GRUB

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GRUB — not to be confused with GRUB Legacy — is the next generation of the GRand Unified Bootloader. GRUB is derived from PUPA which was a research project to develop the next generation of what is now GRUB Legacy. GRUB has been rewritten from scratch to clean up everything and provide modularity and portability [1].

Contents

1 Preface

  • A 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.
  • The name GRUB officially refers to version 2 of the software, see [2]. If you are looking for the article on the legacy version, see GRUB Legacy.
  • GRUB supports Btrfs as root (without a separate /boot file system) compressed with either zlib or LZO
    • GRUB currently (Sep 2015) supports booting from Btrfs RAID 0/1/10, but not RAID 5/6. You may use mdadm for RAID 5/6, which is supported by GRUB.
  • GRUB does not support F2FS as root so you will need a separate /boot with a supported file system.

2 UEFI and GPT/BIOS systems

(GPT for GUID Partition Table)

Note:
  • It is recommended to read and understand the UEFI, GPT and UEFI Bootloaders pages.
  • When installing to use UEFI it is important to start the install with your machine in UEFI mode. The Parabola install media must be UEFI bootable.

2.1 GPT/BIOS specific instructions

On a GPT/BIOS configuration a BIOS boot partition is required. GRUB embeds its core.img into this partition.

Note:
  • Before attempting this method keep in mind that not all systems will be able to support this partitioning scheme, read more on GUID partition tables.
  • This additional partition is only needed on a GRUB, GPT/BIOS partitioning scheme. Previously, for a GRUB, MBR partitioning scheme, GRUB used the Post-MBR gap for the embedding the core.img). GRUB for UEFI and GPT/BIOS, however, does not use the Post-GPT gap to conform to GPT specifications that require 1_megabyte/2048_sector disk boundaries.
  • For UEFI and GPT/BIOS systems this extra partition is not required as no embedding of boot sectors is taken place in that case.

Create a mebibyte partition (+1M with fdisk or gdisk) on the disk with no file system and type BIOS boot (BIOS boot in fdisk, ef02 in gdisk, bios_grub in parted). This partition can be in any position order but has to be on the first 2 TiB of the disk. This partition needs to be created before GRUB installation. When the partition is ready, install the bootloader as per the instructions below.

The post-GPT gap can also be used as the BIOS boot partition though it will be out of GPT alignment specification. Since the partition will not be regularly accessed performance issues can be disregarded (though some disk utilities will display a warning about it). In fdisk or gdisk create a new partition starting at sector 34 and spanning to 2047 and set the type. To have the viewable partitions begin at the base consider adding this partition last.

2.2 Check if you have GPT and an ESP

An EFI System Partition (ESP) is needed on every disc you want to boot using EFI. GPT is not strictly necessary, but it is highly recommended and is the only method currently supported in this article. If you are installing Parabola on an EFI-capable computer with an already-working operating system, it is very likely that you already have an ESP. To check for GPT and for an ESP, use parted as root to print the partition table of the disk you want to boot from. (We are calling it /dev/sda.) 

# parted /dev/sda print

For GPT, you are looking for "Partition Table: GPT". For EFI, you are looking for a small (512 MiB or less) partition with a vfat file system and the boot flag enabled. On it, there should be a directory named "EFI". If these criteria are met, this is your ESP. Make note of the partition number. You will need to know which one it is, so you can mount it later on while installing GRUB to it.

2.3 Create an ESP

If you do not have an ESP, you will need to create one. See UEFI#EFI System Partition

2.4 Installation

Note: UEFI firmware are not implemented consistently by hardware manufacturers. The installation examples provided are intended to work on the widest range of UEFI systems possible. Those experiencing problems despite applying these methods are encouraged to share detailed information for their hardware-specific cases, especially where solving these problems. A GRUB/EFI examples article has been provided for such cases.

This section assumes you are installing GRUB for x86_64 systems (x86_64-efi). For i686 systems, replace x86_64-efi with i386-efi where appropriate. For armv7h systems, replace x86_64-efi with arm-efi where appropriate.

Make sure you are in a bash shell. For example, when booting from the Parabola ISO: 

# arch-chroot /mnt /bin/bash

Install the packages grub and efibootmgr. GRUB is the bootloader, efibootmgr creates bootable .efi stub entries used by the GRUB installation script.

The following steps install the GRUB UEFI application to $esp/EFI/grub, install its modules to /boot/grub/x86_64-efi, and place the bootable grubx64.efi stub in $esp/EFI/grub.

First, tell GRUB to use UEFI, set the boot directory and set the bootloader ID. Change $esp to your efi partition (usually /boot): 

# grub-install --target=x86_64-efi --efi-directory=$esp --bootloader-id=grub --recheck

The --bootloader-id is what appears in the boot options to identity the GRUB EFI boot option; make sure this is something you will recognize later. The install will create a directory of the same name under $esp/EFI/ where the EFI binary bootloader will be placed.

After the above install finished the main GRUB directory is located at /boot/grub/.

Remember to #Generate the main configuration file after finalizing further setup dependant #Configuration.

Note:
  • While some distributions require a /boot/efi or /boot/EFI directory, Parabola does not.
  • --efi-directory and --bootloader-id are specific to GRUB UEFI. --efi-directory specifies the mountpoint of the ESP. It replaces --root-directory, which is deprecated.
  • You might note the absence of a <device_path> option (e.g.: /dev/sda) in the grub-install command. In fact any <device_path> provided will be ignored by the GRUB install script, as UEFI bootloaders do not use a MBR or partition boot sector at all.

See UEFI troubleshooting in case of problems.

2.5 Further reading

Below is other relevant information regarding installing Parabola via UEFI

2.5.1 Alternative install method

Usually, GRUB keeps all files, including configuration files, in /boot, regardless of where the EFI System Partition is mounted.

If you want to keep these files inside the EFI System Partition itself, add --boot-directory=$esp to the grub-install command: 

# grub-install --target=x86_64-efi --efi-directory=$esp --bootloader-id=grub --boot-directory=$esp --recheck --debug

This puts all GRUB files in $esp/grub, instead of in /boot/grub. When using this method, make sure you have grub-mkconfig put the configuration file in same place: 

# grub-mkconfig -o $esp/grub/grub.cfg

Configuration is otherwise the same.

2.5.2 UEFI firmware workaround

Some UEFI firmware requires that the bootable .efi stub have a specific name and be placed in a specific location: $esp/EFI/boot/bootx64.efi (where $esp is the UEFI partition mountpoint). Failure to do so in such instances will result in an unbootable installation. Fortunately, this will not cause any problems with other firmware that does not require this.

To do so, first create the necessary directory, and then copy across the grub .efi stub, renaming it in the process: 

# mkdir $esp/EFI/boot
# cp $esp/EFI/grub_uefi/grubx64.efi  $esp/EFI/boot/bootx64.efi

2.5.3 Create a GRUB entry in the firmware boot manager

grub-install automatically tries to create a menu entry in the boot manager. If it does not, then see UEFI#efibootmgr for instructions to use efibootmgr to create a menu entry. However, the problem is likely to be that you have not booted your CD/USB in UEFI mode, as in UEFI#Create UEFI bootable USB from ISO.

2.5.4 GRUB standalone

This section assumes you are creating a standalone GRUB for x86_64 systems (x86_64-efi). For i686 systems, replace x86_64-efi with i386-efi where appropriate.

It is possible to create a grubx64_standalone.efi application which has all the modules embedded in a tar archive within the UEFI application, thus removing the need for having a separate directory populated with all of the GRUB UEFI modules and other related files. This is done using the grub-mkstandalone command (included in grub) as follows: 

# echo 'configfile ${cmdpath}/grub.cfg' > /tmp/grub.cfg
# grub-mkstandalone -d /usr/lib/grub/x86_64-efi/ -O x86_64-efi --modules="part_gpt part_msdos" --fonts="unicode" --locales="en@quot" --themes="" -o "$esp/EFI/grub/grubx64_standalone.efi"  "boot/grub/grub.cfg=/tmp/grub.cfg" -v

Then copy the GRUB config file to $esp/EFI/grub/grub.cfg and create a UEFI Boot Manager entry for $esp/EFI/grub/grubx64_standalone.efi using efibootmgr.

Note:

The option --modules="part_gpt part_msdos" (with the quotes) is necessary for the ${cmdpath} feature to work properly.

Warning: You may find that the grub.cfg file is not loaded due to ${cmdpath} missing a slash (i.e. (hd1,msdos2)EFI/Boot instead of (hd1,msdos2)/EFI/Boot) and so you are dropped into a GRUB shell. If this happens determine what ${cmdpath} is set to (echo ${cmdpath} ) and then load the config file manually (e.g. configfile (hd1,msdos2)/EFI/Boot/grub.cfg).

2.5.5 Technical information

The GRUB EFI file always expects its config file to be at ${prefix}/grub.cfg. However in the standalone GRUB EFI file, the ${prefix} is located inside a tar archive and embedded inside the standalone GRUB EFI file itself (inside the GRUB environment, it is denoted by "(memdisk)", without quotes). This tar archive contains all the files that would be stored normally at /boot/grub in case of a normal GRUB EFI install.

Due to this embedding of /boot/grub contents inside the standalone image itself, it does not rely on actual (external) /boot/grub for anything. Thus in case of standalone GRUB EFI file ${prefix}==(memdisk)/boot/grub and the standalone GRUB EFI file reads expects the config file to be at ${prefix}/grub.cfg==(memdisk)/boot/grub/grub.cfg.

Hence to make sure the standalone GRUB EFI file reads the external grub.cfg located in the same directory as the EFI file (inside the GRUB environment, it is denoted by ${cmdpath} ), we create a simple /tmp/grub.cfg which instructs GRUB to use ${cmdpath}/grub.cfg as its config (configfile ${cmdpath}/grub.cfg command in (memdisk)/boot/grub/grub.cfg). We then instruct grub-mkstandalone to copy this /tmp/grub.cfg file to ${prefix}/grub.cfg (which is actually (memdisk)/boot/grub/grub.cfg) using the option "boot/grub/grub.cfg=/tmp/grub.cfg".

This way, the standalone GRUB EFI file and actual grub.cfg can be stored in any directory inside the EFI System Partition (as long as they are in the same directory), thus making them portable.

3 MBR systems

Usually the post-MBR gap (after the 512 byte MBR region and before the start of the first partition) in many MBR (or 'msdos' disklabel) partitioned systems is 31 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 GRUB's core.img (Issue#24103). It is advisable to use a partitioning tool that supports 1 MiB partition alignment to obtain this space as well as to satisfy other non-512 byte sector issues (which are unrelated to embedding of core.img).

3.1 Installation

Install the grub package. It will replace grub-legacy, where already installed.

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

3.1.1 Install boot files

There are 4 ways to install GRUB boot files in BIOS booting:

3.1.1.1 Install to disk
Note: The method is specific to installing GRUB to a partitioned (MBR or GPT) disk, with GRUB files installed to /boot/grub and its first stage code installed to the 440-byte MBR boot code region (not to be confused with MBR partition table).

The following commands will:

  • Set up GRUB in the 440-byte Master Boot Record boot code region
  • Populate the /boot/grub directory
  • Generate the /boot/grub/i386-pc/core.img file
  • Embed it in the 31 KiB (minimum size - varies depending on partition alignment) post-MBR gap in case of MBR partitioned disk
  • In the case of a GPT partitioned disk it will embed it in the BIOS Boot Partition , denoted by bios_grub flag in parted and EF02 type code in gdisk
# grub-install --recheck /dev/sdx
# grub-mkconfig -o /boot/grub/grub.cfg
Note: --target=i386-pc is not needed since it is the default value, and besides GRUB will print for which platform it is installing providing an easy way to verify it.

If you use LVM for your /boot, you can install GRUB on multiple physical disks.

3.1.1.2 Install to external USB stick

Assume your USB stick's first partition is FAT32 and its partition is /dev/sdy1 

# mkdir -p /mnt/usb ; mount /dev/sdy1 /mnt/usb
# grub-install --target=i386-pc --recheck --debug --boot-directory=/mnt/usb/boot /dev/sdy
# grub-mkconfig -o /mnt/usb/boot/grub/grub.cfg

# optional, backup config files of grub.cfg
# mkdir -p /mnt/usb/etc/default
# cp /etc/default/grub /mnt/usb/etc/default
# cp -a /etc/grub.d /mnt/usb/etc

# sync; umount /mnt/usb
3.1.1.3 Install to partition or partitionless disk
Warning: GRUB strongly discourages installation to a partition boot sector or a partitionless disk as GRUB Legacy or Syslinux does. This setup is prone to breakage, especially during updates, and is not supported by the Arch developers.

To set up grub to a partition boot sector, to a partitionless disk (also called superfloppy) or to a floppy disk, run (using for example /dev/sdaX as the /boot partition): 

# chattr -i /boot/grub/i386-pc/core.img
# grub-install --target=i386-pc --recheck --debug --force /dev/sdaX
# chattr +i /boot/grub/i386-pc/core.img
Note:
  • /dev/sdaX used for example only.
  • --target=i386-pc instructs grub-install to install for BIOS systems only. It is recommended to always use this option to remove ambiguity in grub-install.

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 GRUB relies on embedded blocklists in the partition bootsector to locate the /boot/grub/i386-pc/core.img file and the prefix directory /boot/grub. The sector locations of core.img may change whenever the file system 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 GRUB is installed to a partition boot sector or a partitionless disk, not in case of installation to MBR or simple generation of core.img without embedding any bootsector (mentioned above).

Unfortunately, the grub.cfg file that is created will not contain the proper UUID in order to boot, even if it reports no errors. see https://bbs.archlinux.org/viewtopic.php?pid=1294604#p1294604. In order to fix this issue the following commands: 

# mount /dev/sdxY /mnt        #Your root partition.
# mount /dev/sdxZ /mnt/boot   #Your boot partition (if you have one).
# arch-chroot /mnt
# pacman -S linux
# grub-mkconfig -o /boot/grub/grub.cfg
3.1.1.4 Generate core.img alone

To populate the /boot/grub directory and generate a /boot/grub/i386-pc/core.img file without embedding any GRUB bootsector code in the MBR, post-MBR region, or the partition bootsector, add --grub-setup=/bin/true to grub-install: 

# grub-install --target=i386-pc --grub-setup=/bin/true --recheck --debug /dev/sda
Note:
  • /dev/sda used for example only.
  • --target=i386-pc instructs grub-install to install for BIOS systems only. It is recommended to always use this option to remove ambiguity in grub-install.

You can then chainload GRUB's core.img from GRUB Legacy or syslinux as a Linux kernel or as a multiboot kernel (see also Syslinux#Chainloading).

4 Generate the main configuration file

After the installation, the main configuration file grub.cfg needs to be generated. The generation process can be influenced by a variety of options in /etc/default/grub and scripts in /etc/grub.d/; see #Configuration.

If you have not done additional configuration, the automatic generation will determine the root filesystem of the system to boot for the configuration file. For that to succeed it is important that the system is either booted or chrooted into.

Note: Remember that grub.cfg has to be re-generated after any change to /etc/default/grub or files in /etc/grub.d/.

Use the grub-mkconfig tool to generate grub.cfg: 

# grub-mkconfig -o /boot/grub/grub.cfg
Note:
  • The default file path is /boot/grub/grub.cfg, not /boot/grub/i386-pc/grub.cfg. The grub includes a sample /boot/grub/grub.cfg; ensure your intended changes were written to this file.
  • If you are trying to run grub-mkconfig in a chroot or systemd-nspawn container, you might notice that it does not work, complaining that grub-probe cannot get the "canonical path of /dev/sdaX". In this case, try using arch-chroot as described in the BBS post.

By default the generation scripts automatically add menu entries for Parabola to any generated configuration. See #Dual-booting for configuration with other systems.

5 Configuration

This section only covers editing the /etc/default/grub configuration file. See GRUB/Tips and tricks for more information.

Remember to always #Generate the main configuration file after making changes to /etc/default/grub.

5.1 Additional arguments

To pass custom additional arguments to the Linux image, you can set the GRUB_CMDLINE_LINUX + GRUB_CMDLINE_LINUX_DEFAULT variables in /etc/default/grub. The two are appended to each other and passed to kernel when generating regular boot entries. For the recovery boot entry, only GRUB_CMDLINE_LINUX is used in the generation.

It is not necessary to use both, but can be useful. For example, you could use GRUB_CMDLINE_LINUX_DEFAULT="resume=/dev/sdaX quiet" where sdaX is your swap partition to enable resume after hibernation. This would generate a recovery boot entry without the resume and without quiet suppressing kernel messages during a boot from that menu entry. Though, the other (regular) menu entries would have them as options.

By default grub-mkconfig determines the UUID of the root filesystem for the configuration. To disable this, uncomment GRUB_DISABLE_LINUX_UUID=true.

For generating the GRUB recovery entry you also have to comment out #GRUB_DISABLE_RECOVERY=true in /etc/default/grub.

You can also use GRUB_CMDLINE_LINUX="resume=UUID=uuid-of-swap-partition"

See Kernel parameters for more info.

5.2 Multi-booting

Tip: To have grub-mkconfig search for other installed systems, install os-prober.

5.2.1 Automatically generating using /etc/grub.d/40_custom and grub-mkconfig

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

or, for UEFI-GPT Mode (As per #Alternative install method): 

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

to generate an updated grub.cfg.

5.2.1.1 GNU/Linux menu entry

Assuming that the other distro is on partition sda2: 

menuentry "Other GNU/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)
}

Alternatively let grub search for the right partition by UUID or label: 

menuentry "Other GNU/Linux" {
        # assuming that UUID is 763A-9CB6
	search --set=root --fs-uuid 763A-9CB6

        # search by label OTHER_LINUX (make sure that partition label is unambiguous)
        #search --set=root --label OTHER_LINUX

	linux /boot/vmlinuz (add other options here as required, for example: root=UUID=763A-9CB6)
	initrd /boot/initrd.img (if the other kernel uses/needs one)
}
5.2.1.2 "Shutdown" menu entry
menuentry "System shutdown" {
	echo "System shutting down..."
	halt
}
5.2.1.3 "Restart" menu entry
menuentry "System restart" {
	echo "System rebooting..."
	reboot
}

5.3 LVM

If you use LVM for your /boot, make sure that the lvm module is preloaded: 

/etc/default/grub
GRUB_PRELOAD_MODULES="lvm"

and specify the root kernel parameter: 

/etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="root=/dev/mapper/lvm_group_name-lvm_logical_root_partition_name ..."

5.4 RAID

GRUB provides convenient handling of RAID volumes. You need to add insmod mdraid which allows you to address the volume natively. For example, /dev/md0 becomes: 

set root=(md/0)

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

set root=(md/0,1)

To install grub when using RAID1 as the /boot partition (or using /boot housed on a RAID1 root partition), on devices with GPT ef02/'BIOS boot partition', simply run grub-install on both of the drives, such as: 

# grub-install --target=i386-pc --recheck --debug /dev/sda
# grub-install --target=i386-pc --recheck --debug /dev/sdb

Where the RAID 1 array housing /boot is housed on /dev/sda and /dev/sdb.

5.5 Multiple entries

For tips on managing multiple GRUB entries, for example when using both linux-libre and linux-libre-lts kernels, see GRUB/Tips and tricks#Multiple entries.

5.6 Encryption

5.6.1 Root partition

For an encrypted root filesystem, it is necessary to edit /etc/default/grub with the parameters required to unlock the encrypted filesystem during boot. For example, if the mkinitcpio encrypt hook is used, the cryptdevice parameter must be added to GRUB_CMDLINE_LINUX="" command. In the example below, the sda2 partition has been encrypted as /dev/mapper/cryptroot: 

/etc/default/grub
GRUB_CMDLINE_LINUX="cryptdevice=/dev/sda2:cryptroot"

Once /etc/default/grub has been amended, it will then be necessary to #Generate the main configuration file.

For further information about bootloader configuration for encrypted devices, see Dm-crypt/System configuration#Boot loader.

Tip: If you are upgrading from a working GRUB Legacy configuration, check /boot/grub/menu.lst.pacsave for the correct device/label to add. Look for them after the text kernel /vmlinuz-linux.

5.6.2 Boot partition

The GRUB parameter GRUB_ENABLE_CRYPTODISK can be used to enable GRUB to ask for a password to open a LUKS blockdevice in order to read its configuration and load any initramfs and kernel from it. This option tries to solve the issue of having an unencrypted boot partition.

The feature is enabled by adding: 

GRUB_ENABLE_CRYPTODISK=y

to /etc/default/grub. After this configuration a subsequent run of grub-mkconfig to #Generate the main configuration file is required while the encrypted /boot is mounted.

Note: GRUB_ENABLE_CRYPTODISK=1 will not work as opposed to the request shown in GRUB 2.02-beta2.

Depending on the system's setup, note the following:

  • For the feature to work it is not required that /boot is kept in a separate partition, it may also stay under the system's root / directory tree.
  • Without further changes you will be prompted twice for a passhrase: the first for GRUB to unlock the /boot mount point in early boot, the second to unlock the root filesystem itself as described in #Root partition.
  • In order to perform system updates involving the /boot mount point, it must be ensured that the encrypted /boot is unlocked to be re-mounted by the initramfs and kernel during boot. With a separate /boot partition, this may be accomplished by adding an entry to /etc/crypttab with a keyfile. See Dm-crypt/System configuration#crypttab.
  • If you use a special keymap, a default GRUB installation will not know it. This is relevant for how to enter the passphrase to unlock the LUKS blockdevice.

6 Using the command shell

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

GRUB 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: 

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/i386-pc/normal.mod
rescue:grub> normal

6.1 Pager support

GRUB 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 GRUB command shell type: 

sh:grub> set pager=1

6.2 Using the command shell environment to boot operating systems

grub>

The GRUB's command shell environment can be used to boot operating systems. A common scenario may be to boot Windows / Linux stored on a drive/partition via chainloading.

Chainloading means to load another boot-loader from the current one, ie, chain-loading.

The other bootloader may be embedded at the starting of the disk(MBR) or at the starting of a partition.

6.2.1 Chainloading a partition

set root=(hdX,Y)
chainloader +1
boot

X=0,1,2... Y=1,2,3...

6.2.2 Chainloading a disk/drive

set root=hdX
chainloader +1
boot

6.2.3 Normal loading

See the examples in #Using the rescue console

6.3 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 i386-pc/linux.mod
Note: With a separate boot partition, omit /boot from the path, (i.e. type set prefix=(hdX,Y)/grub).

This introduces the linux-libre and initrd commands, which should be familiar.

An example, booting Parabola: 

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 GRUB.

To reinstall GRUB and fix the problem completely, changing /dev/sda if needed. See #Installation for details.

7 Troubleshooting

7.1 Intel BIOS not booting GPT

7.1.1 MBR

Some Intel BIOS's require at least one bootable MBR partition to be present at boot, causing GPT-partitioned boot setups to be unbootable.

This can be circumvented by using (for instance) fdisk to mark one of the GPT partitions (preferably the 1007 KiB partition you have created for GRUB already) bootable in the MBR. This can be achieved, using fdisk, by the following commands: Start fdisk against the disk you are installing, for instance fdisk /dev/sda, then press a and select the partition you wish to mark as bootable (probably #1) by pressing the corresponding number, finally press w to write the changes to the MBR.

Note: The bootable-marking must be done in fdisk or similar, not in GParted or others, as they will not set the bootable flag in the MBR.

With cfdisk, the steps are similar, just cfdisk /dev/sda, choose bootable (at the left) in the desired hard disk, and quit saving.

More information is available here

7.1.2 EFI path

Some UEFI firmwares require a bootable file at a known location before they will show UEFI NVRAM boot entries. If this is the case, grub-install will claim efibootmgr has added an entry to boot GRUB, however the entry will not show up in the VisualBIOS boot order selector. The solution is to place a file at one of the known locations. Assuming the EFI partition is at /boot/efi/ this will work: 

mkdir /boot/efi/EFI/boot
cp /boot/efi/EFI/grub/grubx64.efi /boot/efi/EFI/boot/bootx64.efi

This solution worked for an Intel DH87MC motherboard with firmware dated Jan 2014.

7.2 Enable debug messages

Note: This change is overwritten when #Generate the main configuration file.

Add: 

set pager=1
set debug=all

to grub.cfg.

7.3 "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 GRUB graphical terminal (gfxterm) with proper video mode (gfxmode) in GRUB. This video mode is passed by GRUB to the linux-libre kernel via 'gfxpayload'. In case of UEFI systems, if the GRUB 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 ${GRUB_PREFIX_DIR} (/boot/grub/ in case of BIOS and UEFI systems). If GRUB UEFI was installed with --boot-directory=$esp/EFI set, then the directory is $esp/EFI/grub/: 

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

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

7.4 msdos-style error message

grub-setup: warn: This msdos-style partition label has no post-MBR gap; embedding will not 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.

7.5 UEFI

7.5.1 Common installation errors

  • If you have a problem when running grub-install with sysfs or procfs and it says you must run modprobe efivars, try Unified Extensible Firmware Interface#Switch to efivarfs.
  • Without --target or --directory option, grub-install cannot determine for which firmware to install. In such cases grub-install will print source_dir does not exist. Please specify --target or --directory.
  • If after running grub-install you are told your partition does not look like an EFI partition then the partition is most likely not Fat32.

7.5.2 Drop to rescue shell

If GRUB loads but drops you into the rescue shell with no errors, it may be because of a missing or misplaced grub.cfg. This will happen if GRUB 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).

7.5.3 GRUB UEFI not loaded

An example of a working EFI: 

# efibootmgr -v
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 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)

7.6 Boot freezes

If booting gets stuck without any error message after GRUB loading the kernel and the initial ramdisk, try removing the add_efi_memmap kernel parameter.

7.7 Parabola not found from other OS

Some have reported that other distributions have trouble finding Parabola automatically with os-prober. If this problem arises, it has been reported that detection can be improved with the presence of /etc/lsb-release. This file and updating tool is available with the package lsb-release in the official repositories.

7.8 Warning when installing in chroot

When installing GRUB on a LVM system in a chroot environment (e.g. during system installation), you may receive warnings like /run/lvm/lvmetad.socket: connect failed: No such file or directory or WARNING: failed to connect to lvmetad: No such file or directory. Falling back to internal scanning. This is because /run is not available inside the chroot. These warnings will not prevent the system from booting, provided that everything has been done correctly, so you may continue with the installation.

7.9 GRUB loads slowly

GRUB can take a long time to load when disk space is low. Check if you have sufficient free disk space on your /boot or / partition when you are having problems.

7.10 error: unknown filesystem

GRUB may output error: unknown filesystem and refuse to boot for a few reasons. If you are certain that all UUIDs are correct and all filesystems are valid and supported, it may be because your BIOS Boot Partition is located outside the first 2TB of the drive [3]. Use a partitioning tool of your choice to ensure this partition is located fully within the first 2TB, then reinstall and reconfigure GRUB.

7.11 grub-reboot not resetting

GRUB seems to be unable to write to root BTRFS partitions [4]. If you use grub-reboot to boot into another entry it will therefore be unable to update its on-disk environment. Either run grub-reboot from the other entry (for example when switching between various distributions) or consider a different file system. You can reset a "sticky" entry by executing grub-editenv create and setting GRUB_DEFAULT=0 in your /etc/default/grub (don't forget grub-mkconfig).

8 See also

9 Acknowledgement

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