map (@xref{Commands}), like this:

grub> map (hd0) (hd1)
grub> map (hd1) (hd0)

This performs a virtual swap between your first and second hard drive.

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 hides a DOS (or Windows) partition, it ignores the partition. If GRUB unhides a DOS (or Windows) partition, it detects the partition. Thus, if you have installed DOS (or Windows) on the first partition and the second of the first hard disk, and boot the first copy, do the following:

grub> unhide (hd0,0)
grub> hide (hd0,1)
grub> rootnoverify (hd0,0)
grub> chainloader +1
grub> makeactive
grub> boot

SCO UnixWare

It is known that the signature in the boot loader for SCO UnixWare is wrong, so you will have to specify the option @option{--force} to chainloader, like this:

grub> rootnoverify (hd1,0)
grub> chainloader --force +1
grub> makeactive
grub> boot


You probably noticed that you need to type several commands to boot your OS. There's a solution to that - GRUB provides a menu interface (@xref{Menu interface}) from which you can select an item (using arrow keys) that will do everything to boot an OS.

To enable the menu, you need a configuration file, `/boot/grub/menu.lst'. We'll analyze an example file.

The file first contains some general settings, the menu interface related options. You can put these commands (@xref{Menu-specific commands}) before any of the items (starting with title).

# Sample boot menu configuration file

As you may have guessed, these lines are comments. Lines starting with a hash character (`#'), and blank lines, are ignored by GRUB.

# By default, boot the first entry.
default 0

The first entry (here, counting starts with number zero, not one!) will be the default choice.

# Boot automatically after 30 secs.
timeout 30

As the comment says, GRUB will boot automatically in 30 seconds, unless interrupted with a keypress.

# Fallback to the second entry.
fallback 1

If, for any reason, the default entry doesn't work, fall back to the second one (this is rarely used, for obvious reasons).

Note that the complete descriptions of these commands, which are menu interface specific, can be found in @xref{Menu-specific commands}. Other descriptions can be found in @xref{Commands}.

Now, on to the actual OS definitions. You will see that each entry begins with a special command, title, and the action is described after it. Note that there is no command boot at the end of each item. That is because GRUB automatically executes boot if it loads other commands successfully.

The argument for the command title is used to display a short title/description of the entry in the menu. Since title displays the argument as is, you can write basically anything in there.

# For booting the GNU Hurd
title  GNU/Hurd
root   (hd0,0)
kernel /boot/gnumach.gz root=hd0s1
module /boot/serverboot.gz

This boots GNU/Hurd from the first hard disk.

# For booting Linux
title  GNU/Linux
kernel (hd1,0)/vmlinuz root=/dev/hdb1

This boots GNU/Linux, but from the second hard disk.

# For booting Mach (getting kernel from floppy)
title  Utah Mach4 multiboot
root   (hd0,2)
pause  Insert the diskette now^G!!
kernel (fd0)/boot/kernel root=hd0s3
module (fd0)/boot/bootstrap

This boots Mach with a kernel on a floppy, but the root filesystem at hd0s3. It also contains a pause line, which will cause GRUB to display a prompt and delay, before actually executing the rest of the commands and booting.

# For booting FreeBSD
title  FreeBSD
root   (hd0,2,a)
kernel /boot/loader

This item will boot FreeBSD kernel loaded from the `a' partition of the third PC slice of the first hard disk.

# For booting OS/2
title OS/2
root  (hd0,1)
# chainload OS/2 bootloader from the first sector
chainloader +1
# This is similar to "chainload", but loads a specific file
#chainloader /boot/chain.os2

This will boot OS/2, using a chain-loader.

# For booting Windows NT or Windows95
title Windows NT / Windows 95 boot menu
root        (hd0,0)
chainloader +1
# For loading DOS if Windows NT is installed
# chainload /bootsect.dos

The same as the above, but for Windows.

# For installing GRUB into the hard disk
title Install GRUB into the hard disk
root    (hd0,0)
setup   (hd0)

This will just (re)install GRUB onto the hard disk.

# Change the colors.
title Change the colors
color light-green/brown blink-red/blue

In the last entry, the command color is used, to change the menu colors (try it!). This command is somewhat special, because it can be used both in the command-line and in the menu (@xref{Command-line and menu commands}). GRUB has several such commands, see the user reference.

We hope that you now understand how to use the basic features of GRUB. To learn more about GRUB, see the following chapters and the User Reference Manual (@xref{Introduction}).

Downloading OS images from a network

Although GRUB is a disk-based boot loader, it does provide some network support. To use the network support, you need to enable at least one network driver in the GRUB build process. For more information please see `netboot/README.netboot' in the source distribution.

How to set up your network

GRUB requires a server that will assign an IP address to the machine on which GRUB is running, and a file server. The former is either BOOTP, DHCP or a RARP server (RARP is deprecated, since it cannot serve much information). For the latter, only TFTP is supported at the moment. It is not necessary to run both the servers on one computer. How to configure these servers is beyond the scope of this document, so please refer to the manuals specific to those protocols/servers.

Once you have set up the servers, run bootp, dhcp or rarp for BOOTP, DHCP or RARP, respectively. Each command will show an assigned IP address, a netmask, an IP address for your TFTP server and a gateway. If any of the addresses is wrong or it causes an error, probably the configuration of your servers isn't set up properly.

Finally, download your OS images from your network. The network can be accessed using the network drive `(nd)'. Everything else is very similar to the normal instructions (see section Booting).

Here is an example:

grub> bootp
Probing... [NE*000]
NE2000 base ...
Address:    Netmask:
Server:     Gateway:

grub> root (nd)
grub> kernel /tftproot/gnumach.gz root=sd0s1
grub> module /tftproot/serverboot.gz
grub> boot

Booting from a network

It is sometimes very useful to boot from a network, especially, when you use a machine which has no local disk. In this case, you need to obtain a kind of Net Boot ROM, such as a PXE ROM or a free software package like Etherboot. Such a Boot ROM first boots the machine, sets up the network card installed into the machine, and downloads a second stage boot image from the network. Then, the second image will try to boot an operating system from the network actually.

GRUB provides two second stage images, `nbgrub' and `pxegrub'. Those images are the same as the normal Stage 2, except that they set up a network automatically, and try to load a configuration file from the network, if specified. The usage is very simple: If the machine has a PXE ROM, use `pxegrub'. If the machine has a NBI loader such as Etherboot, use `nbgrub'. There is no difference between them but their formats. As how to load a second stage image you want to use should be described in the manual on your Net Boot ROM, please refer to the manual, for more details. The topic is beyond the scope of this documentation.

However, there is one thing specific to GRUB. Namely, how to specify a configuration file in a BOOTP/DHCP server. For now, GRUB uses the tag `150', to get the name of a configuration file. This below is an example about a BOOTP configuration:



See the manual about your BOOTP/DHCP server, for more information. The exact syntax should differ from the example, more or less.

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 would be very useful to control the computers with serial communications. To connect a 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. First of all, make sure that you haven't specified the option @option{--disable-serial} to the configure script when you built your GRUB images. If you get them in binary form, probably they have serial terminal support already.

Then, initialize your serial terminal after GRUB starts up. Here is an example:

grub> serial --unit=0 --speed=9600
grub> terminal serial

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

The command terminal chooses which type of terminal you want to use. In that case above, the terminal will be a serial terminal, but you can also pass console to the command, like `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 the option, then GRUB doesn't provide you with the menu interface, because the menu requires several fancy features for your terminal. Instead, GRUB only gives you the hidden menu interface and the command-line interface.


You may be interested in how to prevent ordinary users from doing whatever they like, if you share your computer with other people. So this chapter describes how to improve the security of GRUB.

One thing which could be a security hole is that the user can do too many things with GRUB, because GRUB allows to modify its configuration and run arbitrary commands at run-time. For example, the user can read even `/etc/passwd' in the command-line interface by the command cat. So it is necessary to disable all the interactive operations.

Thus, GRUB provides password feature, so that only administrators can start the interactive operations (i.e. editing menu entries and entering the command-line interface). To use this feature, you need to run the command password in your configuration file, like this:

password PASSWORD

If this is specified, GRUB disallows any interactive control, until you press the key p and enter `PASSWORD'.

Also, you can specify an optional argument to password. See this example:

password PASSWORD /boot/grub/menu-admin.lst

In this case, GRUB will load `/boot/grub/menu-admin.lst' as the configuration file when you enter the valid password.

Another thing which may be dangerous is that any user can choose any menu entry. Usually, this wouldn't be problematic, but you might want to permit only administrators to run some of your menu entries, such as an entry for booting an insecure OS like DOS.

The solution used by GRUB is the command lock. This command always fails until you enter a valid password, so you can use it, like this:

title Boot DOS
rootnoverify (hd0,1)
chainload +1

You should insert lock right after title, because any user can execute commands in an entry, until GRUB encounters lock.

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