GRUB - GRand Unified Bootloader

The GNU GRand Unified Boot loader (GRUB) is a program which enables the selection of the installed operating system or kernel to be loaded at system boot time. It also allows the user to pass arguments to the kernel.

GRUB  which provides a user the choice to boot one of multiple operating systems installed on a computer or select a specific kernel configuration available on a particular operating system's partitions.

 GRUB was developed from a package called the Grand Unified Bootloader (a play on Grand Unified Theory). It is predominantly used for Unix-like systems. The operating system uses  GRUB as its boot loader, as do most Linux distributions. The Solaris operating system has used GRUB as its boot loader on x86 systems, starting with the Solaris 10 1/06 release.

GRUB and the x86 Boot Process

This section discusses the specific role GRUB plays when booting an x86 system. For a look at the overall boot process, refer to Section 33.2, “A Detailed Look at the Boot Process”.

GRUB loads itself into memory in the following stages:

1. The Stage 1 or primary boot loader is read into memory by the BIOS from the MBR ⁠[4]. The primary boot loader exists on less than 512 bytes of disk space within the MBR and is capable of loading either the Stage 1.5 or Stage 2 boot loader.
2. The Stage 1.5 boot loader is read into memory by the Stage 1 boot loader, if necessary. Some hardware requires an intermediate step to get to the Stage 2 boot loader. This is sometimes true when the /boot/ partition is above the 1024 cylinder head of the hard drive or when using LBA mode. The Stage 1.5 boot loader is found either on the /boot/ partition or on a small part of the MBR and the /boot/ partition.
3. The Stage 2 or secondary boot loader is read into memory. The secondary boot loader displays the GRUB menu and command environment. This interface allows the user to select which kernel or operating system to boot, pass arguments to the kernel, or look at system parameters.
4. The secondary boot loader reads the operating system or kernel as well as the contents of /boot/sysroot/ into memory. Once GRUB determines which operating system or kernel to start, it loads it into memory and transfers control of the machine to that operating system.

Configure the GRUB boot loader

GRUB is a boot loader designed to boot a wide range of operating systems from a wide range of filesystems. GRUB is becoming popular due to the increasing number of possible root filesystems that can Linux can reside upon.

GRUB is documented in a GNU info file. Type info grub to view the documentation.

The GRUB configuration file is /boot/grub/menu.lst. Some distributions use another configuration file; for example, Red Hat Linux uses the file /boot/grub/grub.conf.

GRUB configuration files are interpreted. Syntax errors will not be detected until the machine is rebooted, so take care not to make typing errors.

Edit the GRUB configuration file and remove any splashimage entries. If these entries are not removed GRUB 0.90 behaves very oddly, transferring control between the serial console and the attached monitor and keyboard.

If there is not already a password command in the GRUB configuration file then create a hashed password, see Figure 4-4. The password should be good, as it can be used to gain root access.

Figure 4-4. Using md5crypt to create a hashed password for GRUB

grub> md5crypt Password: ********** Encrypted: $1$U$JK7xFegdxWH6VuppCUSIb.

Use that hashed password in the GRUB configuration file, this is shown in Figure 4-5.

Figure 4-5. GRUB configuration to require a password

password --md5 $1$U$JK7xFegdxWH6VuppCUSIb.

Define the serial port and configure GRUB to use the serial port, as shown in Figure 4-6.

Figure 4-6. GRUB configuration for serial console

serial --unit=0 --speed=9600 --word=8 --parity=no --stop=1 terminal serial

--unit is the number of the serial port, counting from zero, unit 0 being COM1.

Note that the values of --parity are spelt out in full: no, even and odd. The common abbreviations n, e and o are not accepted.

If there is mysteriously no output on the serial port then suspect a syntax error in the serial or terminal commands.

If you also want to use and attached monitor and keyboard as well as the serial port to control the GRUB boot loader then use the alternative configuration in Figure 4-7.

Figure 4-7. GRUB configuration for serial console and attached monitor and keybaord console

password --md5 $1$U$JK7xFegdxWH6VuppCUSIb. serial --unit=0 --speed=9600 --word=8 --parity=no --stop=1 terminal --timeout=10 serial console

When both the serial port and the attached monitor and keyboard are configured they will both ask for a key to be pressed until the timeout expires. If a key is pressed then the boot menu is displayed to that device. Disconcertingly, the other device sees nothing.

If no key is pressed then the boot menu is displayed on the whichever of serial or console is listed first in the terminal command. After the timeout set by the timeout the default option set by default is booted.

Figure 4-8. GRUB output to default device when configured for serial and attached monior output

Press any key to continue. Press any key to continue. Press any key to continue. Press any key to continue. Press any key to continue. Press any key to continue. Press any key to continue. Press any key to continue. Press any key to continue. Press any key to continue. GRUB version 0.90 (639K lower / 162752K upper memory) +-------------------------------------------------------------------------+ | [ Red Hat Linux (2.4.9-21) ] | | | | | +-------------------------------------------------------------------------+ Use the ^ and v keys to select which entry is highlighted. Press enter to boot the selected OS or 'p' to enter a password to unlock the next set of features. The highlighted entry will be booted automatically in 10 seconds.

If you are not using a VT100 terminal then the cursor keys may not work to select a GRUB menu item. The instructions shown in Figure 4-8 are literally correct: Use the ^ and v keys means that the caret key (Shift-6) moves the cursor up and letter vee key (V) moves the cursor down.

Note when configuring GRUB that there are two timeouts involved. Press any key to continue is printed for terminal --timeout=10 seconds, waiting for someone on the keyboard or terminal to press a key to get the input focus. Then the menu is displayed for timeout 10 seconds before the default boot option is taken.

If the terminal attached to the serial port is not a real or emulated VT100, then force GRUB to use it's command line interface. This interface is much more difficult to use than GRUB's menu interface; however, the command line interface does not assume the VT100's terminal language.

Figure 4-9. GRUB configuration for command line interface for terminals other than VT100

terminal --timeout=10 --dumb serial console

This HOWTO does not discuss the use of GRUB's command line. It is far too complex and error-prone to recommend for use on production machines. Wizards will know to consult GRUB's info manual for the commands required to boot the kernel.

GRUB's menu's can be edited interactively after P is pressed and the password supplied. A better approach is to add menu items to boot the machine into alternative run levels. A sample configuration showing a menu entry for the default run level and an alternative menu entry for single user mode (run level s) is shown in Figure 4-10. Remember to use the lock command to require a password for single user mode, as single user mode does not ask for a Linux password.

Figure 4-10. Adding a single user mode option to the GRUB menu

password --md5 $1$U$JK7xFegdxWH6VuppCUSIb. default 0 title Red Hat Linux (2.4.9-21) root (hd0,0) kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6 initrd /initrd-2.4.9-21.img title Red Hat Linux (2.4.9-21) single user mode lock root (hd0,0) kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6 s initrd /initrd-2.4.9-21.img

File names in the kernel and initrd commands are relative to the GRUB installation directory, which is usually /boot/grub. So /vmlinuz-2.4.9-21 is actually the file /boot/grub/vmlinuz-2.4.9-21.

GRUB is now configured to use the serial console. The kernels booted from GRUB are yet to be configured to use the serial console.