Difference between revisions of "GRUB"

Revision as of 00:01, 27 December 2011

Template:Article summary text Template:Article summary heading Template:Article summary text Template:Article summary heading Template:Article summary wiki - Burg is a brand-new boot loader based on GRUB2. It uses a new object format which allows it to be built in a wider range of OS, including Linux/Windows/OSX/Solaris/FreeBSD, etc. It also has a highly configurable menu system which works in both text and graphic mode. Template:Article summary heading Template:Article summary link Template:Article summary end

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].

In brief, 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.

Notes for current GRUB Legacy 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, /dev/sda1 would be referred to as (hd0,msdos1) (for MBR) or (hd0,gpt1) (for GPT) using GRUB2.

Preliminary Requirements for GRUB2

BIOS systems

GPT specific instructions

GRUB2 in BIOS-GPT configuration requires a BIOS Boot Partition to embed its core.img in the absence of post-MBR gap in GPT partitioned systems (which is taken over by the GPT Primary Header and Primary Partition table). 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 of bootsectors takes place in that case. Syslinux does not require this partition.

For a BIOS-GPT configuration, create a 2 MiB 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 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 set <BOOT_PART_NUM> bios_grub on in GNU Parted.

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

UEFI systems

Create and Mount the UEFI SYSTEM PARTITION

Follow Unified_Extensible_Firmware_Interface#Create_an_UEFI_SYSTEM_PARTITION_in_Linux for instructions on creating a UEFI SYSTEM PARTITION. Then mount the UEFI SYSTEM PARTITION at /boot/efi. It should be FAT32 formatted and should be >=200 MiB in size. If you have mounted the UEFISYS partition in some other mountpoint, replace /boot/efi in the below instructions with that mountpoint:

# mkdir -p /boot/efi
# mount -t vfat <UEFISYS_PART_DEVICE> /boot/efi

Create a <UEFI_SYSTEM_PARTITION>/efi directory if it does not exist:

# mkdir -p /boot/efi/efi

Installation

During Arch Linux installation

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

# aif -p partial-configure-network

This will bring up a prompt; put in the network interface to use, (e.g., eth0) and use DHCP for easy configuration.

• If you did not configure the installed system's /etc/resolv.conf 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 /tmp/install/etc/resolv.conf

• If you run into network issues in the pacman update step below, you may have needed to install the net-tools package.
• Load the dm-mod module (you might need grub2-bios; install the package if needed):

# modprobe dm-mod

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

# mount -o bind /dev /mnt/dev
# mount -t proc /proc /mnt/proc/
# mount -t sysfs /sys /mnt/sys/
# 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

Backup Important Data

In general, a grub installation should run smoothly. Sometimes it could mess up your system. You're strongly advised to make a backup before installing grub2-bios.

• copy grub modules and configuration

# cp -a /boot/grub /path/to/backup/

• backup the MBR and GRUB-Legacy stage 1.5

# dd if=/dev/sdX of=/path/to/backup/first-sectors bs=512 count=63

Replace /dev/sdaX with you disk path (mostly /dev/sda).

To backup only the MBR boot code use:

# dd if=/dev/sdX of=/path/to/backup/mbr-boot-code bs=440 count=1

You could now lightly remove /boot/grub with:

# rm -rf /boot/grub

and follow the instructions below. You know that if things get nasty, you could reboot your system thanks to an installation media and:

• move old grub-legacy or grub2 files out of the way

# mv /boot/grub /boot/grub.nonfunctional

• copy grub back to /boot

# cp -a /path/to/backup/grub /boot/

• replace MBR and next 62 sectors of sda with backed up copy (DANGEROUS!)

# dd if=/path/to/backup/first-sectors of=/dev/sdX bs=512 count=63

To restore only the MBR boot code use:

# dd if=/path/to/backup/mbr-boot-code of=/dev/sdX bs=440 count=1

Install grub2-bios package

The GRUB2 package can be installed with pacman (and will replace grub, if it is installed). Simply installing the package won't update the /boot/grub/core.img file and the grub2 modules in /boot/grub. You need to update them manually using grub_bios-install as explained below.

# pacman -S grub2-bios

Also load the device-mapper kernel module without which grub-probe does not reliably detect disks and partitions:

# modprobe dm-mod

Install grub2-bios boot files

There are 3 ways to install grub2 boot files in BIOS booting：

• #Install_to_440-byte_MBR_boot_code_region (recommended) ,
• #Install_to_Partition_or_Partitionless_Disk (not recommended),
• #Generate_core.img_alone (safest method, but requires another BIOS bootloader like grub-legacy or syslinux to be installed to chainload /boot/grub/core.img ).

Install to 440-byte MBR boot code region

To setup grub2-bios in the 440-byte Master Boot Record boot code region, populate the /boot/grub directory, generate the /boot/grub/core.img file, and embed it in the 32 KiB (minimum size - varies depending on partition alignment) post-MBR gap (MBR disks) or in BIOS Boot Partition (GPT disks), run:

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

where /dev/sda is the destination of the installation (in this case the MBR of the first SATA disk). If you use LVM for your /boot, you can install GRUB2 on multiple physical disks.

The --no-floppy tells grub2-bios utilities not to search for any floppy devices which reduces the overall execution time of grub_bios-install on many systems (it will also prevent the issue below from occurring). Otherwise you get an error like this:

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.

Install to Partition or Partitionless Disk

To setup grub2-bios to a partition boot sector, to a partitionless disk (also called superfloppy) or to a floppy disk, run (using for example /dev/sda1 as the /boot partition)

# chattr -i /boot/grub/core.img
# grub_bios-install --boot-directory=/boot --no-floppy --recheck --force --debug /dev/sda1
# chattr +i /boot/grub/core.img

You need to use the --force option to allow usage of blocklists and should not use --grub-setup=/bin/true (which is similar to simply generating core.img).

grub_bios-install 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.

Without --force you may get the below error and grub-setup will not setup its boot code in the partition boot sector.

/sbin/grub-setup: error: will not proceed with blocklists

With --force you should get

Installation finished. No error reported.

The reason why grub-setup does not by default allow this is because in case of partition or a partitionless disk is that grub2-bios relies on embedded blocklists in the partition bootsector to locate the /boot/grub/core.img file and the prefix dir /boot/grub . The sector locations of core.img may change whenever the filesystem in the partition is being altered (files copied, deleted etc.). For more info see https://bugzilla.redhat.com/show_bug.cgi?id=728742 and https://bugzilla.redhat.com/show_bug.cgi?id=730915 .

The workaround for this is to set the immutable flag on /boot/grub/core.img (using chattr command as mentioned above) so that the sector locations of the core.img file in the disk is not altered. The immutable flag on /boot/grub/core.img needs to be set only if grub2-bios is installed to a partition boot sector or a partitionless disk, not in case of installtion to MBR or simple generation of core.img without embedding any bootsector (mentioned above).

Generate core.img alone

To populate the /boot/grub directory and generate a /boot/grub/core.img file WITHOUT embedding any grub2-bios bootsector code in the MBR, post-MBR region, or the partition bootsector, add --grub-setup=/bin/true to grub_bios-install:

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

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

Generate GRUB2 BIOS Config file

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

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

If grub2 complains about "no suitable mode found" while booting, go to #Correct_GRUB2_No_Suitable_Mode_Found_Error .

If grub-mkconfig fails, convert your /boot/grub/menu.lst file to /boot/grub/grub.cfg using:

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

For example:

/boot/grub/menu.lst

default=0
timeout=5

title  Arch Linux Stock Kernel
root   (hd0,0)
kernel /vmlinuz-linux root=/dev/sda2 ro
initrd /initramfs-linux.img

title  Arch Linux Stock Kernel Fallback
root   (hd0,0)
kernel /vmlinuz-linux root=/dev/sda2 ro
initrd /initramfs-linux-fallback.img

/boot/grub/grub.cfg

set default='0'; if [ x"$default" = xsaved ]; then load_env; set default="$saved_entry"; fi
set timeout=5

menuentry 'Arch Linux Stock Kernel' {
  set root='(hd0,1)'; set legacy_hdbias='0'
  legacy_kernel   '/vmlinuz-linux' '/vmlinuz-linux' 'root=/dev/sda2' 'ro'
  legacy_initrd '/initramfs-linux.img' '/initramfs-linux.img'
  
}

menuentry 'Arch Linux Stock Kernel Fallback' {
  set root='(hd0,1)'; set legacy_hdbias='0'
  legacy_kernel   '/vmlinuz-linux' '/vmlinuz-linux' 'root=/dev/sda2' 'ro'
  legacy_initrd '/initramfs-linux-fallback.img' '/initramfs-linux-fallback.img'
}

If you forgot to create a GRUB2 /boot/grub/grub.cfg configfile and simply rebooted into GRUB2 Command Shell, type:

sh:grub> insmod legacycfg
sh:grub> legacy_configfile ${prefix}/menu.lst

Boot into Arch and re-create the proper GRUB2 /boot/grub/grub.cfg configfile.

Multiboot in BIOS

Boot Microsoft Windows installed in BIOS-MBR mode

Find the UUID of the NTFS filesystem of the Windows's SYSTEM PARTITION where the bootmgr and its files reside. For example, if Windows bootmgr exists at /media/Windows/bootmgr:

# grub-probe --target=fs_uuid /media/Windows/bootmgr
69B235F6749E84CE

Then, add the below code to /etc/grub.d/40_custom and regenerate grub.cfg with grub-mkconfig as explained above to chainload Windows (Vista, 7 or 8) installed in BIOS-MBR mode:

menuentry "Microsoft Windows 7 BIOS-MBR" {
    insmod part_msdos
    insmod ntfs
    insmod search_fs_uuid
    insmod ntldr     
    search --fs-uuid --no-floppy --set=root 69B235F6749E84CE
    ntldr (${root})/bootmgr
}

For Windows XP

menuentry "Microsoft Windows XP" {
    insmod part_msdos
    insmod ntfs
    insmod search_fs_uuid
    insmod ntldr     
    search --fs-uuid --no-floppy --set=root 69B235F6749E84CE
    ntldr (${root})/ntldr
}

UEFI systems

Install grub2-uefi package

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

Also load the device-mapper kernel module without which grub-probe does not reliably detect disks and partitions

# modprobe dm-mod

Install grub2-uefi boot files

Install to UEFI SYSTEM PARTITION

# grub_efi_x86_64-install --root-directory=/boot/efi --boot-directory=/boot/efi/efi --bootloader-id=grub --no-floppy --recheck --debug

The grub_efi_x86_64-install automatically generates a grub.efi while setting up the /boot/efi/efi/grub/ directory.

If you notice carefully, there is no <device_path> option (Eg: /dev/sda) at the end of the grub_efi_x86_64-install 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.

You may now be able to UEFI boot your system by creating a grub.cfg file by following #Generate_GRUB2_UEFI_Config_file and copying /boot/efi/efi/grub/grubx64.efi to /boot/efi/efi/boot/bootx64.efi.

Create GRUB2 entry in the Firmware Boot Manager

Non-Mac UEFI systems

grub_efi_${UEFI_ARCH}-install will ensure /boot/efi/efi/grub/grub.efi is launched by default if it detects efibootmgr and if it is able to access UEFI Runtime Services. efibootmgr 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 /boot/efi/efi/grub/grub.efi from the firmware itself (using maybe the UEFI Shell) if grub2-efi was installed in BIOS mode. Then efibootmgr 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 /dev/sda1 (used as example in the remaining steps). Load 'efivars' kernel module:

# 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 efibootmgr 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 efibootmgr and reboot:

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

In the above command, /boot/efi/efi/grub/grub.efi can be split up as /boot/efi and /efi/grub/grub.efi, which translates to (/dev/sda) -> partition 1 -> \\EFI\\grub\\grub.efi.

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 \\EFI\\grub\\grub.efi or \\efi\\grub\\grub.efi 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 \EFI\grub\grub.efi is passed to efibootmgr.

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/grubx64.efi /boot/efi/shellx64.efi

or

# cp /boot/efi/efi/grub/grubx64.efi /boot/efi/efi/shellx64.efi

or

# cp /boot/efi/efi/grub/grubx64.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).

Apple Mac EFI systems

Use bless command from within Mac OS X to set grub.efi as the default boot option. You can also boot from the Mac OS X install disc and launch a Terminal there if you only have Linux installed. In the Terminal, create a directory and mount the EFI System Partition:

# cd /Volumes
# mkdir efi
# mount -t msdos /dev/disk0s1 /Volumes/efi

Then run bless on grub.efi and on the EFI partition to set them as the default boot options.

# bless --folder=/Volumes/efi --file=/Volumes/efi/efi/grub/grub.efi --setBoot
# bless --mount=/Volumes/efi --file=/Volumes/efi/efi/grub/grub.efi --setBoot

More info at https://help.ubuntu.com/community/UEFIBooting#Apple_Mac_EFI_systems_.28both_EFI_architecture.29 .

Generate GRUB2 UEFI Config file

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

If grub2-uefi complains about "no suitable mode found" while booting, go to #Correct_GRUB2_No_Suitable_Mode_Found_Error .

Create GRUB2 Standalone UEFI Application

It is possible to create a grub_standalone.efi application which has all the modules embeddded in a memdisk within the uefi application, thus removing the need for having a separate directory populated with all the grub2 uefi modules and other related files.

The easiest way to do this would be with the install command already mentioned before, but specifying the modules to include. For example:

# grub_efi_x86_64-install --root-directory=/boot/efi --boot-directory=/boot/efi/efi --bootloader-id=grub --no-floppy --recheck --debug \
--modules="normal help ext2 video efi_gop gfxterm gzio"

However, you might want to embed certain configuration as well, for which you might want to use a script like this:

grub_image_helper

#!/bin/bash

ARCH=${1:-x86_64-efi}

cat << EOF > "grub.cfg"
search --no-floppy --fs-uuid --set=grub_root 5EF2-55E6
set prefix=(\$grub_root)/efi/grub
EOF

grub-mkimage --format=$ARCH --compression=xz --output=/boot/efi/efi/grub/grub_standalone.efi --config grub.cfg '--prefix=(,gpt1)/efi/grub' fat part_gpt search

You would need to create an entry with efibootmgr for grub_standalone.efi, just use grub.efi instead to replace the original, or you can use the UEFI shell to try it out first. See #Create GRUB2 entry in the Firmware Boot Manager.

Multiboot in UEFI

Chainload Microsoft Windows x86_64 UEFI-GPT

Find the UUID of the FAT32 filesystem in the UEFI SYSTEM PARTITION where the Windows UEFI Bootloader files reside. For example, if Windows bootmgfw.efi exists at /boot/efi/efi/Microsoft/Boot/bootmgfw.efi (ignore the upper-lower case differences since that is immaterial in FAT filesystem):

# grub-probe --target=fs_uuid /boot/efi/efi/Microsoft/Boot/bootmgfw.efi
1ce5-7f28

Then, add this code to /boot/efi/efi/grub/grub.cfg to chainload Windows x86_64 (Vista SP1+, 7 or 8) installed in UEFI-GPT mode:

menuentry "Microsoft Windows x86_64 UEFI-GPT" {
    insmod part_gpt
    insmod fat
    insmod search_fs_uuid
    insmod chain
    search --fs-uuid --no-floppy --set=root 1ce5-7f28
    chainloader (${root})/efi/Microsoft/Boot/bootmgfw.efi
}

Configuration

You can also choose to automatically generate or manually edit grub.cfg.

Automatically generating using grub-mkconfig (Recommended)

The GRUB2 menu.1st equivalent configuration files are /etc/default/grub and /etc/grub.d/*. grub-mkconfig uses these files to generate grub.cfg. By default the script outputs to stdout. To generate a grub.cfg file run the command:

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

/etc/grub.d/10_linux is set to automatically add menu items for Arch linux that work out of the box, to any generated configuration. Other operating systems may need to be added manually by editing /etc/grub.d/40_custom

Manually creating grub.cfg

A basic grub file uses the following options

• (hdX,Y) is the partition Y on disk X, partition numbers starting at 1, disk numbers starting at 0
• set default=N is the default boot entry that is chosen after timeout for user action
• set timeout=M is the time M to wait in seconds for a user selection before default is booted
• menuentry "title" {entry options} is a boot entry titled title
• set root=(hdX,Y) 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 (/)

An example configuration:

/boot/grub/grub.cfg

# Config file for GRUB2 - The GNU GRand Unified Bootloader
# /boot/grub/grub.cfg

# DEVICE NAME CONVERSIONS
#
#  Linux           Grub
# -------------------------
#  /dev/fd0        (fd0)
#  /dev/sda        (hd0)
#  /dev/sdb2       (hd1,2)
#  /dev/sda3       (hd0,3)
#

# Timeout for menu
set timeout=5

# Set default boot entry as Entry 0
set default=0

# (0) Arch Linux
menuentry "Arch Linux" {
set root=(hd0,1)
linux /vmlinuz-linux root=/dev/sda3 ro
initrd /initramfs-linux.img
}

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

Dual-booting

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

Using grub-mkconfig

The best way to add other entries is editing the /etc/grub.d/40_custom. 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 grub.cfg.

With GNU/Linux

Assuming that the other distro is on partition sda2:

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 FreeBSD

Requires that FreeBSD is installed on a single partition with UFS. Assuming it is installed on sda4:

menuentry "FreeBSD" {
set root=(hd0,4)
chainloader +1
}

With Windows

This assumes that your Windows partition is sda3.

# (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 map and is now done with drivemap. Assume grub is on hd0 and windows on hd2, you need to add the following after set root:

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 C:\NST\menu.lst 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. Make sure to initialize, if not done already, grub2 graphical terminal, gfxterm, with proper video mode, gfxmode, in grub2. This can be seen in the section #Correct_GRUB2_No_Suitable_Mode_Found_Error. This video mode is passed by grub2 to the linux kernel via 'gfxpayload' so any visual configurations need this mode in order to be in effect.

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 /etc/default/grub as the following sample:

GRUB_GFXMODE=1024x768x32
GRUB_GFXPAYLOAD_LINUX=keep

To generate the changes, run:

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

The gfxpayload 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 /etc/default/grub for eg: "GRUB_CMDLINE_LINUX_DEFAULT="quiet splash vga=792" will give you a 1024x768 resolution.

You can choose one of these resolutions: 640×480, 800×600, 1024×768, 1280×1024, 1600×1200

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 unicode.pf2, or, as only ascii characters under the name ascii.pf2.

Image formats supported include tga, png and jpeg, providing the correct modules are loaded. The maximum supported resolution depends on your hardware.

Make sure you have set up the proper framebuffer resolution.

Edit /etc/default/grub like this:

GRUB_BACKGROUND="/boot/grub/archlinux.tga"
#GRUB_THEME="/path/to/gfxtheme"

(archlinux.tga is a placeholder; put your file name there)

To generate the changes and add the information into grub.cfg, run:

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

If adding the splash image was successful, the user will see "Found background image..." in the terminal as the command is executed. If this phrase is not seen, the image information was probably not incorporated into the grub.cfg file.

If the image is not displayed, check:

• The path and the filename in /etc/default/grub are correct.
• The image is of the proper size and format (tga, png, 8-bit jpg).
• The image was saved in the RGB mode, and is not indexed.
• The console mode is not enabled in /etc/default/grub.
• The command grub-mkconfig must be executed to place the background image information into the /boot/grub/grub.cfg file.

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 /etc/default/grub:

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 /etc/default/grub 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

Other Options

LVM

If you use LVM for your /boot, 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. /dev/md0 becomes:

set root=(md0)

whereas a partitioned raid-volume (e.g. /dev/md0p1) becomes:

set root=(md0,1)

Persistent block device naming

You can use UUIDs to detect partitions instead of the "old" /dev/sd* and /dev/hd* 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 grub.cfg 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 false 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 --label option to search. 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 /etc/default/grub and change the setting of GRUB_DEFAULT:

GRUB_DEFAULT=saved

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

GRUB_SAVEDEFAULT=true

Note that manually added menu items, eg Windows in /etc/grub.d/40_custom, will need savedefault added. 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 grub-mkpasswd_pbkdf2. Enter a password and confirm it. The output will look like this:

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

cat << EOF

set superusers="username"
password_pbkdf2 username <password>

EOF

where <password> is the string generated by grub-mkpasswd_pbkdf2.

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

Booting an Ubuntu ISO Image from the GRUB2 Screen

Edit /etc/grub.d/40_custom to add an entry pointing to an Ubuntu iso:

menuentry "ubuntu-11.04-desktop-amd64.iso" {
set isofile="/path/to/ubuntu-11.04-desktop-amd64.iso"
loopback loop (hdX,Y)$isofile
linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=$isofile quiet noeject noprompt splash --
initrd (loop)/casper/initrd.lz
}

Be sure to adjust the "hdX,Y" in the third line to point to the correct disk/partition number of the isofile. When finished, update grub2's configuration file via:

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

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 /boot/grub, 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 /boot 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

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 grub.cfg as needed and then run:

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

to reinstall GRUB2 and fix the problem completely, changing /dev/sda 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.

Enable GRUB2 debug messages

Add

set pager=1
set debug=all

to grub.cfg.

Correct GRUB2 No Suitable Mode Found Error

If you get this error when booting any menuentry

error: no suitable mode found
Booting however

Then you need to initialize grub2 graphical terminal (gfxterm) with proper video mode (gfxmode) in grub2. This video mode is passed by grub2 to the linux kernel via 'gfxpayload'. In case of UEFI systems, if the grub2 video mode is not initialized, no kernel boot messages will be shown in the terminal (atleast until KMS kicks in)

Copy /usr/share/grub/unicode.pf2 to ${GRUB2_PREFIX_DIR} (/boot/grub/ in case of BIOS systems and /boot/efi/efi/grub/ in case of UEFI systems).

# cp /usr/share/grub/unicode.pf2 ${GRUB2_PREFIX_DIR}

If /usr/share/grub/unicode.pf2 does not exist, install bdf-unifont, create the unifont.pf2 file and then copy it to ${GRUB2_PREFIX_DIR} .

# grub-mkfont -o unicode.pf2 /usr/share/fonts/misc/unifont.bdf

Then, in the grub.cfg 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:

BIOS systems

insmod vbe

UEFI systems

insmod efi_gop
insmod efi_uga

After that add the following code (common to both BIOS and UEFI)

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, unicode.pf2 font file should exist in ${GRUB2_PREFIX_DIR}.

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

UEFI GRUB2 drops to shell

If grub loads but drop you into the rescue shell with no errors, it may be because of a missing or misplaced grub.cfg. For UEFI, grub.cfg must be copied into the UEFI system partition alongside grub.efi (/boot/efi/EFI/grub)

UEFI GRUB2 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

1. Official GRUB2 Manual - http://www.gnu.org/software/grub/manual/grub.html
2. Ubuntu wiki page for Grub2 - https://help.ubuntu.com/community/Grub2
3. GRUB2 wiki page describing steps to compile for UEFI systems - https://help.ubuntu.com/community/UEFIBooting
4. Wikipedia's page on BIOS Boot Partition

External Links

1. A Linux Bash Shell script to compile and install GRUB2 for BIOS from BZR Source
2. A Linux Bash Shell script to compile and install GRUB2 for UEFI from BZR Source