Grand Unified Boot Loader - GRUB 2 on Linux

The Grand Unified Boot Loader - Grub 2 has been the standard boot system for Ubuntu since version 9.10 and other distributions with newer versions, especially Kernel 2.6 and 3.x. Meet the powerful GRUB 2.

Grub 2 has been rewritten completely with the purpose of allowing great flexibility and gain of performance. Among the various improvements, the main ones are:

Script support with conditional expressions and functions;
Loading modules dynamically;
Rescue Mode; Personalized Menus;
Theme support ; - Graphical menu support with “splash” screen;
Loading ISO images from Live CDs;
New structure of configuration file;
Non-x86 platform support (e.g. PowerPC);
Universal UUID (Universally Unique Identifier) support — identifier used in distributed systems.

Upgrade from Grub 1 to Grub 2

The upgrade from Grub 1 to Grub 2 is relatively easy. You must first install the Grub 2 package with the command:

# sudo apt-get install grub-pc

Once installed, Grub 2 will present a test menu, with the kernel images found and also allows the menu be edited.

Once the menu is defined, Grub2 can be permanently installed with the command:

# sudo upgrade-from-grub-legacy /boot/grub/grub.cfg

Main Changes from Grub 1 to Grub 2

It is important that you know what the changes are important from Grub 1 to Grub 2, namely:

The /boot/grub/menu.lst file has been replaced by /boot/grub/grub.cfg;
the main configuration file is /etc/default/grub;
The partition numbering system has changed;
Searches for other operating systems are performed automatically every time update-grub is executed and are placed in the menu;

Changes In the configuration they have no effect until the update-grub command is executed.

Normally the /boot/grub/grub.cfg should not be edited manually because it is rewritten in updates to the Grub 2 package, when a kernel is added or removed, or the user triggers the update-grub command. This command remakes the load menu from other configuration files and rewrite /boot/grub/grub.cfg.

Create a default configuration in grub

The grub-mkconfig utility can also be used to create a configuration for Grub:

# grub-mkconfig -o /boot/grub/grub.cfg

The main menu presentation configuration file is located in /etc/default/grub.

/etc/default/grub file

This file contains the main information for that Grub 2 can assemble the menu. It defines what the default menu option will be, the waiting times for a user action, if the menu will be displayed in mode graphic or text mode, among other options.

Example from /etc/default/grub:

GRUB_DEFAULT=0

#GRUB_SAVEDEFAULT = true

#GRUB_HIDDEN_TIMEOUT =0 GRUB_HIDDEN_TIMEOUT_QUIET=true

GRUB_TIMEOUT=10 GRUB_DISTRIBUTOR=‘LSB_RELEASE

-i -s 2> /dev/null || echo Debian’ GRUB_CMDLINE_LINUX_DEFAULT=

” quiet splash” GRUB_CMDLINE_LINUX_DEFAULT=” quiet splash” GRUB_CMDLINE_LINUX_DEFAULT=

” quiet splash” GRUB_CMDLINE_LINUX_DEFAULT=” quiet splash”

GRUB_CMDLINE_LINUX_DEFAULT=” quiet splash” GRUB_

#GRUB_TERMINAL =console

#GRUB_GFXMODE =640x480

#GRUB_DISABLE_LINUX_RECOVERY =” true”

#GRUB_INIT_TUNE =” 480 440 1”

Grub Configuration File

See grub’s option comment:

GRUB_DEFAULT

Defines what will be the default Grub menu option. THE GRUB_DEFAULT parameter can assume a numeric value, the name of the option in the menu or the word “saved”.

If the value is numeric, indicate the menu position. at /boot/grub/grub.cfg. You can also indicate the name of the menu “Ubuntu 2.6.31”.

If the value is “saved”, the default will be set by the grub-set-default or grub-reboot command.

GRUB_SAVEDEFAULT

If this parameter is “true”, it tells Grub that the default menu option will always be the last operating system selected on the menu.

GRUB_HIDDEN_TIMEOUT

This parameter defines the time in seconds that Grub will wait for a user action until the default menu option is chosen automatically. No menu will be displayed unless The user presses a key (usually the ESC).

If it equals zero, the default operating system will load automatically without waiting for any intervention.

GRUB_HIDDEN_TIMEOUT_QUIET

When this option is “true”, no counter of The weather will be presented. If it is “false”, a countdown counter will be presented with the time set in GRUB_HIDDEN_TIMEOUT.

GRUB_TIMEOUT

This option will only be effective if the directive GRUB_HIDDEN_TIMEOUT is commented, or active, when the user presses some key during the GRUB_HIDDEN_TIMEOUT wait time.

GRUB_TIMEOUT defines the menu wait time until the user makes their choice. Otherwise, it will execute the option standard.

If the value is -1, Grub will wait for the user indefinitely.

GRUB_DISTRIBUTOR

Determines the description of system that will be used in the menu. If nothing is defined, Grub will use the default of the system, which is generally defined by the command:

# lsb_release -i -s 2> /dev/null || echo Debian

SUSE LINUX

GRUB_CMDLINE_LINUX_DEFAULT

This directive passes parameters for the Kernel during normal system load.

GRUB_CMDLINE_LINUX

This directive passes parameters to the Kernel during system load, either for normal kernel load or in mode “recovery”.

GRUB_TERMINAL

This option, if uncommented, disables the menu in graphic mode.

GRUB_GFXMODE

Defines the resolution of the menu graphic terminal.

GRUB_DISABLE_LINUX_RECOVERY

Disable Kernel emergency mode if it is equal to “true”.

GRUB_INIT_TUNE

Defines a “sound” that grub will make before present the menu. The format is [note tone] [length].

Example: Immediate Third Party Contacts Theme Grade:grub_init_tune=” 480 900 2 1000 2 800 2 400 2 600 3”

The /etc/grub.d directory

The scripts in this directory are read during the The update-grub command call and its instructions are incorporated in the file /boot/grub/grub.cfg.

The order of the items in the grub menu is determined by the order of the scripts in this directory. The files are read according to number that precedes your name, in ascending order.

00_header

Configures the initial parameters, such as the mode graphic, standard menu option. These settings are generally imported from /etc/default/grub.

05_debian_theme

Configure the background screen, text colors, etc.

10_Hurd

Used to locate Kernels Hurd. It is not used in most distributions.

10_Linux

Identifies the Kernels on the root device and creates the menu items. All existing kernel images in /boot will be placed on the menu.

20_memtest86+

Search for the image in /boot/memtest86+.bin used for testing memory. To remove this item, you must remove the image in the directory /boot.

30_os-prober

This script searches for Linux and other systems existing operations on the disk and create the menu items. The variables in this The file determines how the names will appear in the menu.

40_custom

Allows you to add customized entries to the menu of Grub.

The settings are usually done in the /etc/default/grub file.
It is not common to directly edit scripts from the /etc/grub.d directory.
Any change made to the /boot/grub/grub.cfg is rewritten when the command update-grub is executed.

Load Sequence of the System

Grub2 runs before any operating system. It will generally load the operating system directly by setting it as the default or if there is only one operating system present.

If there is more than one operating system, it will display the menu to choose from.

The default wait time for the menu is 10s. If the user doesn’t do anything, it will start loading the default menu option.

The time count can be paralyzed if any key is pressed. The user can force the menu to be displayed by pressing the SHIFT key while loading grub2.

Partition Names In Grub 2

The partition naming system in grub2 was simplified compared to grub1. The device count remains starting from scratch.

An sda device will be named hd0, and an sdb device will be named hd1. However, the partitions follow the “normal” numbering, so the partition sda5 will be named hd0.5 and sda3 will be hd0.3.

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