GRUB

GRUB2 is a boot loader with support for modern day computer systems. It is meant to replace the older GRUB boot loader (now referred to as "GRUB Legacy"). GRUB2 has an entirely separate code base from GRUB Legacy and boasts features such as a new shell-like syntax which permits advanced scripting capabilities. Current GRUB Legacy users are encouraged to migrate their systems to GRUB2.

For a shotgun approach, see GRUB2 Quick Start.

Installation
The package is slotted. Both grub-0.97 (GRUB Legacy) and grub-2.00 may be installed at the same time, however only one version of GRUB may be installed in a MBR at a time. In this article, we focus on GRUB2.

By default, GRUB2 will guess a default platform target at build time. This may be overridden by setting the  variable in.

The following platforms are supported depending on the target CPU:

To install GRUB2 use the normal emerge syntax:

Optionally, install the os-prober utility (provided through the package) to have GRUB2 probe for other operating systems when running the grub2-mkconfig command. In most instances, this will enable GRUB2 to automatically detect other operating systems (Windows 7, Windows 8.1, etc.).

The GRUB2 (and optionally ) installations do not automatically enable the boot loader. These only install the software on the operating system, but to install the boot loader to the system itself (so that it is used when booting the system), additional steps need to be taken, which are covered in the Configuration section.

Configuration
There are two important aspects to the configuration of GRUB2:
 * 1) installation of GRUB2 software as the boot loader of the system
 * 2) configuration of the GRUB2 boot loader itself

The installation of GRUB2 software is specific to the type of system, and is covered in Installing the boot loader. First we cover the configuration of the boot loader itself.

Main configuration file
GRUB2 does not require the administrator to manually maintain a boot option configuration (as is the case with boot loaders such as GRUB Legacy and LILO). Instead it can generate its configuration file using the grub2-mkconfig command. This utility will use the scripts in and the settings in.

After modifying one or more settings, run the grub2-mkconfig utility with the -o option pointing to the output file located at (this is GRUB2's default output location):

Each time the grub2-mkconfig utility is called a new configuration will be generated.

Setting configuration parameters
GRUB2 uses as its configuration file. However, it is advised to automatically generate this file based on the configuration entries in. The file uses key/value pairs which are used by the scripts in to generate a working configuration file.

The following variables are the most common ones to set to control how GRUB2 will function:

For a more complete list, please refer to the GRUB2 configuration variables sub-page.

After modifying the parameters, regenerate the GRUB2 configuration file with grub2-mkconfig.

Enabling or disabling configuration scripts
The directory contains the scripts that grub2-mkconfig uses to generate a  file. By default the contents of this directory should be similar to the following:

GRUB2 will use all installed scripts that are marked as executable (which by default, they all are). To disable any of the scripts simply remove the executable bit from the script's file permissions using the chmod command. In the following example every script but and  are disabled:

After modifying the scripts (or removing the executable bit), regenerate the GRUB2 configuration file using grub2-mkconfig.

Manipulating configuration scripts
Some features or GRUB2 functionalities are only possible to be exploited by modifying the configuration scripts. For instance, to support dual-booting with FreeBSD, the following manipulation needs to be done.

Change the script to:

or  is the partition in which FreeBSD resides. If the normal UFS install was used for the FreeBSD partition then is a container (something like a logical partition). It consists of the swap and root partition. Verify the script is executable by running ls -la /etc/grub.d/40_custom. If the executable bit is not set then set it using the chmod u+x 40_custom command.

Next install GRUB2 using the grub2-install command and update GRUB2's configuration file:

Installing the boot loader
Installing GRUB2 as the system's boot loader depends on how the system is meant to boot (through BIOS or UEFI) and how the disk on which the boot loader should be installed is partitioned (using MBR or GPT partition layout).

This article covers the following situations:


 * BIOS with MBR
 * BIOS with GPT
 * UEFI with GPT

Select the installation instructions appropriate for the system.

BIOS with MBR
Make sure that the location is available - if this uses a separate partition, make sure that it is mounted:

Run the grub2-install command to copy the relevant files to. On the PC platform, this also installs a boot image to the Master Boot Record (MBR) or a partition's boot sector. If all goes well, after running the grub2-install command an output such as the one below is to be expected:

grub2-install accepts a --target option to set the CPU architecture and system platform. If unspecified, grub2-install will attempt to guess the proper values; on an AMD64/x86 system it will use i386-pc</tt> by default. grub2-install</tt> also accepts a --boot-directory</tt> option to tell the GRUB2 installer which directory to look for GRUB2's boot files. This defaults to the current but is useful when trying to move a root partition.

Partitioning for BIOS with MBR
Be sure to leave enough free space before the first partition. Starting the first partition at sector 2048 leaves at least 1 MiB of disk space for the master boot record. It is recommended (but not mandatory) to create an additional partition for GRUB called the BIOS boot partition. This partition just needs to be defined, but not formatted.

If the Gentoo installation instructions were followed, this BIOS boot partition will already be avaialble.

BIOS with GPT
If a partition is needed, start by mounting the  partition:

If all goes well, after running the grub2-install command an output such as the one below is to be expected:

grub2-install</tt> accepts a --target</tt> option to set the CPU architecture and system platform. If unspecified, grub2-install</tt> will attempt to guess the correct at the correct values; on an AMD64/x86 system it will use i386-pc</tt> by default. grub2-install</tt> also accepts a --boot-directory</tt> option to tell the GRUB2 installer which directly to look for GRUB2's boot files. This defaults to the current but is useful when trying to move a root partition.

Partitioning for BIOS with GPT
When a GPT partition table is present on the system, a small BIOS boot partition (with type, which is different from the EFI System Partition (ESP) which has type  ) will need to be available. 1 MiB will be enough to work but 2-4 MiB is a more safe option. This BIOS boot partition will hold the stage 2 of the bootloader. BIOS boot partitions do not need formatted with a filesystem; the grub2-install</tt> command will overwrite any pre-existing filesystem with one of it's own.

To set a partition as a BIOS partition use the command line tool parted by typing (change   to the number of the partition to mark as a BIOS Boot partition!):

With 's cgdisk</tt> utility, this is accomplished by setting the partition type to  and give it a label of.

Next to a BIOS partition, an EFI System Partition must be available (which is FAT-formatted).

The following is the output of pressing the key using the gdisk utility on a GPT partitioned disk with both a BIOS boot [0xEF02] partition and an EFI [0xEF00] partition:

Using the same setup, the parted utility gives an output with slightly different syntax:

Creating partitions in gdisk</tt> is straight forward for users already familiar with the fdisk</tt> partitioning utility. After starting gdisk, type (for new) in the main menu, provide beginning and end sectors (if needed), then set the partition type   for an EFI system partition.

Users who have followed the Gentoo installation instructions will already have the proper partitioning layout set up.

UEFI with GPT
Make sure that the location is available - if this uses a separate partition, make sure that it is mounted:

Run the grub2-install command to copy the relevant files to. This should install Grub2 in, copy the core image to , and call efibootmgr to add a boot entry.

grub2-install</tt> accepts a --target</tt> option to set the CPU architecture and system platform. If unspecified, grub2-install</tt> will attempt to guess the proper values; on an AMD64/x86 system it will use <tt>i386-pc</tt> by default. <tt>grub2-install</tt> also accepts a <tt>--boot-directory</tt> option to tell the GRUB2 installer which directory to look for GRUB2's boot files. This defaults to the current but is useful when trying to move a root partition.

Partitioning for UEFI with GPT
For UEFI GPT boot using GRUB2, the system must have a dedicated EFI partition containing a FAT filesystem.

The EFI partition can replace having a partition on  by having a  partition on. This is to say a successful EFI boot scenario using GRUB2 can operate with two partitions total (three total if a swap partition is needed): a root partition and an EFI partition. Using this configuration, the folder will be located in the root  partition (at ) and the EFI partition will mount in the boot folder (at ). For further clarification, see the example file below.

Generating a 100MB partition for should provide plenty of space for holding multiple  files (multiple entries will most likely not be needed; most systems will only use one).

Create the partition using the partitioning tool of choice. The <tt>gdisk</tt> and <tt>parted</tt>  tools fit nicely for this purpose. When using the <tt>gdisk</tt> utility, be sure to use type.

Proceed to create a FAT filesystem on the EFI system partition using mkdosfs and add it to by following the example below:

Alternative: using the default UEFI firmware location
If the system's UEFI firmware fails to find GRUB2's EFI bootloader file, using the default boot loader location should provide a working solution. This circumvents the boot menu managed by efibootmgr and thus offers reduced functionality, but is less error prone. Do do this, verify the EFI partition is mounted at then copy the file  located at  to. This example assumes a 64-bit EFI system, adjust accordingly for 32-bit EFI systems.

Extended features
GRUB2 has many features that make it a very powerful boot loader. It supports:
 * booting from UEFI platforms.
 * booting from GPT partitioned drives without needing a hybrid MBR (hybrid MBR can enabled as needed for compatibility or portability).
 * booting from a btrfs formatted partition.
 * booting directly from a btrfs raid set without needing an initramfs for early mount setup.
 * booting directly from logical volume management (such as LVM2).
 * booting with support for DM-RAID (RAID 0, 1, 4, 5, 6, 9 and 10).
 * booting from encrypted devices (LUKS).

Some specific features are explained in more detail next.

Chainloading
GRUB2 was built with a truly improved chainload mode when compared to GRUB Legacy. To chainload another boot loader, use the  option.

For more information on chainloading, please see the Chainloading sub-page.

Using framebuffer display
To have GRUB2 use a framebuffer graphical display, re-emerge GRUB with the  USE flag enabled. This will install a default True Type font as well as a font conversion utility.

Proceed to configure the default GRUB2 configuration file located at. For example:

In order to find out what display modes the system's graphics card supports, use the following commands on the GRUB2 shell:

Troubleshooting
Most of the issues can be resolved by ensuring that the partition layout is correct. Make sure enough space is available before the first partition of the disk, or optionally make sure that a BIOS boot partition is available. Also verify that was correctly generated with grub2-mkconfig, or generate one with a custom menu entry.

For more GRUB2 troubleshooting, please refer to the Troubleshooting sub-page.

External resources

 * GNU GRUB 2 manual page
 * Network (PXE) section of GRUB2
 * Legacy BIOS issues with GPT article
 * GPT and Hybrid MBR article
 * GPT fdisk utility page
 * Arch Linux GRUB2 wiki article
 * Fedora GRUB2 wiki article : Encountering the dreaded GRUB2 boot prompt
 * ubuntu UEFI booting help
 * http://unix.stackexchange.com/questions/109272/dualboot-freebsd-gentoo-with-grub2-mbr