Difference between revisions of "GRUB"

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{{Note|
 
{{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]].
 
* 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]].
* This additional partition is only needed on a GRUB, BIOS/GPT partitioning scheme. Previously, for a GRUB, BIOS/MBR partitioning scheme, GRUB used the post-MBR gap for the embedding the {{ic|core.img}}). On GPT, however, there is no guaranteed unused space before the first partition.
+
* 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 {{ic|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]].
 
* 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]].
 
}}
 
}}
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Create a mebibyte partition ({{ic|1=+1M}} with ''fdisk'' or ''gdisk'') on the disk with no file system and with partition type GUID {{ic|21686148-6449-6E6F-744E-656564454649}}.
 
Create a mebibyte partition ({{ic|1=+1M}} with ''fdisk'' or ''gdisk'') on the disk with no file system and with partition type GUID {{ic|21686148-6449-6E6F-744E-656564454649}}.
  
* Select partition type {{ic|BIOS boot}} for [[fdisk]], {{ic|ef02}} for [[gdisk]].
+
* Select partition type {{ic|BIOS boot}} for [[fdisk]].
 +
* Select partition type code {{ic|ef02}} for [[gdisk]].
 
* For [[parted]] set/activate the flag {{ic|bios_grub}} on the partition.
 
* For [[parted]] set/activate the flag {{ic|bios_grub}} on the partition.
  
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=== Master Boot Record (MBR) specific instructions ===
 
=== 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 {{ic|core.img}} ({{bug|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 {{ic|core.img}}).
+
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 {{ic|core.img}} ({{Bug|24103}}). It is advisable to use a partitioning tool that supports 1 MiB [[Partitioning#Partition alignment|partition alignment]] to obtain this space as well as to satisfy other non-512-byte-sector issues (which are unrelated to embedding of {{ic|core.img}}).
  
 
=== Installation ===
 
=== Installation ===
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{{Note|
 
{{Note|
 
* It is recommended to read and understand the [[Unified Extensible Firmware Interface]], [[Partitioning#GUID Partition Table]] and [[Arch boot process#Under UEFI]] pages.
 
* 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 start the install with your machine in UEFI mode. The Arch Linux install media must be UEFI bootable.
+
* 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 [[#Default/fallback boot path|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.
 
* 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.
 
}}
 
}}
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{{Note|
 
{{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.
 
* 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) EFI systems (not to be confused with 32-bit CPUs), replace {{ic|x86_64-efi}} with {{ic|i386-efi}} where appropriate.
+
* 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 {{ic|x86_64-efi}} with {{ic|i386-efi}} where appropriate.
 
}}
 
}}
  
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# [[EFI system partition#Mount the partition|Mount the EFI system partition]] and in the remainder of this section, substitute {{ic|''esp''}} with its mount point.
 
# [[EFI system partition#Mount the partition|Mount the EFI system partition]] and in the remainder of this section, substitute {{ic|''esp''}} with its mount point.
# Choose a bootloader identifier, here named {{ic|GRUB}}. A directory of that name will be created to store the EFI binary in the ESP and this is the name that will appear in the UEFI boot menu to identify the GRUB boot entry.
+
# Choose a bootloader identifier, here named {{ic|GRUB}}. A directory of that name will be created in {{ic|''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 {{ic|grubx64.efi}} to {{ic|''esp''/EFI/GRUB/}} and install its modules to {{ic|/boot/grub/x86_64-efi/}}.
 
# Execute the following command to install the GRUB EFI application {{ic|grubx64.efi}} to {{ic|''esp''/EFI/GRUB/}} and install its modules to {{ic|/boot/grub/x86_64-efi/}}.
  
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After the above install completed the main GRUB directory is located at {{ic|/boot/grub/}}. Note that {{ic|grub-install}} also tries to [[GRUB/Tips and tricks#Create a GRUB entry in the firmware boot manager|create an entry in the firmware boot manager]], named {{ic|GRUB}} in the above example.
 
After the above install completed the main GRUB directory is located at {{ic|/boot/grub/}}. Note that {{ic|grub-install}} also tries to [[GRUB/Tips and tricks#Create a GRUB entry in the firmware boot manager|create an entry in the firmware boot manager]], named {{ic|GRUB}} in the above example.
  
Remember to [[#Generate the main configuration file]] after finalizing [[#Configuration]].  
+
Remember to [[#Generate the main configuration file]] after finalizing the configuration.
  
{{Tip|If you use the option {{ic|--removable}} then GRUB will be installed to {{ic|''esp''/EFI/BOOT/BOOTX64.EFI}} (or {{ic|''esp''/EFI/BOOT/BOOTIA32.EFI}} for the {{ic|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 has a {{ic|BOOT}} folder inside the {{ic|EFI}} folder of the EFI system partition, but its only purpose is to recreate the UEFI boot entry for Windows.}}
+
{{Tip|If you use the option {{ic|--removable}} then GRUB will be installed to {{ic|''esp''/EFI/BOOT/BOOTX64.EFI}} (or {{ic|''esp''/EFI/BOOT/BOOTIA32.EFI}} for the {{ic|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|
 
{{Note|
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See [[#UEFI|UEFI troubleshooting]] in case of problems. Additionally see [[GRUB/Tips and tricks#UEFI further reading]].
 
See [[#UEFI|UEFI troubleshooting]] in case of problems. Additionally see [[GRUB/Tips and tricks#UEFI further reading]].
  
== Generate the main configuration file ==
+
== Configuration ==
  
After the installation, the main configuration file {{ic|grub.cfg}} needs to be generated. The generation process can be influenced by a variety of options in {{ic|/etc/default/grub}} and scripts in {{ic|/etc/grub.d/}}; see [[#Configuration]].  
+
On an installed system, GRUB loads the {{ic|/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.
  
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.  
+
=== Generated grub.cfg ===
  
{{Note|Remember that {{ic|grub.cfg}} has to be re-generated after any change to {{ic|/etc/default/grub}} or files in {{ic|/etc/grub.d/}}.}}
+
This section only covers editing the {{ic|/etc/default/grub}} configuration file. See [[GRUB/Tips and tricks]] for more information.
  
Use the ''grub-mkconfig'' tool to generate {{ic|grub.cfg}}:
+
Remember to always [[#Generate the main configuration file]] after making changes to {{ic|/etc/default/grub}} and/or files in {{ic|/etc/grub.d/}}.
  
# grub-mkconfig -o /boot/grub/grub.cfg
+
==== Generate the main configuration file ====
  
To automatically add entries for other installed operating systems, see [[#Dual booting/Multiple operating systems]].  
+
After the installation, the main configuration file {{ic|/boot/grub/grub.cfg}} needs to be generated. The generation process can be influenced by a variety of options in {{ic|/etc/default/grub}} and scripts in {{ic|/etc/grub.d/}}.  
  
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.
+
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|
 
{{Note|
* The default file path is {{ic|/boot/grub/grub.cfg}}, not {{ic|/boot/grub/i386-pc/grub.cfg}}. The {{Pkg|grub}} package includes a sample {{ic|/boot/grub/grub.cfg}}; ensure your intended changes are written to this file.
+
* Remember that {{ic|/boot/grub/grub.cfg}} has to be re-generated after any change to {{ic|/etc/default/grub}} or files in {{ic|/etc/grub.d/}}.
 +
* The default file path is {{ic|/boot/grub/grub.cfg}}, not {{ic|/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 [https://bbs.archlinux.org/viewtopic.php?pid=1225067#p1225067 BBS post].
 
* 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 [https://bbs.archlinux.org/viewtopic.php?pid=1225067#p1225067 BBS post].
 
}}
 
}}
  
=== Dual booting/Multiple operating systems ===
+
Use the ''grub-mkconfig'' tool to generate {{ic|/boot/grub/grub.cfg}}:
  
To have ''grub-mkconfig'' search for other installed systems and automatically add them to the menu, [[install]] the {{Pkg|os-prober}} package and [[mount]] the partitions that contain the other systems. Then re-run ''grub-mkconfig''.
+
# grub-mkconfig -o /boot/grub/grub.cfg
 +
 
 +
By default the generation scripts automatically add menu entries for all installed Arch Linux [[kernel]]s 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 {{Pkg|linux}} and {{Pkg|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 {{ic|/etc/grub.d/40_custom}} and re-generating {{ic|/boot/grub/grub.cfg}}. Or you can create {{ic|/boot/grub/custom.cfg}} and add them there. Changes to {{ic|/boot/grub/custom.cfg}} do not require re-running ''grub-mkconfig'', since {{ic|/etc/grub.d/40_custom}} adds the necessary {{ic|source}} statement to the generated configuration file.
 +
 
 +
{{Tip|{{ic|/etc/grub.d/40_custom}} can be used as a template to create {{ic|/etc/grub.d/''nn''_custom}}. Where {{ic|''nn''}} defines the precedence, indicating the order the script is executed.  The order scripts are executed determine the placement in the GRUB boot menu. {{ic|''nn''}} should be greater than {{ic|06}} to ensure necessary scripts are executed first.}}
  
==== MS Windows ====
+
See [[#Boot menu entries]] for custom menu entry examples.
  
Partitions containing Windows should be automatically discovered by {{Pkg|os-prober}}. However, if the partition is encrypted, you may need to decrypt the partition before mounting. For BitLocker, this can be done with {{AUR|dislocker}}. This should be sufficient for {{Pkg|os-prober}} to add the correct entry.
+
==== Detecting other operating systems ====
  
== Configuration ==
+
To have ''grub-mkconfig'' search for other installed systems and automatically add them to the menu, [[install]] the {{Pkg|os-prober}} package and [[mount]] the partitions that contain the other systems. Then re-run ''grub-mkconfig''.
  
This section only covers editing the {{ic|/etc/default/grub}} configuration file. See [[GRUB/Tips and tricks]] for more information.
+
===== MS Windows =====
  
Remember to always [[#Generate the main configuration file]] after making changes to {{ic|/etc/default/grub}}.
+
Partitions containing Windows should be automatically discovered by {{Pkg|os-prober}}. However, if the partition is encrypted, you may need to decrypt the partition before mounting. For BitLocker, this can be done with {{AUR|dislocker}}. This should be sufficient for {{Pkg|os-prober}} to add the correct entry.
  
=== Additional arguments ===
+
==== Additional arguments ====
  
 
To pass custom additional arguments to the Linux image, you can set the {{ic|GRUB_CMDLINE_LINUX}} + {{ic|GRUB_CMDLINE_LINUX_DEFAULT}} variables in {{ic|/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 {{ic|GRUB_CMDLINE_LINUX}} is used in the generation.
 
To pass custom additional arguments to the Linux image, you can set the {{ic|GRUB_CMDLINE_LINUX}} + {{ic|GRUB_CMDLINE_LINUX_DEFAULT}} variables in {{ic|/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 {{ic|GRUB_CMDLINE_LINUX}} is used in the generation.
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See [[Kernel parameters]] for more info.
 
See [[Kernel parameters]] for more info.
  
=== LVM ===
+
==== LVM ====
  
 
{{Warning|GRUB does not support thin-provisioned logical volumes.}}
 
{{Warning|GRUB does not support thin-provisioned logical volumes.}}
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}}
 
}}
  
=== RAID ===
+
==== RAID ====
  
 
GRUB provides convenient handling of [[RAID]] volumes. You need to load GRUB modules {{ic|mdraid09}} or {{ic|mdraid1x}} to allow you to address the volume natively:
 
GRUB provides convenient handling of [[RAID]] volumes. You need to load GRUB modules {{ic|mdraid09}} or {{ic|mdraid1x}} to allow you to address the volume natively:
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{{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.}}
 
{{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.}}
  
=== Encryption ===
+
==== Encrypted /boot ====
 
 
==== Root partition ====
 
 
 
{{Merge|#Additional arguments|Encrypted root without {{ic|/boot}} is not special enough to warrant a separate section. Required kernel parameters are already listed in [[dm-crypt/System configuration#Kernel parameters]].}}
 
  
To encrypt a root filesystem to be used with GRUB, add the {{ic|encrypt}} hook or the {{ic|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.
+
GRUB also has special support for booting with an encrypted {{ic|/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 [[dm-crypt/Specialties#Securing the unencrypted_boot partition|unencrypted boot partition]].
  
If using the {{ic|encrypt}} hook, add the {{ic|cryptdevice}} parameter to {{ic|/etc/default/grub}}.
+
{{Note|{{ic|/boot}} is '''not''' required to be kept in a separate partition; it may also stay under the system's root {{ic|/}} directory tree.}}
 
 
{{hc|/etc/default/grub|2=
 
GRUB_CMDLINE_LINUX="cryptdevice=UUID=''device-UUID'':cryptroot"
 
}}
 
  
If using the {{ic|sd-encrypt}} hook, add {{ic|rd.luks.name}}:
+
{{Warning|GRUB does not support LUKS2 headers; see [https://savannah.gnu.org/bugs/?55093 GRUB bug #55093]. Make sure you do not specify {{ic|luks2}} for the type parameter when creating the encrypted partition using {{ic|cryptsetup luksFormat}}.}}
 
 
{{hc|/etc/default/grub|2=
 
GRUB_CMDLINE_LINUX="rd.luks.name=''device-UUID''=cryptroot"
 
}}
 
 
 
where ''device-UUID'' is the UUID of the LUKS-encrypted device.
 
 
 
Be sure to [[#Generate the main configuration file|generate the main configuration file]] when done.
 
 
 
For further information about bootloader configuration for encrypted devices, see [[dm-crypt/System configuration#Boot loader]].
 
 
 
{{Note|If you wish to encrypt {{ic|/boot}} either as a separate partition or part of the {{ic|/}} partition, further setup is required. See [[#Boot partition]].}}
 
 
 
{{Tip|If you are upgrading from a working GRUB Legacy configuration, check {{ic|/boot/grub/menu.lst.pacsave}} for the correct device/label to add. Look for them after the text {{ic|kernel /vmlinuz-linux}}.}}
 
 
 
==== 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 [[dm-crypt/Specialties#Securing the unencrypted_boot partition|unencrypted boot partition]]. {{ic|/boot}} is '''not''' required to be kept in a separate partition; it may also stay under the system's root {{ic|/}} directory tree.
 
 
 
{{Warning|GRUB does not support LUKS2 headers. Make sure you do not specify {{ic|luks2}} for the type parameter when creating the encrypted partition using {{ic|cryptsetup luksFormat}}.}}
 
  
 
To enable this feature encrypt the partition with {{ic|/boot}} residing on it using [[LUKS]] as normal. Then add the following option to {{ic|/etc/default/grub}}:
 
To enable this feature encrypt the partition with {{ic|/boot}} residing on it using [[LUKS]] as normal. Then add the following option to {{ic|/etc/default/grub}}:
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}}
 
}}
  
This option is used by grub-install to generate the grub {{ic|core.img}}, so make sure to [[#Installation | install grub]] after modifying this option.
+
This option is used by grub-install to generate the grub {{ic|core.img}}, so make sure to [[#Installation|install grub]] after modifying this option.
  
Without further changes you will be prompted twice for a passhrase: the first for GRUB to unlock the {{ic|/boot}} mount point in early boot, the second to unlock the root filesystem itself as described in [[#Root partition]]. You can use a [[Dm-crypt/Device encryption#With a keyfile embedded in the initramfs|keyfile]] to avoid this.
+
Without further changes you will be prompted twice for a passhrase: the first for GRUB to unlock the {{ic|/boot}} mount point in early boot, the second to unlock the root filesystem itself as implemented by the initramfs. You can use a [[Dm-crypt/Device encryption#With a keyfile embedded in the initramfs|keyfile]] to avoid this.
  
 
{{Warning|
 
{{Warning|
Line 250: Line 237:
 
{{Tip|1=You can use [https://bbs.archlinux.org/viewtopic.php?id=234607 pacman hooks]to automount your {{ic|/boot}} when upgrades need to access related files.}}
 
{{Tip|1=You can use [https://bbs.archlinux.org/viewtopic.php?id=234607 pacman hooks]to automount your {{ic|/boot}} when upgrades need to access related files.}}
  
=== Boot menu entries ===
+
=== Custom grub.cfg ===
 +
 
 +
{{Expansion|Add instructions on how to write a custom {{ic|/boot/grub/grub.cfg}}. See [[User:Eschwartz/Grub]] for a proposed draft.|section=Manually generate grub.cfg}}
 +
 
 +
This section describes the manual creation of GRUB boot entries in {{ic|/boot/grub/grub.cfg}} instead of relying on ''grub-mkconfig''.
 +
 
 +
A basic GRUB config file uses the following options:
 +
* {{ic|(hd''X'',''Y'')}} is the partition ''Y'' on disk ''X'', partition numbers starting at 1, disk numbers starting at 0
 +
* {{ic|1=set default=''N''}} is the default boot entry that is chosen after timeout for user action
 +
* {{ic|1=set timeout=''M''}} is the time ''M'' to wait in seconds for a user selection before default is booted
 +
* {{ic|<nowiki>menuentry "title" {entry options}</nowiki>}} is a boot entry titled {{ic|title}}
 +
* {{ic|1=set root=(hd''X'',''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 ({{ic|/}})
  
{{Merge|Multiboot USB drive|Same topic, substituting USB drives for SATA drives is trivial.}}
+
==== Boot menu entries ====
  
The best way to add other entries is editing {{ic|/etc/grub.d/40_custom}} or {{ic|/boot/grub/custom.cfg}}. The entries in this file will be automatically added after rerunning {{ic|grub-mkconfig}}.
+
{{Tip|These boot entries can also be used when using a {{ic|/boot/grub/grub.cfg}} generated by ''grub-mkconfig''. Add them to {{ic|/etc/grub.d/40_custom}} and [[#Generate the main configuration file|re-generate the main configuration file]] or add them to {{ic|/boot/grub/custom.cfg}}.}}
  
 
For tips on managing multiple GRUB entries, for example when using both {{Pkg|linux}} and {{Pkg|linux-lts}} kernels, see [[GRUB/Tips and tricks#Multiple entries]].
 
For tips on managing multiple GRUB entries, for example when using both {{Pkg|linux}} and {{Pkg|linux-lts}} kernels, see [[GRUB/Tips and tricks#Multiple entries]].
  
==== GRUB commands ====
+
For [[Archiso]] and [[Archboot]] boot menu entries see [[Multiboot USB drive#Boot entries]].
 +
 
 +
===== GRUB commands =====
  
===== "Shutdown" menu entry =====
+
====== "Shutdown" menu entry ======
  
 
{{bc|
 
{{bc|
Line 269: Line 269:
 
}}
 
}}
  
===== "Restart" menu entry =====
+
====== "Restart" menu entry ======
  
 
{{bc|
 
{{bc|
Line 278: Line 278:
 
}}
 
}}
  
===== "Firmware setup" menu entry (UEFI only) =====
+
====== "Firmware setup" menu entry (UEFI only) ======
  
 
{{bc|1=
 
{{bc|1=
Line 288: Line 288:
 
}}
 
}}
  
==== EFI binaries ====
+
===== EFI binaries =====
  
 
When launched in UEFI mode, GRUB can chainload other EFI binaries.
 
When launched in UEFI mode, GRUB can chainload other EFI binaries.
Line 302: Line 302:
 
}}
 
}}
  
===== UEFI Shell =====
+
====== UEFI Shell ======
  
 
You can launch [[Unified Extensible Firmware Interface#UEFI Shell|UEFI Shell]] by using placing it in the root of the [[EFI system partition]] and adding this menu entry:
 
You can launch [[Unified Extensible Firmware Interface#UEFI Shell|UEFI Shell]] by using placing it in the root of the [[EFI system partition]] and adding this menu entry:
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{{bc|1=
 
{{bc|1=
 
menuentry "UEFI Shell" {
 
menuentry "UEFI Shell" {
 +
insmod fat
 
insmod chain
 
insmod chain
search --set=root --file /shellx64.efi
+
search --no-floppy --set=root --file /shellx64.efi
 
chainloader /shellx64.efi
 
chainloader /shellx64.efi
 
}
 
}
 
}}
 
}}
  
===== gdisk =====
+
====== gdisk ======
  
 
Download the [[gdisk#gdisk EFI application|gdisk EFI application]] and copy {{ic|gdisk_x64.efi}} to {{ic|''esp''/EFI/tools/}}.
 
Download the [[gdisk#gdisk EFI application|gdisk EFI application]] and copy {{ic|gdisk_x64.efi}} to {{ic|''esp''/EFI/tools/}}.
Line 320: Line 321:
 
{{bc|1=
 
{{bc|1=
 
menuentry "gdisk" {
 
menuentry "gdisk" {
 +
insmod fat
 
insmod chain
 
insmod chain
search --set=root --file /EFI/tools/gdisk_x64.efi
+
search --no-floppy --set=root --file /EFI/tools/gdisk_x64.efi
 
chainloader /EFI/tools/gdisk_x64.efi
 
chainloader /EFI/tools/gdisk_x64.efi
 
}
 
}
 
}}
 
}}
  
===== Chainloading an Arch Linux .efi file =====
+
====== 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:
 
If you have an ''.efi'' file generated from following [[Secure Boot]] or other means, you can add it to the boot menu. For example:
Line 332: Line 334:
 
{{bc|1=
 
{{bc|1=
 
menuentry "Arch Linux .efi" {
 
menuentry "Arch Linux .efi" {
 +
insmod fat
 
insmod chain
 
insmod chain
search --set=root --fs-uuid ''FILESYSTEM_UUID''
+
search --no-floppy --set=root --fs-uuid ''FILESYSTEM_UUID''
 
chainloader /EFI/arch/vmlinuz.efi
 
chainloader /EFI/arch/vmlinuz.efi
 
}
 
}
 
}}
 
}}
  
==== Dual-booting ====
+
===== Dual-booting =====
  
===== GNU/Linux menu entry =====
+
====== GNU/Linux ======
  
 
Assuming that the other distribution is on partition {{ic|sda2}}:
 
Assuming that the other distribution is on partition {{ic|sda2}}:
Line 357: Line 360:
 
menuentry "Other Linux" {
 
menuentry "Other Linux" {
 
         # assuming that UUID is 763A-9CB6
 
         # assuming that UUID is 763A-9CB6
search --set=root --fs-uuid 763A-9CB6
+
search --no-floppy --set=root --fs-uuid 763A-9CB6
  
 
         # search by label OTHER_LINUX (make sure that partition label is unambiguous)
 
         # search by label OTHER_LINUX (make sure that partition label is unambiguous)
         #search --set=root --label OTHER_LINUX
+
         #search --no-floppy --set=root --label OTHER_LINUX
  
 
linux /boot/vmlinuz (add other options here as required, for example: root=UUID=763A-9CB6)
 
linux /boot/vmlinuz (add other options here as required, for example: root=UUID=763A-9CB6)
Line 367: Line 370:
 
}}
 
}}
  
===== Windows installed in UEFI/GPT Mode menu entry =====
+
====== 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.
+
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.}}
 
{{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.}}
Line 378: Line 381:
 
insmod part_gpt
 
insmod part_gpt
 
insmod fat
 
insmod fat
insmod search_fs_uuid
 
 
insmod chain
 
insmod chain
search --fs-uuid --set=root $hints_string $fs_uuid
+
search --no-floppy --fs-uuid --set=root $hints_string $fs_uuid
 
chainloader /EFI/Microsoft/Boot/bootmgfw.efi
 
chainloader /EFI/Microsoft/Boot/bootmgfw.efi
 
}
 
}
Line 404: Line 406:
 
These two commands assume the ESP Windows uses is mounted at {{ic|''esp''}}. There might be case differences in the path to Windows's EFI file, what with being Windows, and all.
 
These two commands assume the ESP Windows uses is mounted at {{ic|''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 =====
+
====== Windows installed in BIOS/MBR mode ======
  
 
{{Note|GRUB supports booting {{ic|bootmgr}} directly and [https://www.gnu.org/software/grub/manual/grub.html#Chain_002dloading chainloading] of partition boot sector is no longer required to boot Windows in a BIOS/MBR setup.}}
 
{{Note|GRUB supports booting {{ic|bootmgr}} directly and [https://www.gnu.org/software/grub/manual/grub.html#Chain_002dloading chainloading] of partition boot sector is no longer required to boot Windows in a BIOS/MBR setup.}}
  
{{Warning|It is the '''system partition''' that has {{ic|/bootmgr}}, not your "real" Windows partition (usually {{ic|C:}}). In {{ic|blkid}} output, the system partition is the one with {{ic|1=LABEL="SYSTEM RESERVED"}} or {{ic|1=LABEL="SYSTEM"}} and is only about 100 to 200 MiB in size (much like the boot partition for Arch). See [[Wikipedia:System partition and boot partition]] for more info.}}
+
{{Warning|It is the '''system partition''' that has {{ic|/bootmgr}}, not your "real" Windows partition (usually {{ic|C:}}). The system partition's [[Persistent block device naming#by-label|filesystem label]] is {{ic|System Reserved}} or {{ic|SYSTEM}} and the partition is only about 100 to 549 MiB in size. See [[Wikipedia:System partition and boot partition]] for more information.}}
  
Throughout this section, it is assumed your Windows partition is {{ic|/dev/sda1}}. A different partition will change every instance of {{ic|hd0,msdos1}}. Add the below code to {{ic|/etc/grub.d/40_custom}} or {{ic|/boot/grub/custom.cfg}} and regenerate {{ic|grub.cfg}} with {{ic|grub-mkconfig}} as explained above to boot Windows (XP, Vista, 7, 8 or 10) installed in BIOS/MBR mode:
+
Throughout this section, it is assumed your Windows partition is {{ic|/dev/sda1}}. A different partition will change every instance of {{ic|hd0,msdos1}}.
  
 
{{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]] .}}
 
{{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]] .}}
Line 423: Line 425:
 
insmod part_msdos
 
insmod part_msdos
 
insmod ntfs
 
insmod ntfs
insmod search_fs_uuid
 
 
insmod ntldr     
 
insmod ntldr     
search --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 ''XXXXXXXXXXXXXXXX''
+
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 ''XXXXXXXXXXXXXXXX''
 
ntldr /bootmgr
 
ntldr /bootmgr
 
}
 
}
Line 438: Line 439:
 
insmod part_msdos
 
insmod part_msdos
 
insmod ntfs
 
insmod ntfs
insmod search_fs_uuid
 
 
insmod ntldr     
 
insmod ntldr     
search --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 ''XXXXXXXXXXXXXXXX''
+
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 ''XXXXXXXXXXXXXXXX''
 
ntldr /ntldr
 
ntldr /ntldr
 
}
 
}
Line 455: Line 455:
 
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.
 
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.
 
}}
 
}}
 
{{ic|/etc/grub.d/40_custom}} can be used as a template to create {{ic|/etc/grub.d/''nn''_custom}}. Where {{ic|''nn''}} defines the precedence, indicating the order the script is executed.  The order scripts are executed determine the placement in the grub boot menu.
 
 
{{Note|{{ic|nn}} should be greater than 06 to ensure necessary scripts are executed first.}}
 
  
 
== Using the command shell ==
 
== Using the command shell ==
Line 520: Line 516:
 
==== Chainloading Windows/Linux installed in UEFI mode ====
 
==== Chainloading Windows/Linux installed in UEFI mode ====
  
  insmod ntfs
+
  insmod fat
 
  set root=(hd0,gpt4)
 
  set root=(hd0,gpt4)
 
  chainloader (${root})/EFI/Microsoft/Boot/bootmgfw.efi
 
  chainloader (${root})/EFI/Microsoft/Boot/bootmgfw.efi
 
  boot
 
  boot
  
{{ic|insmod ntfs}} is used for loading the ntfs file system module for loading Windows.
+
{{ic|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 my EFI system partition (ESP).
+
{{ic|(hd0,gpt4)}} or {{ic|/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.
+
The entry in the {{ic|chainloader}} line specifies the path of the ''.efi'' file to be chain-loaded.
  
 
==== Normal loading ====
 
==== Normal loading ====
Line 623: Line 619:
  
 
Then you need to initialize GRUB graphical terminal ({{ic|gfxterm}}) with proper video mode ({{ic|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).
 
Then you need to initialize GRUB graphical terminal ({{ic|gfxterm}}) with proper video mode ({{ic|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).
 +
 +
{{Remove|''grub-install'' copies {{ic|unicode.pf2}} to {{ic|/boot/grub/fonts/}}, there is no need to do it manually.}}
  
 
Copy {{ic|/usr/share/grub/unicode.pf2}} to {{ic|${GRUB_PREFIX_DIR}}} ({{ic|/boot/grub/}} in case of BIOS and UEFI systems). If GRUB UEFI was installed with {{ic|1=--boot-directory=''esp''/EFI}} set, then the directory is {{ic|''esp''/EFI/grub/}}:
 
Copy {{ic|/usr/share/grub/unicode.pf2}} to {{ic|${GRUB_PREFIX_DIR}}} ({{ic|/boot/grub/}} in case of BIOS and UEFI systems). If GRUB UEFI was installed with {{ic|1=--boot-directory=''esp''/EFI}} set, then the directory is {{ic|''esp''/EFI/grub/}}:
Line 633: Line 631:
  
 
Then, in the {{ic|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.
 
Then, in the {{ic|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):
 
After that add the following code (common to both BIOS and UEFI):
  
insmod font
+
{{bc|1=
 
+
loadfont "unicode"
if loadfont ${prefix}/fonts/unicode.pf2
+
set gfxmode=auto
then
+
set gfxpayload=keep
    insmod gfxterm
+
insmod all_video
    set gfxmode=auto
+
insmod gfxterm
    set gfxpayload=keep
+
terminal_output gfxterm
    terminal_output gfxterm
+
}}
fi
 
 
 
As you can see for gfxterm (graphical terminal) to function properly, {{ic|unicode.pf2}} font file should exist in {{ic|1=${GRUB_PREFIX_DIR}}}.
 
  
 
=== msdos-style error message ===
 
=== msdos-style error message ===

Latest revision as of 22:26, 8 December 2018

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.

Contents

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-legacyAUR, where 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:

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:

  1. Mount the EFI system partition and in the remainder of this section, substitute esp with its mount point.
  2. 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.
  3. 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.

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.

Remember to always #Generate the main configuration file after making changes to /etc/default/grub and/or files in /etc/grub.d/.

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.

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:

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 entries 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

Partitions containing Windows should be automatically discovered by os-prober. However, if the partition is encrypted, you may need to decrypt the partition before mounting. For BitLocker, this can be done with dislockerAUR. 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.

LVM

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

If you use LVM for your /boot or / root partition, 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 load GRUB modules mdraid09 or mdraid1x to allow you to address the volume natively:

/etc/default/grub
GRUB_PRELOAD_MODULES="... mdraid09 mdraid1x"

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 BIOS systems, 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.

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 you do not specify luks2 for the type parameter 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.

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

Custom grub.cfg

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Add instructions on how to write a custom /boot/grub/grub.cfg. See User:Eschwartz/Grub for a proposed draft. (Discuss in Talk:GRUB#Manually generate grub.cfg)

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" {entry options} is a boot entry titled title
  • set root=(hdX,Y) sets the boot partition, where the kernel and GRUB modules are stored (boot need not be a separate partition, and may simply be a directory under the "root" partition (/)

Boot menu entries

Tip: These boot entries can also be used when using a /boot/grub/grub.cfg generated by grub-mkconfig. 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
GNU/Linux

Assuming that the other distribution 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 --no-floppy --set=root --fs-uuid 763A-9CB6

        # search by label OTHER_LINUX (make sure that partition label is unambiguous)
        #search --no-floppy --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)
}
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.
if [ "${grub_platform}" == "efi" ]; then
	menuentry "Microsoft Windows Vista/7/8/8.1 UEFI/GPT" {
		insmod part_gpt
		insmod fat
		insmod chain
		search --no-floppy --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.

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

# grub-probe --target=fs_uuid esp/EFI/Microsoft/Boot/bootmgfw.efi
1ce5-7f28

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:

# 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
Note: GRUB supports booting bootmgr directly and chainloading of partition boot sector is no longer required to boot Windows in a BIOS/MBR setup.
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.

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

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 .

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

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

if [ "${grub_platform}" == "pc" ]; then
	menuentry "Microsoft Windows Vista/7/8/8.1/10 BIOS/MBR" {
		insmod part_msdos
		insmod ntfs
		insmod ntldr     
		search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 XXXXXXXXXXXXXXXX
		ntldr /bootmgr
	}
fi

For Windows XP:

if [ "${grub_platform}" == "pc" ]; then
	menuentry "Microsoft Windows XP" {
		insmod part_msdos
		insmod ntfs
		insmod ntldr     
		search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 XXXXXXXXXXXXXXXX
		ntldr /ntldr
	}
fi
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 starting of the disk (MBR) or at the starting of a partition or as an EFI binary 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 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-gitAUR 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).

Tango-edit-cut.pngThis section is being considered for removal.Tango-edit-cut.png

Reason: grub-install copies unicode.pf2 to /boot/grub/fonts/, there is no need to do it manually. (Discuss in Talk: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 [2]. 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 [3]. 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.

See also VirtualBox#Installation in EFI mode.

See also