GRUB2 - ParabolaWiki

# GRUB2

Summary
Covers various aspects of the next generation of the GRand Unified Bootloader (GRUB2).
Overview
In order to boot Parabola, a Linux-capable boot loader such as GRUB, GRUB2, LILO, or Syslinux must be installed to the Master Boot Record, or the GUID Partition Table. The boot loader is responsible for loading the kernel and initial ramdisk before initiating the boot process.
Related
Burg - 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 GNU/Linux, etc. It also has a highly configurable menu system which works in both text and graphic mode.
Resources
GNU GRUB -- GNU Project

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. GRUB2 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.

Note: From 1.99-6 onwards, GRUB2 supports Btrfs as root (without a separate /boot filesystem) compressed with either zlib or LZO.

## Preface

Although, GRUB (i.e. version 0.9x) is the standard bootloader of Parabola, it is considered 'legacy' by upstream. It is being replaced by GRUB2 and Syslinux in many distributions. Upstream recommends GRUB2 >=1.99 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.

Note: This partition should be created before grub-install or grub-setup is run or before the Install Bootloader step of the Parabola installer (if GRUB2 BIOS 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 ([2]). 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).

Note: Create the 2MiB partition mentioned above BEFORE you convert to GPT. If you do not, gparted will not resize your boot partition to allow its creation, and when you reboot GRUB2 will not know where to look.

#### 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. 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 Parabola 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 /mnt/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.
- Check and see if the dm_mod module is loaded. If it is not, load it manually (you might need grub2-bios):

    # lsmod | grep dm_mod
    # 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 Parabola "net" installations.

- 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 Parabola (Note that the dm-mod module has already been loaded, no need to do that again).

### From a running Parabola

#### BIOS systems

##### Backup Important Data

Though a GRUB2 installation should run smoothly, it is strongly recommended to keep the GRUB-legacy files before installing grub2-bios.

    mv /boot/grub /boot/grub-legacy

Backup the MBR which contains the boot code and partition table (Replace /dev/sdX with your actual disk path)

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

To backup only the MBR boot code use:

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

If unable to install GRUB2 correctly see GRUB2#Restore_GRUB_Legacy.

##### Install grub2-bios package

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

    # pacman -S grub2-bios

Note: Simply installing the package won't update the /boot/grub/i386-pc/core.img file and the GRUB2 modules in /boot/grub/i386-pc. You need to update them manually using grub-install as explained below.

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/i386-pc/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/i386-pc/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-install --directory=/usr/lib/grub/i386-pc --target=i386-pc --boot-directory=/boot --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-install on many systems (it will also prevent the issue below from occurring). Otherwise you get an error that looks 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.

Note: --no-floppy has been removed from grub-install in 2.00~beta2 upstream release, and replaced with --allow-floppy.
Warning: Make sure to check the /boot directory if you use the latter. Sometimes the boot-directory parameter creates another /boot folder inside of /boot. A wrong install would look like: /boot/boot/grub/.

###### Install to Partition or Partitionless Disk

Note: grub2-bios (any version - including upstream Bazaar repo) does not encourage installation to a partition boot sector or a partitionless disk like GRUB Legacy or syslinux does. This kind of setup is prone to breakage, especially during updates, and is not supported by Parabola devs.

To set up 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/i386-pc/core.img
    # grub-install --directory=/usr/lib/grub/i386-pc --target=i386-pc --boot-directory=/boot --recheck --force --debug /dev/sda1
    # chattr +i /boot/grub/i386-pc/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-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/i386-pc/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/i386-pc/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/i386-pc/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/i386-pc/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-install:

    # grub-install --directory=/usr/lib/grub/i386-pc --target=i386-pc --grub-setup=/bin/true --boot-directory=/boot --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

Note: The file path is /boot/grub/grub.cfg, NOT /boot/grub/i386-pc/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  Parabola GNU/Linux-libre Stock Kernel
    root   (hd0,0)
    kernel /vmlinuz-linux-libre root=/dev/sda2 ro
    initrd /initramfs-linux-libre.img

    title  Parabola GNU/Linux-libre Stock Kernel Fallback
    root   (hd0,0)
    kernel /vmlinuz-linux-libre root=/dev/sda2 ro
    initrd /initramfs-linux-libre-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 'Parabola GNU/Linux-libre Stock Kernel' {
      set root='(hd0,1)'; set legacy_hdbias='0'
      legacy_kernel   '/vmlinuz-linux-libre' '/vmlinuz-linux-libre' 'root=/dev/sda2' 'ro'
      legacy_initrd '/initramfs-linux-libre.img' '/initramfs-linux-libre.img'

    }

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

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

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

Boot into Parabola and re-create the proper GRUB2 /boot/grub/grub.cfg config file.

Note: This option works only in BIOS systems, not in UEFI systems.

#### UEFI systems

##### Install grub2-uefi package

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. This kind of firmware does not fall under any one UEFI Specification version and is therefore not a standard UEFI firmware.

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 do this manually using grub-install as explained below.

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

Note: The below commands assume you are using grub2-efi-x86_64 (for grub2-efi-i386 replace x86_64 with i386 in the below commands).

The UEFI system partition will need to be mounted at /boot/efi/ for the GRUB2 install script to detect it:

    # mkdir -p /boot/efi
    # mount -t vfat /dev/sdXY /boot/efi

Install GRUB UEFI application to /boot/efi/efi/libre and its modules to /boot/grub/x86_64-efi (recommended) using:

    # grub-install --directory=/usr/lib/grub/x86_64-efi --target=x86_64-efi --efi-directory=/boot/efi \
     --bootloader-id=libre --boot-directory=/boot --recheck --debug

If you want to install grub2 modules and grub.cfg at the directory /boot/efi/efi/grub and the grubx64.efi application at /boot/efi/efi/libre use:

    # grub-install --directory=/usr/lib/grub/x86_64-efi --target=x86_64-efi --efi-directory=/boot/efi \
     --bootloader-id=libre --boot-directory=/boot/efi/efi --recheck --debug

In this case grub2-efi-x86_64 will be installed into /boot/grub, making the behavior consistent with the BIOS verion of GRUB2, but this is not recommended if you use both grub2-bios and grub2-efi-x86_64 in your system, as this will overwrite grub2-bios modules in /boot/grub.

The --efi-directory option mentions the mountpoint of UEFI SYSTEM PARTITION , --bootloader-id mentions the name of the directory used to store the grubx64.efi file and --boot-directory mentions the directory wherein the actual modules will be installed (and into which grub.cfg should be created).

The actual paths are:

    In --efi-directory=/boot/efi --boot-directory=/boot/efi/efi --bootloader-id=grub:

    <boot-directory>/grub/x86_64-efi/<all modules, grub.efi, core.efi, grub.cfg>

Note: the --bootloader-id option does not change <boot-directory>/grub, i.e. you cannot install the modules to <boot-directory>/<bootloader-id>, the path is hard-coded to be <boot-directory>/grub.

In --root-directory=/boot/efi --boot-directory=/boot/efi/efi --bootloader-id=grub:

    <efi-directory>/<efi or EFI>/<bootloader-id> == <boot-directory>/grub == /boot/efi/efi/grub

In --efi-directory=/boot/efi --boot-directory=/boot/efi/efi --bootloader-id=libre:

    <efi-directory>/<efi or EFI>/<bootloader-id> == /boot/efi/efi/libre
    <boot-directory>/grub == /boot/efi/efi/grub

In --efi-directory=/boot/efi --boot-directory=/boot --bootloader-id=libre:

    <efi-directory>/<efi or EFI>/<bootloader-id> == /boot/efi/efi/libre
    <boot-directory>/grub == /boot/grub

In --root-directory=/boot/efi --boot-directory=/boot --bootloader-id=grub:

    <efi-directory>/<efi or EFI>/<bootloader-id> == /boot/efi/efi/grub
    <boot-directory>/grub == /boot/grub

The <efi-directory>/<efi or EFI>/<bootloader-id>/grubx64.efi is an exact copy of <boot-directory>/grub/x86_64-efi/core.efi.

Note: In GRUB2 2.00~beta4, the grub-install option --efi-directory replaces --root-directory and the latter is deprecated.
Note: The options --efi-directory and --bootloader-id are specific to GRUB2 UEFI.

In all the cases the UEFI SYSTEM PARTITION should be mounted for grub-install to install grubx64.efi in it, which will be launched by the firmware (using the efibootmgr created boot entry in non-Mac systems).

If you notice carefully, there is no <device_path> option (Eg: /dev/sda) at the end of the grub-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 #Create_GRUB2_entry_in_the_Firmware_Boot_Manager.

##### Create GRUB2 entry in the Firmware Boot Manager

###### Non-Mac UEFI systems

grub-install will ensure that /boot/efi/efi/libre/grubx64.efi is launched by default if it detects efibootmgr and if it is able to access UEFI Runtime Services. Follow Unified_Extensible_Firmware_Interface#efibootmgr for more info.

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

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

or

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

or

    # cp /boot/efi/efi/libre/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).

##### 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-directory>/grub/grub.cfg

Note: The file path is <boot-directory>/grub/grub.cfg, NOT <boot-directory>/grub/x86_64-efi/grub.cfg.

If you used --boot-directory=/boot:

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

If you used --boot-directory=/boot/efi/efi:

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

This is independent of the value of --bootloader-id option.

If GRUB2 complains about "no suitable mode found" while booting, try #Correct_GRUB2_No_Suitable_Mode_Found_Error.

##### Create GRUB2 Standalone UEFI Application

It is possible to create a grubx64_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. This is done using the grub-mkstandalone command which is included in grub2-common >= 1:1.99-6 package.

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

    # grub-mkstandalone --directory="/usr/lib/grub/x86_64-efi/" --format="x86_64-efi" --compression="xz" \
    --output="/boot/efi/efi/libre/grubx64_standalone.efi" <any extra files you want to include>

The grubx64_standalone.efi file expects grub.cfg to be within its $prefix which is (memdisk)/boot/grub. The memdisk is embedded within the efi app. The grub-mkstandlone script allow passing files to be included in the memdisk image to be as the arguments to the script (in <any extra files you want to include>).

If you have the grub.cfg at /home/user/Desktop/grub.cfg, then create a temporary /home/user/Desktop/boot/grub/ directory, copy the /home/user/Desktop/grub.cfg to /home/user/Desktop/boot/grub/grub.cfg, cd into /home/user/Desktop/boot/grub/ and run:

    # grub-mkstandalone --directory="/usr/lib/grub/x86_64-efi/" --format="x86_64-efi" --compression="xz" \
    --output="/boot/efi/efi/libre/grubx64_standalone.efi" "boot/grub/grub.cfg"

The reason to cd into /home/user/Desktop/boot/grub/ and to pass the file path as boot/grub/grub.cfg (notice the lack of a leading slash - boot/ vs /boot/ ) is because dir1/dir2/file is included as (memdisk)/dir1/dir2/file by the grub-mkstandalone script.

If you pass /home/user/Desktop/grub.cfg the file will be included as (memdisk)/home/user/Desktop/grub.cfg. If you pass /home/user/Desktop/boot/grub/grub.cfg the file will be included as (memdisk)/home/user/Desktop/boot/grub/grub.cfg. That is the reason for cd'ing into /home/user/Desktop/boot/grub/ and passing boot/grub/grub.cfg, to includ the file as (memdisk)/boot/grub/grub.cfg, which is what grub.efi expects the file to be.

You need to create an UEFI Boot Manager entry for /boot/efi/efi/libre/grubx64_standalone.efi using efibootmgr. Follow #Create GRUB2 entry in the Firmware Boot Manager.

## Configuration

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

Note: If GRUB2 was installed with the --boot-directory option set, the grub.cfg file must be placed in the same directory as grubx64.efi. Otherwise, the grub.cfg file goes in /boot/grub/, just like in the BIOS version of GRUB2.
Note: Here is a quite complete description of how to configure GRUB2: http://members.iinet.net/~herman546/p20/GRUB2%20Configuration%20File%20Commands.html

### Automatically generating using grub-mkconfig (Recommended)

The GRUB2 menu.lst 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 Parabola 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

#### Additional arguments

To pass custom additional arguments to the linux image, you can set the GRUB_CMDLINE_LINUX variable in /etc/default/grub. This is analogous to adding commands to the kernel line in legacy GRUB.

For example, use GRUB_CMDLINE_LINUX="resume=/dev/sdaX" where sdaX is your swap partition to enable resume after hibernation.

You can also use GRUB_CMDLINE_LINUX="resume=/dev/disk/by-uuid/${swap_uuid}", where ${swap_uuid} is the uuid of your swap partition.

### Manually creating grub.cfg

A basic GRUB config 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) Parabola GNU/Linux-libre
    menuentry "Parabola GNU/Linux-libre" {
        set root=(hd0,1)
        linux /vmlinuz-linux-libre root=/dev/sda3 ro
        initrd /initramfs-linux-libre.img
    }

    === Dual-booting ===

    {{Note|If you want GRUB2 to automatically search for other systems, you may wish to install {{Pkg|os-prober}}.}}

    ==== Using grub-mkconfig ====
    The best way to add other entries is editing the {{ic|/etc/grub.d/40_custom}}. The entries in this file will be automatically added when running {{ic|grub-mkconfig}}.
    After adding the new lines, run:
     # grub-mkconfig -o /boot/grub/grub.cfg
    to generate an updated {{ic|grub.cfg}}.

    ===== With GNU/Linux =====

    Assuming that the other distro is on partition {{ic|sda2}}:

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

    ===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 {{ic|1=vga=}} way is deprecated. The preferred method is editing {{ic|/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 {{ic|gfxpayload}} property will make sure the kernel keeps the resolution.

    {{Note|If this example does not work for you try to replace {{ic|1=gfxmode="1024x768x32"}} by {{ic|1=vbemode="0x105"}}. Remember to replace the specified resolution with one suitable for your screen.}}
    {{Note|To show all the modes you can use {{ic|1=# hwinfo --framebuffer}} (hwinfo is available in [community]), while at GRUB2 prompt you can use the {{ic|1=vbeinfo}} command.}}

    If this method does not work for you, the deprecated {{ic|1=vga=}} method will still work. Just
    add it next to the {{ic|1="GRUB_CMDLINE_LINUX_DEFAULT="}} line in {{ic|/etc/default/grub}}
    for eg: {{ic|1="GRUB_CMDLINE_LINUX_DEFAULT="quiet splash vga=792"}} will give you a {{ic|1024x768}} resolution.

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

    ====915resolution hack ====

    Some times for Intel graphic adapters neither {{ic|1=# hwinfo --framebuffer}} nor {{ic|1=vbeinfo}} will show you the desired resolution. In this case you can use {{ic|915resolution}} hack. This hack will temporarily modify video BIOS and add needed resolution. See [http://915resolution.mango-lang.org/ 915resolution's home page]

    In the following I will proceed with the example for my system. Please adjust the recipe for your needs. First you need to find a video mode which will be modified later. For that, run {{ic|915resolution}} in GRUB2 command shell:
     915resolution -l
    The output will be something like:
     Intel 800/900 Series VBIOS Hack : version 0.5.3
     ...
     Mode 30 : 640x480, 8 bits/pixel
     ...
    Next, our purpose is to overwrite mode 30. (You can choose what ever mode you want.) In the file {{ic|/etc/grub.d/00_header}} just before the {{ic|set gfxmode=${GRUB_GFXMODE}}} line insert:
     915resolution 30 1440 900
    Here we are overwriting the mode {{ic|30}} with {{ic|1440x900}} resolution. Lastly we need to set {{ic|GRUB_GFXMODE}} as described earlier, regenerate GRUB2 configuration file and reboot to test changes:
     # grub-mkconfig -o /boot/grub/grub.cfg
     # reboot

    ====Background image and bitmap fonts====

    GRUB2 comes with support for background images and bitmap fonts in {{ic|pf2}} format. The unifont font is included in the {{Pkg|grub2-common}} package under the filename {{ic|unicode.pf2}}, or, as only ASCII characters under the name {{ic|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 [https://wiki.parabolagnulinux.org/index.php/GRUB2#Setting_the_framebuffer_resolution framebuffer resolution].

    Edit {{ic|/etc/default/grub}} like this:
     GRUB_BACKGROUND="/boot/grub/parabola.tga"
     #GRUB_THEME="/path/to/gfxtheme"

    ({{ic|parabola.tga}} is a placeholder; put your file name there)

    {{Note|If you have installed GRUB on a separate partition, {{ic|/boot/grub/parabola.tga}} becomes {{ic|/grub/parabola.tga}}.}}

    To generate the changes and add the information into {{ic|grub.cfg}}, run:
     grub-mkconfig -o /boot/grub/grub.cfg

    If adding the splash image was successful, the user will see {{ic|"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 {{ic|grub.cfg}} file.

    If the image is not displayed, check:
    * The path and the filename in {{ic|/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 {{ic|/etc/default/grub}}.
    * The command {{ic|grub-mkconfig}} must be executed to place the background image information into the {{ic|/boot/grub/grub.cfg}} file.

    ====Theme====

    Here is an example for configuring Starfield theme which was included in GRUB2 package.

    Edit {{ic|/etc/default/grub}}
     GRUB_THEME="/boot/grub/themes/starfield/theme.txt"

    Generate the changes:
     grub-mkconfig -o /boot/grub/grub.cfg

    If configuring the theme was successful, you'll see {{ic|Found theme: /boot/grub/themes/starfield/theme.txt}} in the terminal.

    ====Menu colors====

    As in GRUB Legacy (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 {{ic|/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 {{keypress|Shift}} when needed. You can also adjust whether you want to see the timeout counter.

    Edit {{ic|/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 {{ic|/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) Parabola GNU/Linux-libre
     menuentry "Parabola GNU/Linux-libre" {
     insmod lvm
     set root=(VolumeGroup-lv_boot)
     # you can only set following two lines
     linux /vmlinuz-linux-libre root=/dev/mapper/VolumeGroup-root ro
     initrd /initramfs-linux-libre.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. {{ic|/dev/md0}} becomes:
     set root=(md0)

    whereas a partitioned raid-volume (e.g. {{ic|/dev/md0p1}}) becomes:
     set root=(md0,1)

    ==== Persistent block device naming ====
    You can use UUIDs to detect partitions instead of the "old" {{ic|/dev/sd*}} and {{ic|/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 {{ic|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 {{ic|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 {{ic|--label}} option to {{ic|search}}. First of all, label your existing partition:
     # tune2fs -L a <LABEL> <PARTITION>

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

     menuentry "Parabola GNU/Linux-libre, session texte" {
         search --label --no-floppy --set=root libreroot
         linux /boot/vmlinuz-linux-libre root=/dev/disk/by-label/libreroot ro
         initrd /boot/initramfs-linux-libre.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 LIBRE one and the LTS kernel as a fallback option) or operating systems. To do this, edit {{ic|/etc/default/grub}} and change the setting of {{ic|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 {{ic|/etc/default/grub}}:

     GRUB_SAVEDEFAULT=true

    {{Note|Manually added menu items, in {{ic|/etc/grub.d/40_custom}}, will need {{ic|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 {{ic|grub-mkpasswd_pbkdf2}}. Enter a password and confirm it. The output will look like this:

    {{bc|<nowiki>
    Your PBKDF2 is grub.pbkdf2.sha512.10000.C8ABD3E93C4DFC83138B0C7A3D719BC650E6234310DA069E6FDB0DD4156313DA3D0D9BFFC2846C21D5A2DDA515114CF6378F8A064C94198D0618E70D23717E82.509BFA8A4217EAD0B33C87432524C0B6B64B34FBAD22D3E6E6874D9B101996C5F98AB1746FE7C7199147ECF4ABD8661C222EEEDB7D14A843261FFF2C07B1269A

Then, add the following to /etc/grub.d/00_header:

    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.

#### Root Encryption

To let GRUB2 automatically add the kernel parameters for root encryption, add cryptdevice=/dev/yourdevice:label to GRUB_CMDLINE_LINUX in /etc/defaults/grub.

Example with root mapped to /dev/mapper/root:

    GRUB_CMDLINE_LINUX="cryptdevice=/dev/sda2:root"

Also, disable the usage of UUIDs for the rootfs:

    GRUB_DISABLE_LINUX_UUID=true

Regenerate the configuration.

### Booting an ISO Directly From GRUB2

Edit /etc/grub.d/40_custom to add an entry for the target ISO. When finished, update the GRUB menu as with the usual grub-mkconfig -o /boot/grub/grub.cfg (as root).

#### Parabola ISO

Note: Be sure to adjust the "hdX,Y" in the third line to point to the correct disk/partition number of the isofile. Also adjust the img_dev line to match this same location.

i686:

    menuentry "Parabola ISO" --class iso {
        set isofile="/parabola-2012.10.17-dual.iso"
        loopback loop (hd0,5)$isofile
        linux (loop)/libre/boot/i686/vmlinuz archisobasedir=libre archisolabel=LIBRE_201210 img_dev=/dev/sda5 img_loop=$isofile earlymodules=loop
        initrd (loop)/libre/boot/i686/libreiso.img
    }

x86_64:

    menuentry "Parabola ISO" --class iso {
        set isofile="/parabola-2012.10.17-dual.iso"
        loopback loop (hd0,5)$isofile
        linux (loop)/libre/boot/x86_64/vmlinuz archisobasedir=libre archisolabel=LIBRE_201210 img_dev=/dev/sda5 img_loop=$isofile earlymodules=loop
        initrd (loop)/libre/boot/x86_64/libreiso.img
    }

#### Trisquel ISO

Note: Be sure to adjust the "hdX,Y" in the third line to point to the correct disk/partition number of the isofile.

    menuentry "trisquel_5.5_amd64.iso" {
        set isofile="/path/to/trisquel_5.5_amd64.iso"
        loopback loop (hdX,Y)$isofile
        linux (loop)/casper/vmlinuz-libre boot=casper iso-scan/filename=$isofile quiet noeject noprompt splash --
        initrd (loop)/casper/initrd.lz
    }

## 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

## 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

Note: With a separate boot partition, omit /boot from the path, (i.e. type set prefix=(hdX,Y)/grub and insmod (hdX,Y)/grub/linux.mod).

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

An example, booting Parabola GNU/Linux-libre:

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

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

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

After successfully booting the Parabola installation, users can correct grub.cfg as needed and then reinstall GRUB2.

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 "Parabola GNU/Linux-libre 64" {
         # Set the UUIDs for your boot and root partition respectively
         set the_boot_uuid=ece0448f-bb08-486d-9864-ac3271bd8d07
         set the_root_uuid=c55da16f-e2af-4603-9e0b-03f5f565ec4a

         # (Note: This may be the same as your boot partition)

         # Get the boot/root devices and set them in the root and grub_boot variables
         search --fs-uuid --no-floppy --set=root $the_root_uuid
         search --fs-uuid --no-floppy --set=grub_boot $the_boot_uuid

         # Check to see if boot and root are equal.
         # If they are, then append /boot to $grub_boot (Since $grub_boot is actually the root partition)
         if [ $the_boot_uuid == $the_root_uuid] ; then
             set grub_boot=$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-libre root=/dev/disk/by-uuid/$uuid_os_root ro
         initrd ($grub_boot)/initramfs-linux-libre.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 and UEFI systems). If GRUB2 UEFI was installed with --boot-directory=/boot/efi/efi set, then the directory is /boot/efi/efi/grub/:

    # 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}/fonts/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}.

### 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. This will happen if GRUB2 UEFI was installed with --boot-directory and grub.cfg is missing OR if the partition number of the boot partition changed (which is hard-coded into the grubx64.efi file).

### 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)

### Restore GRUB Legacy

- Move GRUB2 files out of the way:

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

- Copy GRUB Legacy 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

Note: This command also restores the partition table so be careful.

To restore only the MBR boot code use:

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

## References

1. Official GRUB2 Manual - http://www.gnu.org/software/grub/manual/grub.html
2. 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

## Acknowledgement

This wiki article is based on ArchWiki. We may have removed non-FSDG bits from it.
Category:

- Boot loaders