GRUB (Grand Unified Bootloader) comprises two stages. stage1 consists of 512 bytes and is written to the MBR or the boot sector of a hard disk partition or floppy disk. Subsequently, stage2 is loaded. This stage contains the actual program code. The only task of the first stage is to load the second stage of the boot loader.
stage2 is able to access file systems. Currently, Ext2, Ext3, ReiserFS, Minix, and the DOS FAT file system used by Windows are supported. To a certain extent, JFS, XFS, and UFS and FFS used by BSD systems are also supported. Since version 0.95, GRUB is also able to boot from a CD or DVD containing an ISO 9660 standard file system pursuant to the “El Torito” specification. Even before the system is booted, GRUB can access file systems of supported BIOS disk devices (floppy disks or hard disks, CD drives, and DVD drives detected by the BIOS). Therefore, changes to the GRUB configuration file (menu.lst) no longer require a reinstallation of the boot manager. When the system is booted, GRUB reloads the menu file together with the valid paths and partition data of the kernel or the initial RAM disk (initrd) and locates these files.
The actual configuration of GRUB is based on three files that are described below:
This file contains all information about partitions or operating systems that can be booted with GRUB. Without this information, the system control cannot be passed to the operating system.
This file translates the device names from the GRUB and BIOS notation to Linux device names.
This file contains the parameters and options the GRUB shell needs for installing the boot loader correctly.
GRUB can be controlled in various ways. Boot entries from an existing configuration can be selected from the graphical menu (splash screen). The configuration is loaded as it is from the file menu.lst.
In GRUB, all boot parameters can be changed prior to booting. For example, errors made when editing the menu file can be corrected in this way. Boot commands can also be entered interactively over a kind of input prompt (see 126.96.36.199. “Editing Menu Entries during the Boot Procedure”). GRUB offers the possibility of determining the location of the kernel and the initrd prior to booting. In this way, you can even boot an installed operating system for which no entry exists in the boot loader configuration.
The GRUB shell provides an emulation of GRUB in the installed system. It can be used to install GRUB or test new settings before applying them. See 7.4.4. “The GRUB Shell”.
The graphical splash screen with the boot menu is based on the GRUB configuration file /boot/grub/menu.lst, which contains all information about all partitions or operating systems that can be booted by means of the menu.
Every time the system is booted, GRUB loads the menu file from the file system. For this reason, GRUB does not need to be reinstalled after every change to the file. The YaST boot loader module can be used to modify the GRUB configuration as described in 7.5. “Configuring the Boot Loader with YaST”.
The menu file contains commands. The syntax is very simple. Every line contains a command followed by optional parameters separated by spaces like in the shell. For historical reasons, some commands permit an = in front of the first parameter. Comments are introduced by a hash (#).
To identify the menu items in the menu overview, specify a title for every entry. The text (including any spaces) following the keyword title is displayed as a selectable option in the menu. All commands up to the next title are executed when this menu item is selected.
The simplest case is the redirection to boot loaders of other operating systems. The command is chainloader and the argument is usually the boot block of another partition in GRUB block notation. For example:
The device names in GRUB are explained in 188.8.131.52. “Naming Conventions for Hard Disks and Partitions”. The above example specifies the first block of the fourth partition of the first hard disk.
Use the command kernel to specify a kernel image. The first argument is the path to the kernel image in a partition. The other arguments are passed to the kernel on the command line.
If the kernel does not have built-in drivers for access to the root partition, initrd must be specified with a separate GRUB command whose only argument is the path to the initrd file. Because the loading address of the initrd is written to the loaded kernel image, the command initrd must follow immediately after the kernel command.
The command root simplifies the specification of kernel and initrd files. The only argument of root is a GRUB device or a partition on a GRUB device. This device is used for all kernel, initrd, or other file paths for which no device is explicitly specified until the next root command. This command is not used in the menu.lst file generated during the installation. It merely facilitates manual editing.
The boot command is implied at the end of every menu entry, so it does not need to be written into the menu file. However, if you use GRUB interactively for booting, you must enter the boot command at the end. The command itself has no arguments. It merely boots the loaded kernel image or the specified chain loader.
After writing all menu entries, define one of them as the default entry. Otherwise, the first one (entry 0) is used. You can also specify a time-out in seconds after which the default entry should boot. timeout and default usually precede the menu entries. An example file is described in 184.108.40.206. “An Example Menu File”.
The naming conventions GRUB uses for hard disks and partitions differ from those used for normal Linux devices. In GRUB, the numbering of the partitions starts with zero. Thus, (hd0,0) is the first partition of the first hard disk. On a common desktop machine with a hard disk connected as primary master, the corresponding Linux device name is /dev/hda1.
The four possible primary partitions are assigned the partition numbers 0 to 3. The logical partitions are numbered from 4:
(hd0,0) first primary partition of the first hard disk (hd0,1) second primary partition (hd0,2) third primary partition (hd0,3) fourth primary partition (usually an extended partition) (hd0,4) first logical partition (hd0,5) second logical partition
GRUB does not distinguish between IDE, SCSI, and RAID devices. All hard disks recognized by the BIOS or other controllers are numbered according to the boot sequence preset in the BIOS.
Unfortunately, GRUB is not able to map the Linux device names to BIOS device names exactly. It generates this mapping with a help of an algorithm and saves it to the file device.map, which can be edited if necessary. Information about the file device.map is available in 7.4.2. “The File device.map”.
A complete GRUB path consists of a device name written in parentheses and the path to the file in the file system in the specified partition. The path begins with a slash. For example, the bootable kernel could be specified as follows on a system with a single IDE hard disk containing Linux in its first partition:
The following example shows the structure of a GRUB menu file. The example installation comprises a Linux boot partition under /dev/hda5, a root partition under /dev/hda7, and a Windows installation under /dev/hda1.
gfxmenu (hd0,4)/message color white/blue black/light-gray default 0 timeout 8 title linux kernel (hd0,4)/vmlinuz root=/dev/hda7 vga=791 initrd (hd0,4)/initrd title windows chainloader(hd0,0)+1 title floppy chainloader(fd0)+1 title failsafe kernel (hd0,4)/vmlinuz.shipped root=/dev/hda7 ide=nodma \ apm=off acpi=off vga=normal nosmp maxcpus=0 3 initrd (hd0,4)/initrd.shipped
The first block defines the configuration of the splash screen:
The background image message is located in /dev/hda5.
Color scheme: white (foreground), blue (background), black (selection), and light gray (background of the selection). The color scheme has no effect on the splash screen, only on the customizable GRUB menu that you can access by exiting the splash screen with Esc.
The first menu entry title linux is the one to boot by default.
After eight seconds without any user input, GRUB automatically boots the default entry.
The second and largest block lists the various bootable operating systems. The sections for the individual operating systems are introduced by title.
The first entry (title linux) is responsible for booting SUSE LINUX. The kernel (vmlinuz) is located in the first logical partition (the boot partition) of the first hard disk. Kernel parameters, such as the root partition and VGA mode, are appended here. The root partition is specified according to the Linux naming convention (/dev/hda7/), because this information is read by the kernel and has nothing to do with GRUB. The initrd is also located in the first logical partition of the first hard disk.
The second entry is responsible for loading Windows. Windows is booted from the first partition of the first hard disk (hd0,0). The command chainloader +1 causes GRUB to read and execute the first sector of the specified partition.
The next entry enables booting from floppy disk without modifying the BIOS settings.
The boot option failsafe starts Linux with a selection of kernel parameters that enables Linux to boot even on problematic systems.
The menu file can be changed whenever necessary. GRUB then uses the modified settings during the next boot. Edit the file permanently using YaST or an editor of your choice. Alternatively, make temporary changes interactively using the edit function of GRUB (see 220.127.116.11. “Editing Menu Entries during the Boot Procedure”).
In the graphical GRUB boot menu, the operating system to boot can be selected with the arrow keys. If you select a Linux system, you can enter additional boot parameters at the boot prompt. To edit individual menu entries directly, press Esc to exit the splash screen and press E. Changes made in this way only apply to the current boot procedure and are not adopted permanently.
|Keyboard Layout during the Boot Procedure|
The US keyboard layout is the only one available when booting.
After activating the editing mode, use the arrow keys to select the menu entry of which to edit the configuration. To make the configuration editable, press E once more. In this way, edit incorrect partition or path specifications before they have a negative effect on the boot process. Press Enter to exit the editing mode and return to the menu. Then press B to boot this entry. Further possible actions are displayed in the help text at the bottom.
To enter changed boot options permanently and pass them to the kernel, open the file menu.lst as the user root and append the respective kernel parameters to the existing line, separated by spaces:
title linux kernel (hd0,0)/vmlinuz root=/dev/hda3 <additional parameter> initrd (hd0,0)/initrd
GRUB automatically adopts the new parameters the next time the system is booted. Alternatively, this change can also be made with the YaST boot loader module. Append the new parameters to the existing line, separated by spaces.
The file device.map maps GRUB device names to Linux device names. In a mixed system containing IDE and SCSI hard disks, GRUB must try to determine the boot sequence by means of a special procedure, because GRUB does not have access to the BIOS information on the boot sequence. GRUB saves the result of this analysis in the file /boot/grub/device.map. For a system on which the boot sequence in the BIOS is set to IDE before SCSI, the file device.map could appear as follows:
(fd0) /dev/fd0 (hd0) /dev/hda (hd1) /dev/sda
Because the order of IDE, SCSI, and other hard disks depends on various factors and Linux is not able to identify the mapping, the sequence in the file device.map can be set manually. If you encounter problems when booting, check if the sequence in this file corresponds to the sequence in the BIOS and use the GRUB shell to modify it temporarily if necessary (see 7.4.4. “The GRUB Shell”). Once the Linux system has booted, the file device.map can be edited permanently with the YaST boot loader module or an editor of your choice.
After manually changing the file device.map, execute the following command to reinstall GRUB. This command causes the file device.map to be reloaded and the commands listed in grub.conf to be executed:
grub --batch < /etc/grub.conf
The third important GRUB configuration file apart from menu.lst and device.map is /etc/grub.conf. This file contains the parameters and options the command grub needs for installing the boot loader correctly:
root (hd0,4) install /grub/stage1 d (hd0) /grub/stage2 0x8000 (hd0,4)/grub/menu.lst quit
Meaning of the individual entries:
This command tells GRUB to apply the following commands to the first logical partition of the first hard disk (the location of the boot files).
The command grub should be run with the parameter install. stage1 of the boot loader should be installed in the MBR of the first hard disk (/grub/stage1 d (hd0)). stage2 should be loaded to the memory address 0x8000 (/grub/stage2 0x8000). The last entry ((hd0,4)/grub/menu.lst) tells GRUB where to look for the menu file.
GRUB actually exists in two versions: as a boot loader and as a normal Linux program in /usr/sbin/grub. This program is referred to as the GRUB shell. The functionality to install GRUB as boot loader on a hard disk or floppy disk is integrated in GRUB in the form of the commands install and setup. This is available in the GRUB shell when Linux is loaded.
However, the commands setup and install are also available during the boot procedure before Linux is started. This facilitates the repair of a defective system that can no longer be booted, because the faulty configuration file of the boot loader can be circumvented by manually entering parameters. The manual specification of parameters during the boot procedure is also useful for testing new settings to avoid impairing the native system. Simply enter the experimental configuration file with a syntax similar to that in menu.lst. Then test the functionality of this entry without changing the existing configuration file. For example, to test a new kernel, enter the command kernel and the path to the new kernel. If the boot procedure fails, you can continue using the intact menu.lst the next time you boot. Similarly, the command line interface can also be used to boot a system despite a faulty menu.lst file by entering the corrected parameters. In the running system, the correct parameters can be entered in menu.lst to make the system permanently bootable.
The mapping of GRUB devices to Linux device names is only relevant when running the GRUB shell as a Linux program (by entering grub as described in 7.4.2. “The File device.map”). For this purpose, the program reads the file device.map. For more information, see 7.4.2. “The File device.map”.
Even before the operating system is booted, GRUB enables access to file systems. Users without root permissions can access files in your Linux system to which they have no access once the system is booted. To block this kind of access or to prevent users from booting certain operating systems, a boot password can be set.
Proceed as follows as the user root to set a boot password:
At the root prompt, enter grub.
Encrypt the password in the GRUB shell:
grub> md5crypt Password: **** Encrypted: $1$lS2dv/$JOYcdxIn7CJk9xShzzJVw/
Paste the encrypted string into the global section of the file menu.lst:
gfxmenu (hd0,4)/message color white/blue black/light-gray default 0 timeout 8 password --md5 $1$lS2dv/$JOYcdxIn7CJk9xShzzJVw/
Now GRUB commands can only be executed at the boot prompt after pressing P and entering the password. However, users can still boot all operating systems from the boot menu.
To prevent one or several operating systems from being booted from the boot menu, add the entry lock to every section in menu.lst that should not be bootable without entering a password. Example:
title linux kernel (hd0,4)/vmlinuz root=/dev/hda7 vga=791 initrd (hd0,4)/initrd lock
After rebooting the system and selecting the Linux entry from the boot menu, the following error message is displayed:
Error 32: Must be authenticated
Press Enter to enter the menu. Then press P to get a password prompt. After entering the password and pressing Enter, the selected operating system (Linux in this case) should boot.
|Boot Password and Splash Screen|
If you use a boot password for GRUB, the usual splash screen will not be displayed.