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

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

## UEFI systems

Note:

- It is recommended to read and understand the Unified Extensible Firmware Interface, Partitioning#GUID Partition Table and Arch boot process#Under UEFI pages.
- When installing to use UEFI it is important to boot the installation media in UEFI mode, otherwise efibootmgr will not be able to add the GRUB UEFI boot entry. Installing to the fallback boot path will still work even in BIOS mode since it does not touch the NVRAM.
- To boot from a disk using UEFI, an EFI system partition is required. Follow EFI system partition#Check for an existing partition to find out if you have one already, otherwise you need to create it.
- This whole article assumes that inserting additional GRUB2 modules via `insmod` is possible. As discussed in Unified Extensible Firmware Interface/Secure Boot#shim with key and GRUB, this is not the case on UEFI systems with Secure Boot enabled. If you want to use any additional GRUB module that is not included in the standard GRUB EFI file `grubx64.efi` on a Secure Boot system, you have to re-generate the GRUB EFI `grubx64.efi` with `grub-mkstandalone` with the additional GRUB modules included.

### 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 workarounds found, for their hardware-specific case. A /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 boot loader while efibootmgr is used by the GRUB installation script to write boot entries to NVRAM.

Then follow the below steps to install GRUB to your disk:

1. Mount the EFI system partition and in the remainder of this section, substitute `esp` with its mount point.
2. Choose a boot loader 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/`.

    Note:
    - Make sure to install the packages and run the `grub-install` command from the system in which GRUB will be installed as the boot loader. That means if you are booting from the live installation environment, you need to be inside the chroot when running `grub-install`. If for some reason it is necessary to run `grub-install` from outside of the installed system, append the `--boot-directory=` option with the path to the mounted `/boot` directory, e.g `--boot-directory=/mnt/boot`.
    - Some motherboards cannot handle `bootloader-id` with spaces in it.

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

https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot#sbctl After the above installation completed, the main GRUB directory is located at `/boot/grub/`. Read /Tips and tricks#Alternative install method for how to specify an alternative location. Note that `grub-install` also tries to create an entry in the firmware boot manager, named `GRUB` in the above example – this will, however, fail if your boot entries are full; use efibootmgr to remove unnecessary entries.

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

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. If you are installing GRUB on a Mac, you will have to use this option.
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 boot loaders do not use a MBR bootcode or partition boot sector at all.

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

### Secure Boot support

GRUB fully supports secure boot utilising either CA keys or shim, the installation command however is different depending on which you intend to use.

To make use of CA Keys the command is

    # grub-install --target=x86_64-efi --efi-directory=esp --bootloader-id=GRUB --modules="tpm" --disable-shim-lock

To make use of shim-lock the command is

    # grub-install --target=x86_64-efi --efi-directory=esp --bootloader-id=GRUB --modules="normal test efi_gop efi_uga search echo linux all_video gfxmenu gfxterm_background gfxterm_menu gfxterm loadenv configfile tpm"

After installation see Secure Boot for instructions on implementing it

Warning:

- Incorrectly configuring Secure Boot can render your system unbootable. If for any reason you cannot boot after enabling secure boot then you should disable it in firmware and reboot the system.
- Loading unnecessary modules in your bootloader has the potential to present a security risk, only use these commands if you need them.

## 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 boot loader 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 if that is already installed.) Then do:

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

where `i386-pc` is deliberately used regardless of your actual architecture, and `/dev/sdX` is the disk (not a partition) where GRUB is to be installed. For example `/dev/sda` or `/dev/nvme0n1`, or `/dev/mmcblk0`. See Device file#Block device names for a description of the block device naming scheme.

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 /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.

## 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 /Tips and tricks for more information.

Note: Remember to always generate the main configuration file after making changes to `/etc/default/grub` and/or files in `/etc/grub.d/`.
Warning: Update/reinstall the boot loader (see #UEFI systems or #BIOS systems) if a new GRUB version changes the syntax of the configuration file: mismatching configuration can result in an unbootable system.

#### 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:

- 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: `grub-probe: error: failed to get canonical path of /dev/sdaX`. In this case, try using arch-chroot as described in the BBS post.

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

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

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

Tip:

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

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

You can add additional custom menu entries by editing `/etc/grub.d/40_custom` and re-generating `/boot/grub/grub.cfg`. Or you can create `/boot/grub/custom.cfg` and add them there. Changes to `/boot/grub/custom.cfg` do not require re-running grub-mkconfig, since `/etc/grub.d/41_custom` adds the necessary `source` statement to the generated configuration file.

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

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

#### Detecting other operating systems

To have grub-mkconfig search for other installed systems and automatically add them to the menu, install the os-prober package and mount the partitions from which the other systems boot. Then re-run grub-mkconfig. If you get the following output: `Warning: os-prober will not be executed to detect other bootable partitions` then edit `/etc/default/grub` and add/uncomment:

    GRUB_DISABLE_OS_PROBER=false

Then try again.

Note:

- The exact mount point does not matter, os-prober reads the `mtab` to identify places to search for bootable entries.
- Remember to mount the partitions each time you run grub-mkconfig in order to include the other operating systems every time.

Tip: You might also want GRUB to remember the last chosen boot entry, see /Tips and tricks#Recall previous entry.

##### Windows

For Windows installed in UEFI mode, make sure the EFI system partition containing the Windows Boot Manager (`bootmgfw.efi`) is mounted. Run `os-prober` as root to detect and generate an entry for it.

For Windows installed in BIOS mode, mount the Windows system partition (its file system label should be `System Reserved` or `SYSTEM`). Run `os-prober` as root to detect and generate an entry for it.

Note: For Windows installed in BIOS mode:

- NTFS partitions may not always be detected when mounted with the default Linux drivers. If GRUB is not detecting it, try installing NTFS-3G and remounting.

This article or section is out of date.

Reason: Since Windows 7, `bootmgr` is placed in the system partition which is not encrypted. (Discuss in Talk:GRUB)

- Encrypted Windows partitions may need to be decrypted before mounting. For BitLocker, this can be done with cryptsetup or 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.

#### Setting the top-level menu entry

By default, grub-mkconfig sorts the included kernels using `sort -V` and uses the first kernel in that list as the top-level entry. This means that, for example, since `/boot/vmlinuz-linux-lts` is sorted before `/boot/vmlinuz-linux`, if you have both linux-lts and linux installed, the LTS kernel will be the top-level menu entry, which may not be desirable. This can be overridden by specifying `GRUB_TOP_LEVEL=path_to_kernel"` in `/etc/default/grub`. For example, to make the regular kernel be the top-level menu entry, you can use `GRUB_TOP_LEVEL="/boot/vmlinuz-linux"`.

#### LVM

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

Notes: grub-mkconfig is capable of detecting that it needs the `lvm` module, specifying it in `GRUB_PRELOAD_MODULES` is not required. Move warning to #Installation & #Installation_2 or create a Known issues section and document it there. (Discuss in Talk:GRUB)
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

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

Notes: grub-mkconfig is capable of detecting that it needs the `mdraid09` and/or `mdraid1x` modules, specifying them in `GRUB_PRELOAD_MODULES` is not required. Summarize the double grub-install in a note and move it to #Installation; move `set root` stuff to #Custom grub.cfg. (Discuss in Talk:GRUB)

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.

Tip: `/boot` is not required to be kept in a separate partition; it may also stay under the system's root `/` directory tree.
Warning: GRUB 2.06 has limited support for LUKS2. See the #LUKS2 section below for details.

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`.

Make sure to install grub after modifying this option or encrypting the partition.

Without further changes you will be prompted twice for a passphrase: the first for GRUB to unlock the `/boot` mount point in early boot, the second to unlock the root filesystem itself as implemented by the initramfs. You can use a keyfile to avoid this.

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. See /Tips and tricks#Manual configuration of core image for early boot.
- 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 hooks to automount your `/boot` when upgrades need to access related files.

##### LUKS2

- GRUB 2.06 has limited support for LUKS2. See GRUB bug #55093.
- Argon2id (cryptsetup default) and Argon2i PBKDFs are not supported (GRUB bug #59409), only PBKDF2 is.
- `grub-install` does not support creating a core image that could be used for unlocking LUKS2. See the comments below or on grub-gitAUR for a workaround.

Tip: You can use grub-improved-luks2-gitAUR that has been patched for LUKS2 as well as Argon support.

- Use `grub-install` as described in the #Installation section to populate the GRUB module directory. Do not forget to edit `/etc/default/grub` and run `grub-mkconfig -o /boot/grub/grub.cfg`, as described above. The generated EFI binary does not support LUKS2 and will be replaced in the next step.

- Create `/boot/grub/grub-pre.cfg`. Replace `UUID` with the LUKS encrypted device UUID. Use `lvm/NameOfVolumeGroup-NameOfLogicalVolume` instead of `crypto0` if your root drive is on an LVM logical volume inside the LUKS volume, otherwise do not change `crypto0`, this is how GRUB names encrypted volumes.

    /boot/grub/grub-pre.cfg

    set crypto_uuid=UUID
    cryptomount -u $crypto_uuid
    set root=crypto0
    set prefix=($root)/boot/grub
    insmod normal
    normal

Note: Whenever you make changes to `grub-pre.cfg`, you will have to do the following steps again.
Warning: If using Btrfs and `/boot/grub` is located on a subvolume, the prefix must include the full path from the top level Btrfs subvolume (subvolid = `5`). For example, if `/boot/grub` is located on a subvolume called `my_subvolume` (subvolid ≠ `5`), set the line to `set prefix=($root)/my_subvolume/boot/grub`. This is true even if you have set `my_subvolume` as default with `btrfs subvolume set-default` before.

- Add `lvm` if you use LVM. Replace `ext2` with `btrfs` or another filesystem module, if needed. Depending on the settings of your key slot, replace `gcry_sha256` with `gcry_sha512` or `gcry_whirlpool` if need be:

    $ grub-mkimage -p /boot/grub -O x86_64-efi -c grub-pre.cfg -o /tmp/grubx64.efi luks2 part_gpt cryptodisk gcry_rijndael pbkdf2 gcry_sha256 ext2

If it cannot find `grub-pre.cfg`, simply change to `-c /boot/grub/grub-pre.cfg` above.

- Copy to ESP:

    # install -v /tmp/grubx64.efi esp/EFI/GRUB/grubx64.efi

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

If you enter a correct passphrase, but an `Invalid passphrase` error is immediately returned, make sure that the right cryptographic modules are specified. Use `cryptsetup luksDump /dev/nvme0n1p2` and check whether the hash function (SHA-256, SHA-512) matches the modules (`gcry_sha256`, `gcry_sha512`) installed and the PBKDF algorithm is pbkdf2. The hash and PBDKDF algorithms can be changed for existing keys by using `cryptsetup luksConvertKey --hash sha256 --pbkdf pbkdf2 /dev/nvme0n1p2`. Under normal circumstances it should take a few seconds before the passphrase is processed.

### Custom grub.cfg

This article or section needs expansion.

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 entry examples

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 /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
    }

###### "UEFI Firmware Settings" menu entry

    if [ ${grub_platform} == "efi" ]; then
    	menuentry 'UEFI Firmware Settings' --id 'uefi-firmware' {
    		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 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 a unified kernel image

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

    menuentry "Arch Linux" {
    	insmod fat
    	insmod chain
    	search --no-floppy --set=root --fs-uuid FILESYSTEM_UUID
    	chainloader /EFI/Linux/Arch-linux.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 file system 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)
    }

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

    menuentry "configfile hd0,gpt4"  {
            insmod part_gpt
            insmod btrfs
            insmod ext2
            set root='hd0,gpt4'
            configfile /boot/grub/grub.cfg
    }

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

###### Windows installed in UEFI/GPT mode

This mode determines where the Windows boot loader 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 `lsblk --fs` 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 `XXXX-XXXX` 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 XXXX-XXXX
    		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 XXXX-XXXX
    		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 labels

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

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

    menuentry "Arch Linux, session texte" {
      search --label --set=root archroot
      linux /boot/vmlinuz-linux root=/dev/disk/by-label/archroot ro
      initrd /boot/initramfs-linux.img
    }

## Using the command shell

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

GRUB offers multiple shells/prompts. If there is a problem reading the menu but the bootloader is able to find the disk, you will likely be dropped to the "normal" shell:

    grub>

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

    grub rescue>

The rescue shell is a restricted subset of the normal shell, offering much less functionality. If dumped to the rescue shell, first try inserting the "normal" module, then starting the "normal" shell:

    grub rescue> set prefix=(hdX,Y)/boot/grub
    grub rescue> insmod (hdX,Y)/boot/grub/i386-pc/normal.mod
    rescue:grub> normal

### Pager support

GRUB supports pager for reading commands that provide long output (like the `help` command). This works only in normal shell mode and not in rescue mode. To enable pager, in GRUB command shell type:

    sh:grub> set pager=1

### Using the command shell environment to boot operating systems

    grub>

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

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

The other bootloader may be embedded at the start of a partitioned disk (MBR), at the start of a partition or a partitionless disk (VBR), or as an EFI binary in the case of UEFI.

#### Chainloading a partition's VBR

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

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

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

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

Similarly GRUB installed to a partition can be chainloaded.

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

    set root=hdX
    chainloader +1
    boot

#### Chainloading Windows/Linux installed in UEFI mode

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

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

#### Normal loading

See the examples in #Using the rescue console

### Using the rescue console

See #Using the command shell first. If unable to activate the standard shell, one possible solution is to boot using a live CD or some other rescue disk to correct configuration errors and reinstall GRUB. However, such a boot disk is not always available (nor necessary); the rescue console is surprisingly robust.

The available commands in GRUB rescue include `insmod`, `ls`, `set`, and `unset`. This example uses `set` and `insmod`. `set` modifies variables and `insmod` inserts new modules to add functionality.

Before starting, the user must know the location of their `/boot` partition (be it a separate partition, or a subdirectory under their root):

    grub rescue> set prefix=(hdX,Y)/boot/grub

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

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

To expand console capabilities, insert the `linux` module:

    grub rescue> insmod i386-pc/linux.mod

or simply

    grub rescue> insmod linux

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

An example, booting Arch Linux:

    set root=(hd0,5)
    linux /boot/vmlinuz-linux root=/dev/sda5
    initrd /boot/initramfs-linux.img
    boot

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

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

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

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

After successfully booting the Arch Linux installation, users can correct `grub.cfg` as needed and then reinstall GRUB.

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

## GRUB removal

This article or section needs expansion.

Reason: Migrating from BIOS booting to UEFI is not the only case where GRUB could be removed. Section needs to either cover how to remove GRUB installed for UEFI booting or it should be removed altogether as too trivial. (Discuss in Talk:GRUB)

In general, in order to remove grub, one has to do the installation steps in reverse order. Perhaps cleaning any left over at the end. However, before doing anything, one has to decide if, and how, the machine will boot after the removal. Assuming one removes grub because they would like to use another boot loader, a safe, though a bit difficult, method is to make sure the other boot loader is working before removing grub.

In any case, even when removing grub before installing another boot loader, for the UEFI case, run:

    $ efibootmgr

And verify the other boot loader is listed in the `BootOrder` line. If grub was not removed, the other boot loader should be listed before grub. If grub is already removed, grub should not be mentioned in that line. But note this is only a necessary, but not sufficient, condition for the machine to boot with the other boot loader. Neither it is a sufficient condition for the full removal of grub.

For both UEFI and non UEFI machines, `grub-install` was manually run as part of the installation of grub. One of its jobs for the UEFI case was to run the equivalent of `efibootmgr --create`. As part of grub removal, one has to remove the products of `grub-install`. The opposite of `efibootmgr --create` is `efibootmgr --delete-bootnum`, or an equivalent program. One way to obtain the number of the boot entry for the `efibootmgr --delete-bootnum` command is from the output of `efibootmgr` (with no arguments).

`grub-install` creates the `/boot/grub` directory that needs to be removed manually. Though some users will want to keep it, should they want to install grub again.

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

    # dd if=/dev/zero of=/dev/sdX bs=440 count=1

## Troubleshooting

### Unsupported file systems

In case that GRUB does not support the root file system, an alternative `/boot` partition with a supported file system must be created. In some cases, the development version of GRUB grub-gitAUR may have native support for the file system.

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

While GRUB supports F2FS since version 2.0.4, it cannot correctly read its boot files from an F2FS partition that was created with the `extra_attr` flag enabled.

### Enable debug messages

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

Add:

    set pager=1
    set debug=all

to `grub.cfg`.

### 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

- An error that may occur on some UEFI devices is `Could not prepare Boot variable: Read-only file system`. You have to remount `/sys/firmware/efi/efivars` with read-write enabled.

      mount -o remount,rw,nosuid,nodev,noexec --types efivarfs efivarfs /sys/firmware/efi/efivars

  See the Gentoo Wiki on installing the boot loader.
- If you have a problem running grub-install with sysfs or procfs and it says you must run `modprobe efivarfs` try mounting the efivarfs with the command above.
- 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 get `error: esp doesn't look like an EFI partition`, then the partition is most likely not FAT32 formatted.

#### 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.

As another example of creating a GRUB entry in the firmware boot manager, consider `efibootmgr -c`. This assumes that `/dev/sda1` is the EFI System Partition, and is mounted at `/boot/efi`. Which are the default behavior of `efibootmgr`. It creates a new boot option, called "Linux", and puts it at the top of the boot order list. Options may be passed to modify the default behavior. The default OS Loader is `\EFI\arch\grub.efi`.

#### 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 -u

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

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

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

Warning: Make sure to check GRUB support for new file system features before you enable them on your `/boot` file system.

### grub-reboot not resetting

GRUB seems to be unable to write to root Btrfs partitions [2]. If you use grub-reboot to boot into another entry it will therefore be unable to update its on-disk environment. Either run grub-reboot from the other entry (for example when switching between various distributions) or consider a different file system. You can reset a "sticky" entry by executing `grub-editenv create` and setting `GRUB_DEFAULT=0` in your `/etc/default/grub` (do not forget `grub-mkconfig -o /boot/grub/grub.cfg`).

### Old Btrfs prevents installation

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

    # 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

For VirtualBox < 6.1, install GRUB to the default/fallback boot path.

See also VirtualBox#Installation in EFI mode on VirtualBox < 6.1.

### GRUB rescue and encrypted /boot

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

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

    grub rescue> cryptomount <partition>
    grub rescue> insmod normal
    grub rescue> normal

See this blog post for a better description.

### GRUB is installed but the menu is not shown at boot

Check `/etc/default/grub` if `GRUB_TIMEOUT` is set to `0`, in which case set it to a positive number: it sets the number of seconds before the default GRUB entry is loaded. Also check if `GRUB_TIMEOUT_STYLE` is set to `hidden` and set it to `menu`, so that the menu will be shown by default. Then regenerate the main configuration file and reboot to check if it worked.

If it does not work, there may be incompatibility problems with the graphical terminal. Set `GRUB_TERMINAL_OUTPUT` to `console` in `/etc/default/grub` to disable the GRUB graphical terminal.

## See also

- Wikipedia:GNU GRUB
- Official GRUB Manual
- Ubuntu wiki page for GRUB
- GRUB wiki page describing steps to compile for UEFI systems
- Wikipedia:BIOS Boot partition
- How to configure GRUB
- Detecting efi files and booting them from grub

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- GNU

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