Linux update grub config

GRUB (GRand Unified Bootloader) is a boot loader. The current GRUB is also referred to as GRUB 2. The original GRUB, or GRUB Legacy, corresponds to versions 0.9x. This page exclusively describes GRUB 2.

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.

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:

where /dev/sdX is the disk (not a partition) where GRUB is to be installed. For example /dev/sda or /dev/nvme0n1 , or /dev/mmcblk0 . See Device file#Block device names for a description of the block device naming scheme.

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

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

UEFI systems

Installation

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 to your disk:

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/ .

After the above installation 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 – this will, however, fail if your boot entries are full; use efibootmgr to remove unnecessary entries.

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

Configuration

On an installed system, GRUB loads the /boot/grub/grub.cfg configuration file each boot. You can follow #Generated grub.cfg for using a tool, or #Custom grub.cfg for a manual creation.

Generated grub.cfg

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

Generate the main configuration file

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.

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

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

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/41_custom adds the necessary source statement to the generated configuration file.

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 from which the other systems boot. Then re-run grub-mkconfig. If you get the following output: Warning: os-prober will not be executed to detect other bootable partitions then edit /etc/default/grub and add/uncomment:

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 .

This article or section is a candidate for merging with #Installation.

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

This article or section is a candidate for merging with #Installation.

GRUB provides convenient handling of RAID volumes. You need to load GRUB modules mdraid09 or mdraid1x to allow you to address the volume natively:

For example, /dev/md0 becomes:

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

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

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

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.

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 :

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 passphrase: 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.

LUKS2

GRUB 2.06 has limited support for LUKS2. See GRUB bug #55093.

Argon2id (cryptsetup default) and Argon2i PBKDFs are not supported, only PBKDF2 is.
grub-install does not support creating a core image that could be used for unlocking LUKS2. See the comments below or on grub-gitAUR for a workaround.

Use grub-install as described in the #Installation section. However, the generated EFI binary does not support LUKS2 and needs to be replaced.

Create grub-pre.cfg , replace /dev/nvme0n1p2 accordingly. The output is the UUID of the encrypted partition without hyphens.

Add lvm if you use LVM. Replace ext2 by btrfs if needed:

If you enter an invalid passphrase during boot and end up at the GRUB rescue shell, try cryptomount -a to mount all (hopefully only one) encrypted partitions or use cryptomount -u $crypto_uuid to mount a specific one. Then proceed with insmod normal and normal as usual.

If you enter a correct passphrase, but an Invalid passphrase error is immediately returned, make sure that the right cryptographic modules are specified. Use cryptsetup luksDump /dev/nvme0n1p2 and check whether the hash function (SHA-256, SHA-512) matches the modules ( gcry_sha256 , gcry_sha512 ) installed. Under normal circumstances it should take a few seconds before the passphrase is processed.

Custom grub.cfg

This article or section needs expansion.

This section describes the manual creation of GRUB boot entries in /boot/grub/grub.cfg instead of relying on grub-mkconfig.

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» 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 ( / )

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

GRUB commands

«Shutdown» menu entry

«Restart» menu entry

«UEFI Firmware Settings» menu entry

EFI binaries

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

UEFI Shell

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

gdisk

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

Chainloading a unified kernel image

If you have a unified kernel image generated from following Secure Boot or other means, you can add it to the boot menu. For example:

Dual-booting

GNU/Linux

Assuming that the other distribution is on partition sda2 :

Alternatively let GRUB search for the right partition by UUID or file system label:

If the other distribution has already a valid /boot folder with installed GRUB, grub.cfg , kernel and initramfs, GRUB can be instructed to load these other grub.cfg files on-the-fly during boot. For example, for hd0 and the fourth GPT partition:

When choosing this entry, GRUB loads the grub.cfg file from the other volume and displays that menu. Any environment variable changes made by the commands in file will not be preserved after configfile returns. Press Esc to return to the first GRUB menu.

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.

where $hints_string and $fs_uuid are obtained with the following two commands.

The $fs_uuid command determines the UUID of the EFI system partition:

Alternatively one can run blkid (as root) and read the UUID of the EFI system partition from there.

The $hints_string command will determine the location of the EFI system partition, in this case harddrive 0:

These two commands assume the ESP Windows uses is mounted at esp . There might be case differences in the path to Windows’s EFI file, what with being Windows, and all.

Windows installed in BIOS/MBR mode

Throughout this section, it is assumed your Windows partition is /dev/sda1 . A different partition will change every instance of hd0,msdos1 .

In both examples XXXXXXXXXXXXXXXX is the filesystem UUID which can be found with command lsblk —fs .

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

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 labels

It is possible to use file system labels, human-readable strings attached to file systems, by using the —label option to search . First of all, make sure your file system has a label.

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

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:

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

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 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:

Using the command shell environment to boot operating systems

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

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

Similarly GRUB installed to a partition can be chainloaded.

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

Chainloading Windows/Linux installed in UEFI mode

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

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):

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

To expand console capabilities, insert the linux module:

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

An example, booting Arch Linux:

With a separate boot partition (e.g. when using UEFI), again change the lines accordingly:

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.

GRUB removal

This article or section needs expansion.

After migrating to GPT/UEFI one may want to remove the MBR boot code using dd:

Troubleshooting

Unsupported file systems

In case that GRUB does not support the root 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 file system 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

This article or section is a candidate for moving to Arch boot process#Troubleshooting.

The factual accuracy of this article or section is disputed.

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.

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.

More information is available here

Enable debug messages

msdos-style error message

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

Common installation errors

If you have a problem when running grub-install with sysfs or procfs and it says you must run modprobe efivarfs , 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 .

Create a GRUB entry in the firmware boot manager

This article or section needs expansion.

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.

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:

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:

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:

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

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:

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

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

This error might also be caused by an ext4 filesystem having unsupported features set:

large_dir — unsupported.
metadata_csum_seed — will be supported in GRUB 2.11 (commit).

grub-reboot not resetting

GRUB seems to be unable to write to root BTRFS partitions [2]. 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

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

For VirtualBox 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:

GRUB rescue and encrypted /boot

When using an encrypted /boot, and you fail to input a correct password, you will be dropped in grub-rescue prompt.

This grub-rescue prompt has limited capabilities. Use the following commands to complete the boot:

See this blog post for a better description.

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Linux update grub config