GRUB: Difference between revisions

Revision as of 14:21, 20 September 2011

GRUB2 is the next generation of the GRand Unified Bootloader (GRUB). GRUB2 is derived from PUPA which was a research project to investigate the next generation of GRUB. GRUB 2 has been rewritten from scratch to clean up everything and provide modularity and portability [1].

Briefly, the bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the Linux kernel. The kernel, in turn, initializes the rest of the operating system.

Preface

Although, GRUB (i.e. version 0.9x) is the de facto standard bootloader of Linux, it is considered 'legacy' by upstream. It is being replaced by GRUB2 in many distributions. Upstream recommends GRUB2 >=1.99~rc2 over grub-legacy, even for current grub-legacy users.

Note: grub2 >=1.99~rc2 supports btrfs as root (without a separate /boot fs).

Notes for current GRUB users

There are differences in the commands of GRUB and GRUB2. Familiarize yourself with GRUB2 commands before proceeding (e.g. "find" has been replaced with "search").

GRUB2 is now modular and no longer requires "stage 1.5". As a result, the bootloader itself is limited -- modules are loaded from the hard drive as needed to expand functionality (e.g. for LVM or RAID support).

Device naming has changed between GRUB and GRUB2. Partitions are numbered from 1 instead of 0 while drives are still numbered from 0, and prefixed with partition-table type. For example, Template:Filename would be referred to as Template:Codeline (for MBR) or Template:Codeline (for GPT) using GRUB2.

Installation

During Arch Linux installation

Skip the Install Bootloader step and exit the installer.
Configure the network:

# aif -p partial-configure-network

If you did not configure the installed system's Template:Filename file during installation (for instance, if you plan to let DHCP generate it later), you will need to copy the one generated by AIF when it configured the network:

# cp /etc/resolv.conf /mnt/etc/resolv.conf

Load the dm-mod module (you might need grub2-bios; install the package if needed):

# modprobe dm-mod

Note: This is necessary at this point, and cannot be postponed after the chroot. If you try to use modprobe in a chroot environment that has a later kernel version from that of the installing device (at the time of writing, 2.6.33), modprobe will fail. This happens routinely using the Arch "net" installations.

From the installer's live shell, chroot to the installed system:

# mount -o bind /dev /mnt/dev
# chroot /mnt bash

Update pacman's database:

# pacman-db-upgrade

Refresh the package list (with an extra -y flag to force a refresh of all package lists even if they appear to be up to date):

# pacman -Syy

Install the GRUB2 package as mentioned in the section #From a running Arch Linux. Note that the dm-mod module has already been loaded, no need to do that again.

From a running Arch Linux

BIOS systems

The GRUB2 package can be installed with pacman (and will replace Template:Package Official, if it is installed):

# pacman -S grub2-bios

Note: Installing grub2-common (a dependency of grub2-bios) 1.99~rc1 or later, may take forever in some systems since the post_install script runs grub-mkconfig and this script does not provide the option --no-floppy. For more details search this option in the article.

Note: Simply installing the package won't update the core.img file and the grub2 modules in /boot/grub . You need to update the core.img file and the grub2 modules manually using grub_bios-install as explained below.

Then GRUB2 must be installed to the Master Boot Record of a drive :

# modprobe dm-mod

To setup the /boot/grub directory, generate /boot/grub/core.img, embed it in the post MBT gap (MBR disks) or in BIOS Boot Partition (GPT disks) and setup grub2 in the 440-byte Master Boot Record boot code region, run

# grub_bios-install --boot-directory=/boot --no-floppy --recheck /dev/sda

To generate a Template:Filename file alone without setting up grub2 in the MBR or embedding core.img, add --grub-setup=/bin/true to grub_bios-install:

# grub_bios-install --grub-setup=/bin/true --boot-directory=/boot --no-floppy --recheck /dev/sda

You can then chainload grub2's core.img from grub-legacy or syslinux as a Linux kernel or a multiboot kernel.

To setup grub2-bios to a partition boot sector or to a partitionless disk (eg. superfloppy), run (using /dev/sda1 as /boot part in this case)

# grub_bios-install --boot-directory=/boot --no-floppy --recheck --force /dev/sda1

It will give out warnings like which should give you the idea of what might go wrong with this approach.

/sbin/grub-setup: warn: Attempting to install GRUB to a partitionless disk or to a partition.  This is a BAD idea.
/sbin/grub-setup: warn: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  
                        However, blocklists are UNRELIABLE and their use is discouraged.
Installation finished. No error reported.

Set the immutable flag to the Template:Filename (need to check - may or may not work)

# chattr +i /boot/grub/core.img

You need to use Template:Codeline option to allow usage of blocklists and should not use --grub-setup=/bin/true (which is similar to simply generating core.img). Without Template:Codeline you may get an error like

/sbin/grub-setup: warn: Attempting to install GRUB to a partitionless disk or to a partition.  This is a BAD idea.
/sbin/grub-setup: warn: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  
                        However, blocklists are UNRELIABLE and their use is discouraged.
/sbin/grub-setup: error: will not proceed with blocklists

You also need to set the immutable flag to the Template:Filename since the file should not change its sector location so that the embedded bloacklists work. This flag need to be setup only if grub2-bios is installed to a partition boot sector or a partitionless disk, not needed in case of installtion to MBR or simple generaing core.img as mentioned above. For more info see https://bugzilla.redhat.com/show_bug.cgi?id=728742 and https://bugzilla.redhat.com/show_bug.cgi?id=730915 .

Note: grub2-bios (any version - including upstream bzr repo) does not encourage installation to a partition boot sector or a partitionless disk like grub-legacy or syslinux does. Neither do the Arch devs.

The Template:Codeline tells to not search for floppy devices which will vastly improve the command's overall execution time on many systems (it will also prevent the issue below from occuring) and where Template:Filename is the destination of the installation (in this case the MBR of the first SATA disk). If you use LVM for your Template:Filename, you can install GRUB2 on multiple physical disks.

Executing grub_bios-install without the Template:Codeline flag can also lead to this when there is a problem detecting the path of the floppy device:

grub-probe: error: Cannot get the real path of '/dev/fd0'
Auto-detection of a filesystem module failed.
Please specify the module with the option '--modules' explicitly.

Thus it is recommended to use the Template:Codeline switch, if you do not want to have your MBR on a floppy. If you do, you also want to set floppy as the first boot device in BIOS.

Finally, generate a configuration for grub2 (this is explained in greater detail in the Configuration section):

# cp /usr/share/grub/unicode.pf2 /boot/grub/
# grub-mkconfig -o /boot/grub/grub.cfg

GPT specific instructions

GRUB2 in BIOS-GPT configuration requires a BIOS Boot Partition to embed its core.img in the absence of 32 KiB post MBR gap in GPT partitioned systems.

Create a 1 MiB (minimum size - 2 MiB recommended if you use LVM and/or RAID etc.) partition using cgdisk or GNU Parted with no filesystem. The location of the partition in the partition table does not matter but it should be within the first 2 TiB region of the GPT disk. It is advisable to put it somewhere in the beginning of the disk before the /boot partition. Set the partition type to "EF02" in cgdisk or Template:Codeline in GNU Parted.

This partition is used by GRUB2 only in BIOS-GPT setups. No such partition type exists in case of MBR partitioning (at least not for GRUB2). This partition is also not required if the system is UEFI based, as no embedding takes place in that case. Neither GRUB-legacy nor SYSLINUX require this partition.

Note: This partition should be created before grub_bios-install or grub-setup is run or before the Install Bootloader step of the Archlinux installer (if GRUB2 is selected as bootloader).

MBR aka msdos partitioning specific instructions

Usually the post-MBR gap (after the 512 byte MBR region and before the start of the 1st partition) in many MBR (or msdos disklabel) partitioned systems is 32 KiB when DOS compatibility cylinder alignment issues are satisfied in the partition table. However a post-MBR gap of about 1 to 2 MiB is recommended to provide sufficient room for embedding grub2's core.img ( https://bugs.archlinux.org/task/24103 ). It is advisable to use a partitioner which supports 1 MiB partition alignment to obtain this space as well as satisfy other non-512 byte sector issues (which are unrelated to embedding of core.img).

If you do not dual-boot with MS Windows (any version) in BIOS systems, it is advisable to switch to GPT partitioning - GUID_Partition_Table#Convert_from_MBR_to_GPT_without_data_loss

Upgrading from grub-legacy (BIOS systems alone)

Remove grub-legacy traces from the MBR using:

# dd if=/dev/zero of=<your_disk> bs=440 count=1

Follow the #BIOS systems instructions.
Convert your Template:Filename to Template:Filename format using:

# grub-menulst2cfg /boot/grub/menu.lst /boot/grub/grub.cfg

Example:

Template:File

UEFI systems

Note: Unless specified as EFI 1.x , EFI and UEFI terms are used interchangeably to denote UEFI 2.x firmware. Also unless stated explicitely, the instructions are general and not Mac specific. Some of them may not work or may be different in Macs. Apple's EFI implementation is neither a EFI 1.1 version nor UEFI 2.x version but includes features of both. This kind of firmware does not fall under any one UEFI version so it is not a standard EFI firmware.

GRUB2 UEFI bootloader is available in Arch Linux only from version 1.99~rc1 . To install, first Detect which UEFI firmware arch you have (either x86_64 or i386).

Depending on that, install the appropriate package

For 64-bit aka x86_64 UEFI firmware:

# pacman -S grub2-efi-x86_64

For 32-bit aka i386 UEFI firmware:

# pacman -S grub2-efi-i386

Note: Simply installing the package won't update the grub.efi file and the grub2 modules in the UEFI System Partition . You need to update the grub.efi file and the grub2 modules in the UEFI System Partition manually using grub_efi_${UEFI_ARCH}-install as explained below.

Mount the EFI SYSTEM PARTITION in your system at Template:Filename. It should be FAT32 formatted and should be at least 200 MB in size. If you have mounted the EFISYS partition in some other mountpoint, replace Template:Filename in the below commands with that mountpoint:

# mkdir -p /boot/efi/efi  # If the directory does not exist

The below commands assume you are using Template:Codeline (for Template:Codeline, replace Template:Codeline with Template:Codeline in the below commands):

# modprobe dm-mod
# grub_efi_x86_64-install --boot-directory=/boot/efi/efi --bootloader-id=grub --no-floppy --recheck

The Template:Codeline automatically generates a Template:Filename while setting up the Template:Filename directory.

If you notice carefully, there is no <device_path> option (Eg: Template:Filename) at the end of the Template:Codeline command unlike the case of setting up grub2 for BIOS systems. Any <device_path> provided will be ignored by the install script as UEFI bootloaders do not use MBR or Partition boot sectors at all.

Then copy the font files required to enable UEFI video mode (GOP mostly) in grub2 (otherwise no kernel boot messages will be seen in the console):

# cp /usr/share/grub/unicode.pf2 /boot/efi/efi/grub/

Finally, generate a configuration for grub2 (this is explained in greater detail in the Configuration section):

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

Generate a custom GRUB2 UEFI Application

If you want, you can create a custom Template:Filename app using:

# grub-mkimage -d /usr/lib/grub/x86_64-efi -O x86_64-efi -p "" -o /boot/efi/efi/grub/grub_custom.efi <GRUB2_Modules_to_be_included>

Note: The -p option is important for creating a portable Template:Filename app.

There is no file-size restriction on Template:Filename either due to GRUB2 or due to the UEFI firmware. Therefore you can include any number of modules you want.

A "Template:Filename" created for BIOS based GRUB2 will be sufficient for the UEFI GRUB2 as long as all the paths in the config are absolute paths. The Template:Filename file should exist in the same directory as Template:Filename or Template:Filename.

The Template:Filename UEFI application can be launched using the firmware's "Boot Manager", "Boot from file" option or using the UEFI Shell.

Alternatively, you can also setup Template:Filename directory by copying all the files in Template:Filename to Template:Filename and using the above grub-mkimage command to create a Template:Filename application. Just make sure you have the required modules embedded in Template:Filename to enable it to access the EFI SYSTEM PARTITION.

A basic Template:Filename can be generated using the following command:

# sudo grub-mkimage -d /usr/lib/grub/x86_64-efi -O x86_64-efi -p "" -o /boot/efi/efi/grub/grub_custom.efi part_gpt fat iso9660 udf normal chain linux \
ls search search_fs_file search_fs_uuid search_label help boot configfile echo efi_gop

Correct GRUB2 Video Mode problems in UEFI

If you get this error when booting any menuentry

error: no suitable mode found
Booting however

Then you need to initialize grub2-efi's graphical terminal (gfxterm) with proper video mode (gfxmode) in grub2. This video mode is passed by grub2 to the linux kernel via 'gfxpayload'.

Copy /usr/share/grub/unicode.pf2 to /boot/efi/efi/grub/

# cp /usr/share/grub/unicode.pf2 /boot/efi/efi/grub/

and then, in the Template:Filename file, add the following lines to enable grub2 to pass the video mode correctly to the kernel, without of which you will only get a black screen (no output) but booting (actually) proceeds successfully without any system hang:

 insmod efi_gop
 insmod efi_uga
 insmod font

 if loadfont ${prefix}/unicode.pf2
 then
     insmod gfxterm
     set gfxmode=auto
     set gfxpayload=keep
     terminal_output gfxterm
 fi

As you can see for gfxterm (graphical terminal) to function properly, Template:Filename font file should exist in Template:Filename.

Launch GRUB2 as default in UEFI Boot Manager

For non-Mac UEFI systems

NOTE: The below steps will not work if the system has been booted in BIOS mode and will not work if the UEFI processor architecture does not match the kernel one, i.e. x86_64 UEFI + ix86 32-bit Kernel and vice-versa config will not work.

grub_efi_${UEFI_ARCH}-install will ensure Template:Filename is launched by default if it detects Template:Codeline and if it is able to access UEFI Runtime Services. Template:Codeline command will work only if you have booted the system in UEFI mode itself, since it requires access to UEFI Runtime Variables which are available only in UEFI boot mode (with "noefi" kernel parameter NOT being used). Initially the user is required to manually launch Template:Filename from the firmware itself (using maybe the UEFI Shell) if grub2-efi was installed in BIOS mode. Then Template:Codeline should be run to make grub2 as the default entry in the UEFI Boot Manager

First boot into UEFI mode manually either using the firmware boot menu, UEFI shell, or using any UEFI capable bootable iso (Archboot or Ubuntu non-Mac iso). Use grub probe to determine the device path of your EFI System Partition:

# grub-probe --target=device /boot/efi/efi/grub/grub.efi

It should give something like Template:Filename (used as example in the remaining steps). Load 'efivars' kernel module:

# sudo modprobe efivars

If you get no such device found error for this command, that means you have not booted in UEFI mode or due to some reason the kernel is unable to access UEFI Runtime Variables.

Verify whether there are files in /sys/firmware/efi/vars/ directory. This directory and its contents are created by "efivars" kernel module and it will exist only if you have booted in UEFI mode without the "noefi" kernel parameter.

# ls -1 /sys/firmware/efi/vars/

Sample output (x86_64-UEFI 2.3.1 in x86_64 Kernel:

# ls -1 /sys/firmware/efi/vars/
Boot0000-8be4df61-93ca-11d2-aa0d-00e098032b8c/
BootCurrent-8be4df61-93ca-11d2-aa0d-00e098032b8c/
BootOptionSupport-8be4df61-93ca-11d2-aa0d-00e098032b8c/
BootOrder-8be4df61-93ca-11d2-aa0d-00e098032b8c/
ConIn-8be4df61-93ca-11d2-aa0d-00e098032b8c/
ConInDev-8be4df61-93ca-11d2-aa0d-00e098032b8c/
ConOut-8be4df61-93ca-11d2-aa0d-00e098032b8c/
ConOutDev-8be4df61-93ca-11d2-aa0d-00e098032b8c/
ErrOutDev-8be4df61-93ca-11d2-aa0d-00e098032b8c/
Lang-8be4df61-93ca-11d2-aa0d-00e098032b8c/
LangCodes-8be4df61-93ca-11d2-aa0d-00e098032b8c/
MTC-eb704011-1402-11d3-8e77-00a0c969723b/
MemoryTypeInformation-4c19049f-4137-4dd3-9c10-8b97a83ffdfa/
PlatformLang-8be4df61-93ca-11d2-aa0d-00e098032b8c/
PlatformLangCodes-8be4df61-93ca-11d2-aa0d-00e098032b8c/
RTC-378d7b65-8da9-4773-b6e4-a47826a833e1/
del_var
new_var

If /sys/firmware/efi/vars/ directory is empty or does not exist, then Template:Codeline command will not work. If you are unable to make the ISO/CD/DVD/USB boot in UEFI mode try https://gitorious.org/tianocore_uefi_duet_builds/pages/Linux_Windows_BIOS_UEFI_boot_USB .

If /sys/firmware/efi/vars/ directory (along with the exists then run Template:Codeline and reboot:

# sudo efibootmgr --create --gpt --disk /dev/sda --part 1 --write-signature --label "GRUB2" --loader "\\EFI\\grub\\grub.efi"

In the above command, Template:Filename can be split up as Template:Filename and Template:Filename, which translates to Template:Filename -> partition 1 -> Template:Filename.

FAT32 filesystem is case-insensitive since it does not use UTF-8 encoding by default. In that case the firmware uses capital 'EFI' instead of small 'efi', therefore using Template:Filename or Template:Filename does not matter (this will change if the filesystem encoding is UTF-8). UEFI uses backward slash as path separatoir (similar to Windows paths). In the above command "double" backward slashes are used instead of single ones, since a backward slash is used as a escape character by sh shells, the first backward slash escapes the second one so that the path Template:Filename is passed to Template:Codeline.

The 'label' is the name of the menu entry shown in the UEFI boot menu. This name is user's choice and does not affect the booting of the system. More info can be obtained from efibootmgr GIT README

If you have problems running GRUB2 in UEFI mode you can try the following (worked on an ASUS Z68 mainboard):

# cp /boot/efi/efi/grub/grub.efi /boot/efi/shellx64.efi

or

# cp /boot/efi/efi/grub/grub.efi /boot/efi/efi/shellx64.efi

or

# cp /boot/efi/efi/grub/grub.efi /boot/efi/efi/shell/shellx64.efi

After this launch the UEFI Shell from the UEFI setup/menu (in ASUS UEFI BIOS, switch to advanced mode, press Exit in the top right corner and choose "Launch EFI shell from filesystem device"). The grub2 menu will show up and you can boot into your system. Afterwards you can use efibootmgr to setup a menu entry (see above).

For Apple Macs EFI

Use bless command

This article or section needs expansion.

Reason: please use the first argument of the template to provide a brief explanation. (Discuss in Talk:GRUB)

Chainload Windows x86_64 UEFI_GPT

Add this code to Template:Filename to chainload Windows Vista or 7 x86_64 installed in UEFI-GPT mode :

menuentry "Windows x86_64 UEFI-GPT" {
    search --file --no-floppy --set=root /efi/Microsoft/Boot/bootmgfw.efi
    chainloader (${root})/efi/Microsoft/Boot/bootmgfw.efi
}

Manual Compilation

For BIOS Systems

GRUB2 for BIOS systems should be compiled as follows:

./autogen.sh

./configure --with-platform=pc --prefix=/usr

make

make install

The --with-platform=pc generates grub2 for bios alone irrespective of the firmware of the build system.

For UEFI Systems

First Detect which UEFI Firmware arch you have and then follow the compile instructions below:

./autogen.sh

./configure --with-platform=efi --target=TARGET_UEFI_ARCH --prefix=/usr

make

make install

The "--target" option denotes the UEFI firmware arch. for which grub2 should be compiled, not the architecture of the linux kernel grub2 may boot.

It is possible to use UEFI 64-bit firmware + GRUB2 as x86_64-EFI app loading a i686 linux kernel, as long as the kernel does not try to access UEFI Runtime Services. Vice-versa situation is also possible. But a x86_64 UEFI firmware cannot launch GRUB2 i386-efi app (unlike x86_64 Operating Systems), and a i386 UEFI firmware will not launch GRUB2 Template:Codeline app. It is important to compile GRUB2 to match the architecture of the UEFI firmware.

Configuration

The configuration files are Template:Filename and Template:Filename. These files are used to generate the Template:Filename file. You can also choose to manually edit Template:Filename.

grub-mkconfig

The grub-mkconfig script can be used to generate a Template:Filename file. By default the script outputs to stdout. Note that gettext ― an optional dependency of the GRUB2 package ― is required by the grub-mkconfig script. To generate a Template:Filename file run the command:

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

grub.cfg

A basic grub file uses the following options

Template:Codeline is the partition Template:Codeline on disk Template:Codeline, partition numbers starting at 1, disk numbers starting at 0
Template:Codeline is the default boot entry that is chosen after timeout for user action
Template:Codeline is the time Template:Codeline to wait in seconds for a user selection before default is booted
Template:Codeline is a boot entry titled Template:Codeline
Template:Codeline sets the boot partition, where the kernel and GRUB modules are stored (boot need not be a separate partition, and may simply be a directory under the "root" partition (Template:Filename)

An example configuration:

Template:File

Dual-booting

NOTE: If you want GRUB2 to automatically search for other systems, for example as in Ubuntu. Then you may need to download Template:Package AUR from the AUR.

Using grub-mkconfig

The best way to add other entries is editing the Template:Filename. The entries in this file will be automatically added when running grub-mkconfig. After adding the new lines, run:

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

to generate an updated Template:Filename.

With GNU/Linux

Assuming that the other distro is on partition Template:Filename:

menuentry "Other Linux" {
set root=(hd0,2)
linux /boot/vmlinuz (add other options here as required)
initrd /boot/initrd.img (if the other kernel uses/needs one)
}

With Windows

This assumes that your Windows partition is Template:Filename.

# (2) Windows XP
menuentry "Windows XP" {
    set root=(hd0,3)
    chainloader (hd0,3)+1
}

If the windows bootloader is on an entirely different harddrive than grub, it may be necessary to trick Windows into believing that it is in fact the first harddrive. This was possible in the old grub with Template:Codeline and is now done with Template:Codeline. Assume grub is on Template:Filename and windows on Template:Filename, you need to add the following after Template:Codeline:

drivemap -s hd0 hd2

With Windows via EasyBCD and NeoGRUB

Since EasyBCD's NeoGRUB currently does not understand the GRUB2 menu format, chainload to it by replacing the contents of your Template:Filename file with lines similar to the following:

default 0
timeout 1

title       Chainload into GRUB v2
root        (hd0,7)
kernel      /boot/grub/core.img

Visual Configuration

In GRUB2 it is possible, by default, to change the look of the menu.

Background image and bitmap fonts

GRUB2 comes with support for background images and bitmap fonts in pf2 format. The unifont font is included in the grub2 package under the filename Template:Filename, or, as only ascii characters under the name Template:Filename. Image formats supported include tga, png and jpeg, providing the correct modules are loaded. The maximum supported resolution depends on your hardware. There are two ways of setting a Template:Filename file as background. Two sample configurations are shown below.

Edit Template:Filename like this:

GRUB_GFXMODE=1024x768x32
GRUB_GFXPAYLOAD_LINUX=keep
GRUB_BACKGROUND="/boot/grub/archlinux.tga"
#GRUB_THEME="/path/to/gfxtheme"

To generate the changes, run:

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

Note: If this example does not work for you try to replace Template:Codeline by Template:Codeline.

Note: To show all the modes you can use Template:Codeline (hwinfo is available in [community]), while at grub2 prompt you can use the Template:Codeline command.

Note: If you have installed Grub on a separate partition, Template:Filename becomes Template:Filename.

Menu colors

As in Grub (0.9x), you can change the menu colors in Grub2. The available colors for GRUB2 are at http://www.gnu.org/software/grub/manual/html_node/Theme-file-format.html#Theme-file-format . Here is an example:

Edit Template:Filename:

GRUB_COLOR_NORMAL="light-blue/black"
GRUB_COLOR_HIGHLIGHT="light-cyan/blue"

Generate the changes:

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

Hidden menu

One of the unique features of Grub2 is hiding/skipping the menu and showing it by holding "Shift" when needed. You can also adjust whether you want to see the timeout counter.

Edit Template:Filename as you wish. Here is an example where the comments from the beginning of the two lines have been removed to enable the feature, the timeout has been set to five seconds and to be shown to the user:

GRUB_HIDDEN_TIMEOUT=5
GRUB_HIDDEN_TIMEOUT_QUIET=false

and run:

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

Setting the framebuffer resolution

Grub2 can set the framebuffer for both grub2 itself and the kernel. The old vga= way is deprecated. The preferred method is editing Template:Filename as the following sample:

GRUB_GFXMODE=1024x768x32
GRUB_GFXPAYLOAD_LINUX=keep

To generate the changes, run:

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

The Template:Codeline property will make sure the kernel keeps the resolution.

If this method does not work for you, the deprecated vga= method will still work. Just add it next to the "GRUB_CMDLINE_LINUX_DEFAULT=" line in Template:Filename for eg: "GRUB_CMDLINE_LINUX_DEFAULT="quiet splash vga=792" will give you a Template:Codeline resolution.

You can choose one of these resolutions: Template:Codeline, Template:Codeline, Template:Codeline, Template:Codeline, Template:Codeline

Other Options

LVM

If you use LVM for your Template:Filename, add the following before menuentry lines:

insmod lvm

and specify your root in the menuentry as:

set root=(lvm_group_name-lvm_logical_boot_partition_name)

Example:

# (0) Arch Linux
menuentry "Arch Linux" {
insmod lvm
set root=(VolumeGroup-lv_boot)
# you can only set following two lines
linux /vmlinuz-linux root=/dev/mapper/VolumeGroup-root ro
initrd /initramfs-linux.img
}

Raid

Grub2 provides convenient handling of raid-volumes. You need to add:

insmod raid

which allows you to address the volume natively. E.g. Template:Filename becomes:

set root=(md0)

whereas a partitioned raid-volume (e.g. Template:Filename) becomes:

set root=(md0,1)

Persistent block device naming

You can use UUIDs to detect partitions instead of the "old" Template:Filename and Template:Filename scheming. It has the advantage of detecting partitions by their unique UUIDs, which is needed by some people booting with complicated partition setups.

UUIDs are used by default in the recent versions of grub2 - there is no downside in it anyway except that you need to re-generate the Template:Filename file every time you resize or reformat your partitions. Remember this when modifying partitions with Live-CD.

The recent versions of grub2 use UUIDs by default. You can re-enable the use of UUIDS by simply commenting the UUID line (this is also what it looks like by default):

#GRUB_DISABLE_LINUX_UUID=true

you can also just set the value as Template:Codeline as shown here:

GRUB_DISABLE_LINUX_UUID=false

Either way, do not forget to generate the changes:

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

Using Labels

It is possible to use labels, human-readable strings attached to filesystems, by using the Template:Codeline option to Template:Codeline. First of all, label your existing partition:

# tune2fs -L a <LABEL> <PARTITION>

Then, add an entry using labels. An example of this:

menuentry "Arch Linux, session texte" {
    search --label --no-floppy --set=root archroot
    linux /boot/vmlinuz-linux root=/dev/disk/by-label/archroot ro
    initrd /boot/initramfs-linux.img
}

Recall previous entry

Grub2 can remember the last entry you booted from and use this as the default entry to boot from next time. This is useful if you have multiple kernels (i.e., the current Arch one and the LTS kernel as a fallback option) or operating systems. To do this, edit Template:Filename and change the setting of Template:Codeline:

GRUB_DEFAULT=saved

This ensures that grub will default to the saved entry. To enable saving the selected entry, add the following line to Template:Filename:

GRUB_SAVEDEFAULT=true

Remember to regenerate your configuration file.

Security

If you want to secure GRUB2 so it is not possible for anyone to change boot parameters or use the command line, you can add a user/password combination to GRUB2's configuration files. To do this, run the command Template:Codeline. Enter a password and confirm it. The output will look like this:

Your PBKDF2 is grub.pbkdf2.sha512.10000.C8ABD3E93C4DFC83138B0C7A3D719BC650E6234310DA069E6FDB0DD4156313DA3D0D9BFFC2846C21D5A2DDA515114CF6378F8A064C94198D0618E70D23717E82.509BFA8A4217EAD0B33C87432524C0B6B64B34FBAD22D3E6E6874D9B101996C5F98AB1746FE7C7199147ECF4ABD8661C222EEEDB7D14A843261FFF2C07B1269A

Then, add the following to Template:Filename:

cat << EOF

set superusers="username"
password_pbkdf2 username <password>

EOF

where <password> is the string generated by Template:Codeline.

Regenerate your configuration file. Your GRUB2 command line and boot parameters are now protected.

Using the command shell

Since the MBR is too small to store all GRUB2 modules, only the menu and a few basic commands reside there. The majority of GRUB2 functionality remains in modules in Template:Filename, which are inserted as needed. In error conditions (e.g. if the partition layout changes) GRUB2 may fail to boot. When this happens, a command shell may appear.

GRUB2 offers multiple shells/prompts. If there is a problem reading the menu but the bootloader is able to find the disk, you will likely be dropped to the "normal" shell:

sh:grub>

If there is a more serious problem (e.g. GRUB cannot find required files), you may instead be dropped to the "rescue" shell:

grub rescue>

The rescue shell is a restricted subset of the normal shell, offering much less functionality. If dumped to the rescue shell, first try inserting the "normal" module, then starting the "normal" shell:

grub rescue> set prefix=(hdX,Y)/boot/grub
grub rescue> insmod (hdX,Y)/boot/grub/normal.mod
rescue:grub> normal

Pager support

GRUB2 supports pager for reading commands that provide long output (like the help command). This works only in normal shell mode and not in rescue mode. To enable pager, in GRUB2 command shell type:

sh:grub> set pager=1

GUI configuration tools

Following package may be installed from AUR

grub2-editor (requires kdelibs)
A KDE4 control module for configuring the GRUB2 bootloader
kcm-grub2 (requires kdelibs python2-qt kdebindings-python)
This Kcm module manages the most common settings of Grub2.
startupmanager (requires gnome-python imagemagick yelp python2 xorg-xrandr)
GUI app for changing the settings of GRUB, GRUB2, Usplash and Splashy

parttool or legacy hide/unhide

If you have a win9x paradigm with hidden C disks GRUB legacy had the hide/unhide feature. In GRUB2 this has been replaced by parttool. For example, to boot the third C disk of three win9x installations on the CLI enter the CLI and:

parttool hd0,1 hidden+ boot-
parttool hd0,2 hidden+ boot-
parttool hd0,3 hidden- boot+
set root=hd0,3
chainloader +1
boot

Using the rescue console

See #Using the command shell first. If unable to activate the standard shell, one possible solution is to boot using a live CD or some other rescue disk to correct configuration errors and reinstall GRUB. However, such a boot disk is not always available (nor necessary); the rescue console is surprisingly robust.

The available commands in GRUB rescue include "insmod", "ls", "set", and "unset". This example uses "set" and "insmod". "set" modifies variables and "insmod" inserts new modules to add functionality.

Before starting, the user must know the location of their Template:Filename partition (be it a separate partition, or a subdirectory under their root):

grub rescue> set prefix=(hdX,Y)/boot/grub

where X is the physical drive number and Y is the partition number.

To expand console capabilities, insert the "linux" module:

grub rescue> insmod (hdX,Y)/boot/grub/linux.mod

Note: With a separate boot partition, omit Template:Filename from the path, (i.e. type Template:Codeline and Template:Codeline).

This introduces the "linux" and "initrd" commands, which should be familiar (see #Configuration).

An example, booting Arch Linux:

set root=(hd0,5)
linux /boot/vmlinuz-linux root=/dev/sda5
initrd /boot/initramfs-linux.img
boot

With a separate boot partition, again change the lines accordingly:

set root=(hd0,5)
linux /vmlinuz-linux root=/dev/sda6
initrd /initramfs-linux.img
boot

After successfully booting the Arch Linux installation, users can correct Template:Filename as needed and then run:

# grub-install /dev/sda --no-floppy

to reinstall GRUB2 and fix the problem completely, changing Template:Filename if needed. See #Bootloader installation for details.

Combining the use of UUID's and basic scripting

If you like the idea of using UUID's to avoid unreliable BIOS mappings or are struggling with Grub's syntax, here is an example boot menu item that uses UUID's and a small script to direct Grub to the proper disk partitions for your system. All you need to do is replace the UUID's in the sample with the correct UUID's for your system. (The example applies to a system with a boot and root partition. You will obviously need to modify the Grub configuration if you have additional partitions.)

 menuentry "Arch Linux 64" {
   #Enter the UUID of your boot partition (this is where Grub and your kernel reside)
   set uuid_grub_boot=ece0448f-bb08-486d-9864-ac3271bd8d07
   
   #Enter the UUID of the partition containing the root partition of your Arch Linux installation.            
   set uuid_os_root=c55da16f-e2af-4603-9e0b-03f5f565ec4a
   
   #(Note: this may be the same as your boot partition)
   
   #Here we set the Grub "root" variable by locating the UUID of the root partition identified above   
   search --fs-uuid --no-floppy --set=root $uuid_os_root
   
   #Here we set a custom variable grub_boot by locating the UUID of the boot partition identified above          
   search --fs-uuid --no-floppy --set=grub_boot $uuid_grub_boot
   
   #Here is the magic. We test to see if the boot and root partitions have the same UUID.
   #If they do, we append /boot to the $grub_boot variable. For ex. (hd0,1) becomes (hd0,1)/boot.
   if [ $uuid_grub_boot == $uuid_os_root ] ; then
      set grub_boot=$grub_boot/boot
   fi
   
   # $grub_boot now points to the correct location, so the following will properly find the kernel and initrd
   linux ($grub_boot)/vmlinuz-linux root=/dev/disk/by-uuid/$uuid_os_root ro
   initrd ($grub_boot)/initramfs-linux.img
 }

Troubleshooting

Any troubleshooting should be added here.

msdos-style error message

grub-setup: warn: This msdos-style partition label has no post-MBR gap; embedding won't be possible!
grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists.
            However, blocklists are UNRELIABLE and its use is discouraged.
grub-setup: error: If you really want blocklists, use --force.

This error may occur when you try installing GRUB2 in a VMware container. Read more about it here. Hopefully a fix will be provided soon.

It also happens when the first partition starts just after the MBR (block 63), without the usual space of 1MB (2048 blocks) before the first partition.

Grub not loaded

In some cases the EFI may fail to load grub correctly. Provided everything is set up correctly, the output of

efibootmgr -v

might look something like this:

BootCurrent: 0000
Timeout: 3 seconds
BootOrder: 0000,0001,0002
Boot0000* Grub	HD(1,800,32000,23532fbb-1bfa-4e46-851a-b494bfe9478c)File(\efi\grub\grub.efi)
Boot0001* Shell	HD(1,800,32000,23532fbb-1bfa-4e46-851a-b494bfe9478c)File(\EfiShell.efi)
Boot0002* Festplatte	BIOS(2,0,00)P0: SAMSUNG HD204UI           .

If everything works correctly, the EFI would now automatically load grub.
If the screen only goes black for a second and the next boot option is tried afterwards, according to this post, moving grub to the partition root can help. The boot option has to be deleted and recreated afterwards. The entry for grub should look like this then:

Boot0000* Grub	HD(1,800,32000,23532fbb-1bfa-4e46-851a-b494bfe9478c)File(\grub.efi)

Invalid signature

If trying to boot windows results in an "invalid signature" error, e.g. after reconfiguring partitions or adding additional hard drives, (re)move grub's device configuration and let it reconfigure.

# mv /boot/grub/device.map /boot/grub/device.map-old
# grub-mkconfig -o /boot/grub/grub.cfg

Grub-mkconfig should now mention all found boot options including windows. If it works, remove /boot/grub/device.map-old.

References

Official GRUB2 Manual - http://www.gnu.org/software/grub/manual/grub.html
GRUB2 wiki page describing steps to compile for BIOS - http://grub.fsij.org/TestingOnX86 Template:Linkrot
GRUB2 wiki page describing steps to compile for UEFI - http://grub.fsij.org/TestingOnUEFI Template:Linkrot - moved to https://help.ubuntu.com/community/UEFIBooting
Wikipedia's page on BIOS Boot Partition
GRUB2 Wiki page Template:Linkrot describing why BIOS Boot Partition is required.

External Links