User:Eschwartz/Grub

Category:Boot loaders Category:GNU fa:گراب

GRUB (GRand Unified Bootloader) is a multi-boot loader. It is derived from PUPA which was a research project to develop the replacement of what is now known as GRUB Legacy. The latter had become too difficult to maintain and GRUB was rewritten from scratch with the aim to provide modularity and portability [1]. The current GRUB is also referred to as GRUB 2 while GRUB Legacy corresponds to versions 0.9x.

Note: In the entire article esp denotes the mountpoint of the EFI system partition aka ESP.

BIOS systems

GUID Partition Table (GPT) specific instructions

On a BIOS/GPT 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 Partitioning#GUID Partition Table.
The BIOS boot partition is only needed by GRUB on a BIOS/GPT setup. On a BIOS/MBR setup, GRUB uses the post-MBR gap for the embedding the core.img. On GPT, however, there is no guaranteed unused space before the first partition.
For UEFI systems this extra partition is not required, since no embedding of boot sectors takes place in that case. However, UEFI systems still require an EFI system partition.

Create a mebibyte partition (+1M with fdisk or gdisk) on the disk with no file system and with partition type GUID 21686148-6449-6E6F-744E-656564454649.

Select partition type BIOS boot for fdisk.
Select partition type code ef02 for gdisk.
For parted set/activate the flag bios_grub on the partition.

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 space before the first partition 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.

Master Boot Record (MBR) specific instructions

Usually the post-MBR gap (after the 512 byte MBR region and before the start of the first partition) in many MBR 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 (FS#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).

Installation

Install the grub package. (It will replace grub-legacy^AUR if that is already installed.) Then do:

# grub-install --target=i386-pc /dev/sdX

where /dev/sdX is the disk where GRUB is to be installed (for example, disk /dev/sda and not partition /dev/sda1).

Now you must #Generate the main configuration file.

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

Tip: See GRUB/Tips and tricks#Alternative installation methods for other ways to install GRUB, such as to a USB stick.

See grub-install(8) and GRUB Manual for more details on the grub-install command.

UEFI systems

Note:

It is recommended to read and understand the Unified Extensible Firmware Interface, Partitioning#GUID Partition Table and Arch boot process#Under UEFI pages.
When installing to use UEFI it is important to boot the installation media in UEFI mode, otherwise efibootmgr will not be able to add the GRUB UEFI boot entry. Installing to the fallback boot path will still work even in BIOS mode since it does not touch the NVRAM.
To boot from a disk using UEFI, an EFI system partition is required. Follow EFI system partition#Check for an existing partition to find out if you have one already, otherwise you need to create it.

Installation

Note:

UEFI firmwares are not implemented consistently across manufacturers. The procedure described below is intended to work on a wide range of UEFI systems but those experiencing problems despite applying this method are encouraged to share detailed information, and if possible the turnarounds found, for their hardware-specific case. A GRUB/EFI examples article has been provided for such cases.
The section assumes you are installing GRUB for x86_64 systems. For IA32 (32-bit) UEFI systems (not to be confused with 32-bit CPUs), replace x86_64-efi with i386-efi where appropriate.

First, install the packages grub and efibootmgr: GRUB is the bootloader while efibootmgr is used by the GRUB installation script to write boot entries to NVRAM.

Then follow the below steps to install GRUB:

Mount the EFI system partition and in the remainder of this section, substitute esp with its mount point.
Choose a bootloader identifier, here named GRUB. A directory of that name will be created in esp/EFI/ to store the EFI binary and this is the name that will appear in the UEFI boot menu to identify the GRUB boot entry.
Execute the following command to install the GRUB EFI application grubx64.efi to esp/EFI/GRUB/ and install its modules to /boot/grub/x86_64-efi/.

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

After the above install completed the main GRUB directory is located at /boot/grub/. Note that grub-install also tries to create an entry in the firmware boot manager, named GRUB in the above example.

Remember to #Generate the main configuration file after finalizing the configuration.

Tip: If you use the option --removable then GRUB will be installed to esp/EFI/BOOT/BOOTX64.EFI (or esp/EFI/BOOT/BOOTIA32.EFI for the i386-efi target) and you will have the additional ability of being able to boot from the drive in case EFI variables are reset or you move the drive to another computer. Usually you can do this by selecting the drive itself similar to how you would using BIOS. If dual booting with Windows, be aware Windows usually places an EFI executable there, but its only purpose is to recreate the UEFI boot entry for Windows.

Note:

--efi-directory and --bootloader-id are specific to GRUB UEFI, --efi-directory 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 UEFI install script. Indeed, UEFI bootloaders do not use a MBR bootcode or partition boot sector at all.

See UEFI troubleshooting in case of problems. Additionally see GRUB/Tips and tricks#UEFI further reading.

Filesystem-specific installation issues

LVM

Warning: GRUB does not support thin-provisioned logical volumes.

RAID

To install grub when using RAID1 as the /boot partition (or using /boot housed on a RAID1 root partition), on BIOS systems, you must run run grub-install on both of the drives, such as:

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

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

Note: GRUB 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.

Encrypted /boot

GRUB also has special support for booting with an encrypted /boot. This is done by unlocking 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.

Note: /boot is not required to be kept in a separate partition; it may also stay under the system's root / directory tree.

Warning: GRUB does not support LUKS2 headers; see GRUB bug #55093. Make sure to specify --type luks1 when creating the encrypted partition using cryptsetup luksFormat.

To enable this feature encrypt the partition with /boot residing on it using LUKS as normal. Then add the following option to /etc/default/grub:

/etc/default/grub

GRUB_ENABLE_CRYPTODISK=y

This option is used by grub-install to generate the grub core.img, so make sure to install grub after modifying this option.

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 implemented by the initramfs. You can use a keyfile to avoid this.

Note:

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.
If you experience issues getting the prompt for a password to display (errors regarding cryptouuid, cryptodisk, or "device not found"), try reinstalling GRUB and appending --modules="part_gpt part_msdos" to the end of your grub-install command.

If /boot is on a separate partition:

Warning:

If you want to generate the main configuration file, make sure that /boot is mounted.
In order to perform system updates involving the /boot mount point, ensure that the encrypted /boot is unlocked and mounted before performing an update. With a separate /boot partition, this may be accomplished automatically on boot by using crypttab with a keyfile.

Tip: You can use pacman hooksto automount your /boot when upgrades need to access related files.

Configuration

The loader configuration for grub is stored in the file /boot/grub/grub.cfg. This file is interpreted by grub's builtin scripting language, which "has a syntax quite similar to that of GNU Bash" and is documented in info grub --node 'Shell-like scripting'.

There are two approaches to creating grub.cfg: manually writing the configuration file, or generating it via the autodetection script grub-mkconfig.

Write the configuration from scratch

For basic usage, a limited number of commands is sufficient to create and boot from a minimal grub.cfg:

linux /boot/vmlinuz-linux root=UUID=some-uuid-goes-here rw
initrd /boot/initramfs-linux.img
boot

This handles three tasks:

Specify the filepath to the kernel, and kernel commandline, to use.

Specify the path to the initial ramdisk to boot into.

Boot the loaded specification.

For most use cases, grub.cfg and the kernel will share the same partition and no further configuration is needed. However, users will typically want multiple boot entries and a menu to select between them. This can be accomplished by wrapping the commands in a menuentry:

menuentry "The named label for this boot option" {
    linux /boot/vmlinuz-linux root=UUID=some-uuid-goes-here rw
    initrd /boot/initramfs-linux.img
}

For truly pathological cases, advanced scripting can handle a wide variety of edge case needs (including runtime detection of new boot options), but is very rarely needed.

Tip: In order to prevent accidentally overwriting grub.cfg, you can store the true configuration in /boot/grub/custom.cfg and load it from grub.cfg using the directive source ${prefix}/custom.cfg.

Partition loading

The device on which grub.cfg and the GRUB modules reside, embedded by grub-install into the bootloader stub (grub.efi for UEFI, or the MBR core.img) according to the --boot-directory option, is automatically set as the $root environment variable.

Any special handling needed to make the device available, for example, unlocking a LUKS-encrypted device, will be automatically performed without any configuration necessary.

For additional devices which grub needs to access, it may be necessary to explicitly insmod the GRUB device driver module before use, or unlock the partition using cryptomount.

Filenames

Any time a filename is used in grub, it takes the form (device)/unix/path, where device is optional and defaults to the value of the environment variable $root. For the first partition on disk hdX this takes the form hdX,1. Partition numbers are optional and if excluded default to the entire device (for instance, an LVM device would be lvm/vg-boot without a physical disk and partition number).

Command reference

linux - the linux kernel file to load, followed by additional options for the kernel commandline. Required when booting a Linux system.

initrd - the initial ramdisk file(s) to load. Optional, though most modern Linux systems use one. Must come after linux.

boot - boot the last OS or chainloader that has been loaded.

chainloader - load an alternative bootloader. For UEFI installations, execute any EFI binary (bootloaders as well as e.g. the UEFI Shell).

menuentry - the title of a menu entry, with an optional unique identifier specified by --id. If selected, all commands enclosed by {} will be executed, followed by boot.

cryptomount - unlock and make a new LUKS device available to grub.

search - scan for devices and set an environment variable to the location. Defaults to --set root. Can search for devices based on --fs-uuid or --label.

source - execute the contents of another config file in the current GRUB environment.

insmod - can be used to explicitly load a module from /boot/grub/<target>/<modulename>.mod, though most modules should be automatically loaded when needed.

Environment variables

Environment variables can be set using the syntax set key=val, and thereafter used as arguments to commands. One popular use is to store the contents of the kernel command line. Some variables have special meaning to grub, for example:

root - the default location for filenames without an explicit device. Defaults to the device where /boot/grub/ is installed to.

timeout - menu timeout in seconds before the default entry is booted. If this is not set or is equal to -1, grub will wait indefinitely for keyboard input. A timeout of 0 means to skip the menu and boot the default entry immediately.

default - the menuentry selected by default, identified either by number starting from 0, or by menuentry title or --id. In order to select a menuentry inside a submenu, the values for each submenu and menuentry must be listed, separated by the ">" character.

color_normal - the standard foreground and background colors. Defaults to light-gray/black. The menu_color_normal variant governs colors inside a menu.

color_highlight - the highlighted foreground and background colors. Defaults to black/light-gray. The menu_color_highlight variant governs colors inside a menu.

Examples

This article or section needs expansion.

Reason: base IRC example: [2] (Discuss in User talk:Eschwartz/Grub)

A simple grub.cfg will start with the following generic header:

# let menu appear for one second to give user a chance to edit or select a specific option
set timeout=1

# common command-line flags needed on All Arch Linuxes regardless of booted kernel can be expanded via variables
set cmdline="root=UUID=some-uuid-goes-here rw"

Followed by one or more menu options with the following format:

menuentry "The named label for this boot option" {
    linux /boot/vmlinuz-linux $cmdline
    initrd /boot/intel-ucode.img /boot/initramfs-linux.img
}

# wire up mkinitcpio's automatically created fallback initramfs too
menuentry "The named label for this fallback initramfs boot option" {
    linux /boot/vmlinuz-linux $cmdline
    initrd /boot/intel-ucode.img /boot/initramfs-linux-fallback.img
}

Tip: If the boot option is on a filesystem supporting subvolumes (grub, zfs), the path to /boot is the path from outside of the subvolume.

For more general examples, e.g. dual-booting, see #Boot menu entry examples.

Generate the configuration file using grub-mkconfig

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 and/or files in /etc/grub.d/.

Basic Arch Linux installation

After the installation, the main configuration file /boot/grub/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/.

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 /boot/grub/grub.cfg has to be re-generated after any change to /etc/default/grub or files in /etc/grub.d/.
The default file path is /boot/grub/grub.cfg, not /boot/grub/i386-pc/grub.cfg.
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.

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

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

By default the generation scripts automatically add menu entries for all installed Arch Linux kernels to the generated configuration.

Tip:

After installing or removing a kernel, you just need to re-run the above grub-mkconfig command.
For tips on managing multiple GRUB entries, for example when using both linux and linux-lts kernels, see GRUB/Tips and tricks#Multiple entries.

To automatically add entries for other installed operating systems, see #Detecting other operating systems.

You can add additional custom menu entries by editing /etc/grub.d/40_custom and re-generating /boot/grub/grub.cfg. Or you can create /boot/grub/custom.cfg and add them there. Changes to /boot/grub/custom.cfg do not require re-running grub-mkconfig, since /etc/grub.d/40_custom adds the necessary source statement to the generated configuration file.

Tip: /etc/grub.d/40_custom can be used as a template to create /etc/grub.d/nn_custom, where nn defines the precedence, indicating the order the script is executed. The order scripts are executed determine the placement in the GRUB boot menu. nn should be greater than 06 to ensure necessary scripts are executed first.

See #Boot menu entry examples for custom menu entry examples.

Detecting other operating systems

To have grub-mkconfig search for other installed systems and automatically add them to the menu, install the os-prober package and mount the partitions that contain the other systems. Then re-run grub-mkconfig.

MS Windows

Often, partitions containing Windows will be automatically discovered by os-prober. However, NTFS partitions may not always be detected when mounted with the default Linux drivers. If GRUB is not detecting it, try installing ntfs-3g and remounting.

Encrypted Windows partitions may need to be decrypted before mounting. For BitLocker, this can be done with dislocker^AUR. This should be sufficient for os-prober to add the correct entry.

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=UUID=uuid-of-swap-partition quiet" where uuid-of-swap-partition is the UUID of 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 have to ensure that GRUB_DISABLE_RECOVERY is not set to true in /etc/default/grub.

See Kernel parameters for more info.

Boot menu entry examples

Tip: These boot entries can be used for both a handwritten /boot/grub/grub.cfg and one generated by grub-mkconfig. In the latter case, either add them to /etc/grub.d/40_custom and re-generate the main configuration file, or add them to /boot/grub/custom.cfg.

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

For Archiso and Archboot boot menu entries see Multiboot USB drive#Boot entries.

GRUB commands

"Shutdown" menu entry

menuentry "System shutdown" {
	echo "System shutting down..."
	halt
}

"Restart" menu entry

menuentry "System restart" {
	echo "System rebooting..."
	reboot
}

"Firmware setup" menu entry (UEFI only)

if [ ${grub_platform} == "efi" ]; then
	menuentry "Firmware setup" {
		fwsetup
	}
fi

EFI binaries

When launched in UEFI mode, GRUB can chainload other EFI binaries.

Tip: To show these menu entries only when GRUB is launched in UEFI mode, enclose them in the following if statement:

if [ ${grub_platform} == "efi" ]; then
	place UEFI-only menu entries here
fi

UEFI Shell

You can launch UEFI Shell by using placing it in the root of the EFI system partition and adding this menu entry:

menuentry "UEFI Shell" {
	insmod fat
	insmod chain
	search --no-floppy --set=root --file /shellx64.efi
	chainloader /shellx64.efi
}

gdisk

Download the gdisk EFI application and copy gdisk_x64.efi to esp/EFI/tools/.

menuentry "gdisk" {
	insmod fat
	insmod chain
	search --no-floppy --set=root --file /EFI/tools/gdisk_x64.efi
	chainloader /EFI/tools/gdisk_x64.efi
}

Chainloading an Arch Linux .efi file

If you have an .efi file generated from following Secure Boot or other means, you can add it to the boot menu. For example:

menuentry "Arch Linux .efi" {
	insmod fat
	insmod chain
	search --no-floppy --set=root --fs-uuid FILESYSTEM_UUID
	chainloader /EFI/arch/vmlinuz.efi
}

Dual-booting

A dual-boot entry will almost inevitably be on a different filesystem partition from the GRUB installation, so for each menu entry it is necessary to search -u uuid-of-other-partition to set up the paths. See #Command reference for details.

GNU/Linux

menuentry "The named label for this other boot option" {
    search -u uuid-of-other-linux-os-partition
    linux /boot/vmlinuz-kernel-name root=UUID=uuid-of-other-linux-os-partition rw
    initrd /boot/intel-ucode.img /boot/initramfs-kernel-name.img
}

Windows installed in UEFI/GPT mode

This mode determines where the Windows bootloader resides and chain-loads it after GRUB when the menu entry is selected. The main task here is finding the EFI system partition and running the bootloader from it.

Note: This menuentry will work only in UEFI boot mode and only if the Windows bitness matches the UEFI bitness. It will not work in BIOS installed GRUB. See Dual boot with Windows#Windows UEFI vs BIOS limitations and Dual boot with Windows#Bootloader UEFI vs BIOS limitations for more information.

# On UEFI you will have to use the chainloader.
menuentry "Microsoft Windows Vista/7/8/8.1 UEFI/GPT" {
    insmod ntfs
    insmod chain
    search -u uuid-of-windows-esp
    chainloader /EFI/Microsoft/Boot/bootmgfw.efi
}

Windows installed in BIOS/MBR mode

Warning: It is the system partition that has /bootmgr, not your "real" Windows partition (usually C:). The system partition's filesystem label is System Reserved or SYSTEM and the partition is only about 100 to 549 MiB in size. See Wikipedia:System partition and boot partition for more information.

Note: These menu entries will work only in BIOS boot mode. It will not work in UEFI installed GRUB. See Dual boot with Windows#Windows UEFI vs BIOS limitations and Dual boot with Windows#Bootloader UEFI vs BIOS limitations .

For Windows Vista/7/8/8.1/10:

# The ntldr command can load Windows directly on BIOS systems.
menuentry "Microsoft Windows Vista/7/8/8.1/10" {
    insmod ntfs
    search -u uuid-of-windows-system-volume
    ntldr /bootmgr
}

For Windows XP use /ntldr instead:

# The ntldr command can load Windows directly on BIOS systems.
menuentry "Windows XP" {
    insmod ntfs
    search -u uuid-of-windows-system-volume
    ntldr /ntldr
}

Note: In some cases, GRUB may be installed without a clean Windows 8, in which case you cannot boot Windows without having an error with \boot\bcd (error code 0xc000000f). You can fix it by going to Windows Recovery Console (cmd.exe from install disk) and executing:

X:\> bootrec.exe /fixboot
X:\> bootrec.exe /RebuildBcd

Do not use bootrec.exe /Fixmbr because it will wipe GRUB out. Or you can use Boot Repair function in the Troubleshooting menu - it will not wipe out GRUB but will fix most errors. Also you would better keep plugged in both the target hard drive and your bootable device ONLY. Windows usually fails to repair boot information if any other devices are connected.

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

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

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 start of a partitioned disk (MBR), at the start of a partition or a partitionless disk (VBR), or as an EFI binary in the case of UEFI.

Chainloading a partition's VBR

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

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

For example to chainload Windows stored in the first partition of the first hard disk,

set root=(hd0,1)
chainloader +1
boot

Similarly GRUB installed to a partition can be chainloaded.

Chainloading a disk's MBR or a partitionless disk's VBR

set root=hdX
chainloader +1
boot

Chainloading Windows/Linux installed in UEFI mode

insmod fat
set root=(hd0,gpt4)
chainloader (${root})/EFI/Microsoft/Boot/bootmgfw.efi
boot

insmod fat is used for loading the FAT file system module for accessing the Windows bootloader on the EFI system partition. (hd0,gpt4) or /dev/sda4 is the EFI system partition in this example. The entry in the chainloader line specifies the path of the .efi file to be chain-loaded.

Normal loading

See the examples in #Using the rescue console

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.

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

To expand console capabilities, insert the linux module:

grub rescue> insmod i386-pc/linux.mod

or simply

grub rescue> insmod linux

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

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 (e.g. when using UEFI), again change the lines accordingly:

Note: Since boot is a separate partition and not part of your root partition, you must address the boot partition manually, in the same way as for the prefix variable.

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

Note: If you experienced error: premature end of file /YOUR_KERNEL_NAME during execution of linux command, you can try linux16 instead.

After successfully booting the Arch Linux 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.

Troubleshooting

F2FS and other unsupported file systems

GRUB does not support F2FS file system. In case the root partition is on an unsupported file system, an alternative /boot partition with a supported file system must be created. In some cases, the development version of GRUB grub-git^AUR may have native support for the file system.

If GRUB is used with an unsupported filesystem it is not able to extract the UUID of your drive so it uses classic non-persistent /dev/sdXx names instead. In this case you might have to manually edit /boot/grub/grub.cfg and replace root=/dev/sdXx with root=UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX. You can use the blkid command to get the UUID of your device, see Persistent block device naming.

Intel BIOS not booting GPT

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.

With recent version of parted, you can use disk_toggle pmbr_boot option. Afterwards verify that Disk Flags show pmbr_boot.

# parted /dev/sdx disk_toggle pmbr_boot
# parted /dev/sdx print

More information is available here

Enable debug messages

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

Add:

set pager=1
set debug=all

to grub.cfg.

"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 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).

This article or section is being considered for removal.

Reason: grub-install copies unicode.pf2 to /boot/grub/fonts/, there is no need to do it manually. (Discuss in User talk:Eschwartz/Grub)

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.

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

loadfont "unicode"
set gfxmode=auto
set gfxpayload=keep
insmod all_video
insmod gfxterm
terminal_output gfxterm

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.

This error may occur when you try installing GRUB 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 #Master Boot Record (MBR) specific instructions

UEFI

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#Mount 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.

Drop to rescue shell

If GRUB loads but drops into the rescue shell with no errors, it can be due to one of these two reasons:

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,
It also happens if the boot partition, which is hardcoded into the grubx64.efi file, has changed.

GRUB UEFI not loaded

An example of a working UEFI:

# 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\grubx64.efi)
Boot0001* Shell HD(1,800,32000,23532fbb-1bfa-4e46-851a-b494bfe9478c)File(\shellx64.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(\grubx64.efi)

Default/fallback boot 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 install GRUB at the default/fallback boot path:

# grub-install --target=x86_64-efi --efi-directory=esp --removable

Alternatively you can move an already installed GRUB EFI executable to the default/fallback path:

# mv esp/EFI/grub esp/EFI/BOOT
# mv esp/EFI/BOOT/grubx64.efi esp/EFI/BOOT/BOOTX64.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.

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.

Arch not found from other OS

Some have reported that other distributions may have trouble finding Arch Linux 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.

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.

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.

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 2 TiB of the drive [3]. Use a partitioning tool of your choice to ensure this partition is located fully within the first 2 TiB, then reinstall and reconfigure GRUB.

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 (do not forget grub-mkconfig -o /boot/grub/grub.cfg).

Old BTRFS prevents installation

If a drive is formatted with BTRFS without creating a partition table (eg. /dev/sdx), then later has partition table written to, there are parts of the BTRFS format that persist. Most utilities and OS's do not see this, but GRUB will refuse to install, even with --force

# grub-install: warning: Attempting to install GRUB to a disk with multiple partition labels. This is not supported yet..
# grub-install: error: filesystem `btrfs' does not support blocklists.

You can zero the drive, but the easy solution that leaves your data alone is to erase the BTRFS superblock with wipefs -o 0x10040 /dev/sdx

Windows 8/10 not found

A setting in Windows 8/10 called "Hiberboot", "Hybrid Boot" or "Fast Boot" can prevent the Windows partition from being mounted, so grub-mkconfig will not find a Windows install. Disabling Hiberboot in Windows will allow it to be added to the GRUB menu.

VirtualBox EFI mode

Install GRUB to the default/fallback boot path.

Device /dev/xxx not initialized in udev database even after waiting 10000000 microseconds

If grub-mkconfig hangs and gives error: WARNING: Device /dev/xxx not initialized in udev database even after waiting 10000000 microseconds.

You may need to provide /run/lvm/ access to the chroot environment using:

# mkdir /mnt/hostlvm
# mount --bind /run/lvm /mnt/hostlvm
# arch-chroot /mnt
# ln -s /hostlvm /run/lvm

See FS#61040 and workaround.

BIOS systems

GUID Partition Table (GPT) specific instructions

Master Boot Record (MBR) specific instructions

Installation

UEFI systems

Installation

Filesystem-specific installation issues

LVM

RAID

Encrypted /boot

Configuration

Write the configuration from scratch

Partition loading

Filenames

Command reference

Environment variables

Examples

Generate the configuration file using grub-mkconfig

Basic Arch Linux installation

Detecting other operating systems

MS Windows

Additional arguments

Boot menu entry examples

GRUB commands

"Shutdown" menu entry

"Restart" menu entry

"Firmware setup" menu entry (UEFI only)

EFI binaries

UEFI Shell

gdisk

Chainloading an Arch Linux .efi file

Dual-booting

GNU/Linux

Windows installed in UEFI/GPT mode

Windows installed in BIOS/MBR mode

Using the command shell

Pager support

Using the command shell environment to boot operating systems

Chainloading a partition's VBR

Chainloading a disk's MBR or a partitionless disk's VBR

Chainloading Windows/Linux installed in UEFI mode

Normal loading

Using the rescue console

Troubleshooting

F2FS and other unsupported file systems

Intel BIOS not booting GPT

Enable debug messages

"No suitable mode found" error

msdos-style error message

UEFI

Common installation errors

Drop to rescue shell

GRUB UEFI not loaded

Default/fallback boot path

Invalid signature

Boot freezes

Arch not found from other OS

Warning when installing in chroot

GRUB loads slowly

error: unknown filesystem

grub-reboot not resetting

Old BTRFS prevents installation

Windows 8/10 not found

VirtualBox EFI mode

Device /dev/xxx not initialized in udev database even after waiting 10000000 microseconds

See also