GRUB
Related articles
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
- 2 BIOS systems
- 3 UEFI systems
- 4 Generate the main configuration file
- 5 Configuration
- 6 Using the command shell
-
7 Troubleshooting
- 7.1 Intel BIOS not booting GPT
- 7.2 Enable debug messages
- 7.3 "No suitable mode found" error
- 7.4 msdos-style error message
- 7.5 UEFI
- 7.6 Invalid signature
- 7.7 Boot freezes
- 7.8 Arch not found from other OS
- 7.9 Warning when installing in chroot
- 7.10 GRUB loads slowly
- 7.11 error: unknown filesystem
- 7.12 grub-reboot not resetting
- 7.13 Old BTRFS prevents installation
- 7.14 Windows 8 not found
- 8 See also
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 has a few root file system-specific limitations:
- F2FS is not supported
If your root partition is on an unsupported file system, you must create a separate /boot
partition with a supported file system. In some cases, the development version of GRUB grub-gitAUR has native support.
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 BIOS boot. Select BIOS boot
and partition type number 4
for fdisk, ef02
for gdisk, and bios_grub
for 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.
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 (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
(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-legacyAUR, where already installed.
Install boot files
There are 4 ways to install GRUB boot files in BIOS booting:
- Install to disk (recommended)
- Install to external USB stick (for recovery)
- Install to partition or partitionless disk (not recommended)
- Generate core.img alone (safest method, but requires another BIOS bootloader like Syslinux to be installed to chainload
/boot/grub/i386-pc/core.img
)
Install to disk
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 --target=i386-pc /dev/sdx # grub-mkconfig -o /boot/grub/grub.cfg
If you use LVM for your /boot
, you can install GRUB on multiple physical disks.
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 --debug --boot-directory=/mnt/usb/boot /dev/sdy # grub-mkconfig -o /mnt/usb/boot/grub/grub.cfg
Optionally backup configuration 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
Install to partition or partitionless disk
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 --debug --force /dev/sdaX # chattr +i /boot/grub/i386-pc/core.img
You need to use the --force
option to allow usage of blocklists and should not use --grub-setup=/bin/true
(which is similar to simply generating core.img
).
grub-install
will give out warnings like which should give you the idea of what might go wrong with this approach:
/sbin/grub-setup: warn: Attempting to install GRUB to a partitionless disk or to a partition. This is a BAD idea. /sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged.
Without --force
you may get the below error and grub-setup
will not setup its boot code in the partition boot sector:
/sbin/grub-setup: error: will not proceed with blocklists
With --force
you should get:
Installation finished. No error reported.
The reason why grub-setup
does not by default allow this is because in case of partition or a partitionless disk is that 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
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 --debug /dev/sda
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).
UEFI systems
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 Arch Linux on an EFI-capable computer with an already-working operating system, like Windows 8 for example, 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.
Create an ESP
If you do not have an ESP, you will need to create one. See EFI System Partition
Installation
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.
Make sure you are in a bash shell. For example, when booting from the Arch 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. Mount the ESP partition to e.g. /boot
or /boot/efi
and in the following change esp
to that mount point (usually /boot
):
# grub-install --target=x86_64-efi --efi-directory=esp --bootloader-id=grub
The --bootloader-id
is what appears in the boot options to identify 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.
See UEFI troubleshooting in case of problems.
Further reading
Below is other relevant information regarding installing Arch via UEFI
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 --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 the same place:
# grub-mkconfig -o esp/grub/grub.cfg
Configuration is otherwise the same.
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
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.
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.
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.
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.
Use the grub-mkconfig tool to generate grub.cfg
:
# grub-mkconfig -o /boot/grub/grub.cfg
By default the generation scripts automatically add menu entries for Arch Linux to any generated configuration. See Multiboot USB drive#Boot entries and #Dual-booting for custom menu entries for other systems.
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
.
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.
Dual-booting
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
.
menuentry "System shutdown" { echo "System shutting down..." halt }
menuentry "System restart" { echo "System rebooting..." reboot }
Assuming that the other distro is on partition sda2
:
menuentry "Other Linux" { set root=(hd0,2) linux /boot/vmlinuz (add other options here as required) initrd /boot/initrd.img (if the other kernel uses/needs one) }
Alternatively let grub search for the right partition by UUID or label:
menuentry "Other 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) }
menuentry "Other Linux (Encrypted)"{ insmod luks cryptomount (hd0,2) set root=(crypto0) linux /boot/vmlinuz cryptdevice=/dev/sda2:cryptroot root=/dev/mapper/cryptroot (add more options if required) initrd /boot/initrd.img (If the other kernel uses/needs one) }
The following three methods require that FreeBSD is installed on a single partition with UFS(v2). Assuming the nested BSD partition table is on sda4
:
Loading the kernel directly
menuentry 'FreeBSD' { insmod ufs2 set root='hd0,gpt4,bsd1' ## or 'hd0,msdos4,bsd1', if using an IBM-PC (MS-DOS) style partition table kfreebsd /boot/kernel/kernel kfreebsd_loadenv /boot/device.hints set kFreeBSD.vfs.root.mountfrom=ufs:/dev/ada0s4a set kFreeBSD.vfs.root.mountfrom.options=rw }
Chainloading the embedded boot record
menuentry 'FreeBSD' { insmod ufs2 set root='hd0,gpt4,bsd1' chainloader +1 }
Running the traditional BSD 2nd stage loader
menuentry 'FreeBSD' { insmod ufs2 set root='(hd0,4)' kfreebsd /boot/loader }
if [ "${grub_platform}" == "efi" ]; then menuentry "Microsoft Windows Vista/7/8/8.1 UEFI-GPT" { insmod part_gpt insmod fat insmod search_fs_uuid insmod chain search --fs-uuid --set=root $hints_string $fs_uuid chainloader /EFI/Microsoft/Boot/bootmgfw.efi } fi
where $hints_string
and $fs_uuid
are obtained with the following two commands. $fs_uuid
's command:
# grub-probe --target=fs_uuid $esp/EFI/Microsoft/Boot/bootmgfw.efi
1ce5-7f28
$hints_string
's command:
# grub-probe --target=hints_string $esp/EFI/Microsoft/Boot/bootmgfw.efi
--hint-bios=hd0,gpt1 --hint-efi=hd0,gpt1 --hint-baremetal=ahci0,gpt1
These two commands assume the ESP Windows uses is mounted at $esp
. There might be case differences in the path to Windows's EFI file, what with being Windows, and all.
Windows installed in BIOS-MBR mode
Throughout this section, it is assumed your Windows partition is /dev/sda1
. A different partition will change every instance of hd0,msdos1. First, find the UUID of the NTFS file system of the Windows's SYSTEM PARTITION where the bootmgr
and its files reside. For example, if Windows bootmgr
exists at /media/SYSTEM_RESERVED/bootmgr
:
For Windows Vista/7/8/8.1:
# grub-probe --target=fs_uuid /media/SYSTEM_RESERVED/bootmgr 69B235F6749E84CE
# grub-probe --target=hints_string /media/SYSTEM_RESERVED/bootmgr --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1
Then, add the below code to /etc/grub.d/40_custom
or /boot/grub/custom.cfg
and regenerate grub.cfg
with grub-mkconfig
as explained above to boot Windows (XP, Vista, 7 or 8) installed in BIOS-MBR mode:
For Windows Vista/7/8/8.1:
if [ "${grub_platform}" == "pc" ]; then menuentry "Microsoft Windows Vista/7/8/8.1 BIOS-MBR" { insmod part_msdos insmod ntfs insmod search_fs_uuid insmod ntldr search --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 69B235F6749E84CE ntldr /bootmgr } fi
For Windows XP:
if [ "${grub_platform}" == "pc" ]; then menuentry "Microsoft Windows XP" { insmod part_msdos insmod ntfs insmod search_fs_uuid insmod ntldr search --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 69B235F6749E84CE ntldr /ntldr } fi
/etc/grub.d/40_custom
can be used as a template to create /etc/grub.d/nn_custom
. Where nn
defines the precendence, indicating the order the script is executed. The order scripts are executed determine the placement in the grub boot menu.
LVM
If you use LVM for your /boot
, make sure that the lvm
module is preloaded:
/etc/default/grub
GRUB_PRELOAD_MODULES="lvm"
RAID
GRUB provides convenient handling of RAID volumes. You need to add insmod mdraid09
or mdraid1x
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 --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
.
Multiple entries
For tips on managing multiple GRUB entries, for example when using both linux and linux-lts kernels, see GRUB/Tips and tricks#Multiple entries.
Encryption
Root partition
To encrypt a root filesystem to be used with GRUB, add the encrypt
hook or the sd-encrypt
hook (if using systemd hooks) to mkinitcpio. See dm-crypt/System configuration#mkinitcpio for details, and Mkinitcpio#Common hooks for alternative encryption hooks.
If using the encrypt
hook, add cryptdevice
to /etc/default/grub
. In the example below, the sda2
partition has been encrypted as /dev/mapper/cryptroot
:
/etc/default/grub
GRUB_CMDLINE_LINUX="cryptdevice=/dev/sda2:cryptroot"
If using the sd-encrypt
hook, add luks.uuid
:
/etc/default/grub
GRUB_CMDLINE_LINUX="luks.uuid=UUID"
where UUID is the UUID of the LUKS-encrypted device.
Be sure to #Generate the main configuration file when done.
For further information about bootloader configuration for encrypted devices, see Dm-crypt/System configuration#Boot loader.
Boot partition
GRUB can be set 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. /boot
is not required to be kept in a separate partition; it may also stay under the system's root /
directory tree.
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
Be sure to #Generate the main configuration file while the partition containing /boot
is mounted.
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. You can use a keyfile to avoid 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:
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 starting of the disk(MBR) or at the starting of a partition or as an EFI file in the ESP in the case of UEFI.
Chainloading a partition
set root=(hdX,Y) chainloader +1 boot
X=0,1,2... Y=1,2,3...
For example to chainload Windows stored in the first partiton of the first hard disk,
set root=(hd0,1) chainloader +1 boot
Similarly GRUB installed to a partition can be chainloaded.
Chainloading a disk/drive
set root=hdX chainloader +1 boot
Chainloading Windows/Linux installed in UEFI mode
insmod ntfs set root=(hd0,gpt4) chainloader (${root})/EFI/Microsoft/Boot/bootmgfw.efi boot
insmod ntfs used for loading the ntfs file system module for loading Windows. (hd0,gpt4) or /dev/sda4 is my EFI System Partition (ESP). 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.
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 EFI), again change the lines accordingly:
set root=(hd0,5) linux (hdX,Y)/vmlinuz-linux root=/dev/sda6 initrd (hdX,Y)/initramfs-linux.img boot
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
Intel BIOS not booting GPT
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.
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
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.
Enable debug messages
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).
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}
.
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#Switch to efivarfs[broken link: invalid section]. - Without
--target
or--directory
option, grub-install cannot determine for which firmware to install. In such casesgrub-install
will printsource_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 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).
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)
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 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 in the official repositories.
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 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.
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 -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' doesn't 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 not found
A setting in Windows 8 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.
See also
- Official GRUB Manual - https://www.gnu.org/software/grub/manual/grub.html
- Ubuntu wiki page for GRUB - https://help.ubuntu.com/community/Grub2
- GRUB wiki page describing steps to compile for UEFI systems - https://help.ubuntu.com/community/UEFIBooting
- Wikipedia's page on BIOS Boot partition
- http://members.iinet.net/~herman546/p20/GRUB2%20Configuration%20File%20Commands.html - quite complete description of how to configure GRUB