The GRUB2 Bootloader – Installation and Configuration

Michael Wu, The Fedora Documentation Team Versi unknown Last review: 2012-05-11

GRUB2 is the latest version of GNU GRUB, the GRand Unified Bootloader. A bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel. In Fedora, the kernel is Linux. The kernel then initializes the rest of the operating system.

GRUB2 is the follower of the previous version GRUB (version 0.9x). The original version is available under the name GRUB Legacy.

Since Fedora 16, GRUB2 has been the default bootloader on x86 BIOS systems. For upgrades of BIOS systems, the default is also to install GRUB2, but you can opt to skip bootloader configuration entirely.

Discovering the firmware type

To discover what firmware your machine uses, run the following command:

$ [ -d /sys/firmware/efi ] && echo UEFI || echo BIOS

The output returns only UEFI or BIOS, depending on the firmware your machine runs.

Installing GRUB2 on a BIOS system

Normally, GRUB2 will be installed and set up by the installer, Anaconda, during the installation process. You will probably never have to deal with manual installation of GRUB2. However, in certain situations , you will want to install GRUB2 manually, especially if you need to repair the existing GRUB2 installation or you want to change its configuration.

This procedure shows the steps to install GRUB2 on your Master Boot Record (MBR) of your primary hard disk.

Before you start

  • Make sure you have the the GRUB2 packages and the os-prober package installed in your system.

    $ dnf list installed | grep grub

  • To automatically collect information about your disks and operating systems installed on them, the os-prober package needs to be installed on your system.

Procedure

  1. List block devices available on the system.

    $ lsblk

  2. Identify the primary hard disk. Usually, it is the sda device.

  3. Install GRUB2 in the MBR of the primary hard disk.

    # grub2-install /dev/sda

  4. Create a configuration file for GRUB2.

    # grub2-mkconfig -o /boot/grub2/grub.cfg

  5. Reboot your computer to boot with the newly installed bootloader.

More information

  • The grub2-mkconfig command creates a new configuration based on the currently running system. It collects information from the /boot partition (or directory), from the /etc/default/grub file, and the customizable scripts in /etc/grub.d.

  • The configuration format is changing with time, and a new configuration file can become slightly incompatible with the older versions of the bootloader. Always run grub2-install before you create the configuration file with grub2-mkconfig.

  • In Fedora, it is generally safe to edit /boot/grub2/grub.cfg manually. Grubby in Fedora patches the configuration when a kernel update is performed and will try to not make any other changes than what is necessary. Manual changes can be overwritten with grub2-mkconfig when the system gets upgraded with Anaconda. Customizations placed in /etc/grub.d/40_custom or /boot/grub2/custom.cfg files will survive running the grub2-mkconfig command.

Installing GRUB2 on a UEFI system

Normally, GRUB2 will be installed and set up by the installer, Anaconda, during the installation process. You will probably never have to deal with manual installation of GRUB2. However, in certain situations , you will want to install GRUB2 manually, especially if you need to repair the existing GRUB2 installation or you want to change its configuration.

This procedure shows the steps to install GRUB2 on a UEFI system on Fedora 18 or newer. The procedure consists of four parts.

Creating an EFI System Partition

The UEFI firmware requires to boot from an EFI System Partition on a disk with a GPT label. To create such a partition:

  1. List available block devices to find a place to create your ESP.

    $ lsblk -f -p

  2. Create at least a 128 MiB disk partition using a GPT label on the primary hard disk.

    # gdisk /dev/sda

    For the sake of this procedure, we assume that the created partition is recognized as /dev/sda1.

  3. Format the partition with the FAT32 file system.

    # mkfs.vfat /dev/sda1

  4. Create the /boot/efi directory as a mount point for the new partition.

    # mkdir /boot/efi

  5. Mount the partition to the /boot/efi mount point.

    # mount /dev/sda1 /boot/efi

  6. Proceed to the next part.

Install the bootloader files

In order to use GRUB2 with on the UEFI systems, you need to install or re-install appropriate packages:

  1. Re-install the necessary packages.

    # dnf reinstall grub2-efi grub2-efi-modules shim-\*

  2. If the above command ends with an error, install the packages.

    # dnf install grub2-efi grub2-efi-modules shim-\*
More information

  • This installs the signed shim and the GRUB2 binary.

Create a GRUB2 configuration

If you already have a working GRUB2 EFI configuration file, you do not need to do anything else.

Otherwise, create the configuration file using the grub2-mkconfig command.

# grub2-mkconfig -o /boot/grub2/grub.cfg
More information

  • Under EFI, GRUB2 looks for its configuration in /boot/efi/EFI/fedora/grub.cfg, however the postinstall script of grub2-common installs a small shim which chains to the standard configuration at /boot/grub2/grub.cfg which is generated above. To reset this shim to defaults, delete the existing /boot/efi/EFI/fedora/grub.cfg and then reinstall grub2-common.

    # rm -f /boot/efi/EFI/fedora/grub.cfg
# dnf reinstall grub2-common

  • For newly installed kernels to work, grubby expects /etc/grub2-efi.cfg to be a symlink to the real grub.cfg (for example /boot/grub2/grub.cfg).

Solving problems with UEFI bootloader

When you power on your system, your firmware will look for EFI variables that tell it how to boot. On running systems, which have booted into the EFI mode and their EFI runtime services are working correctly, you can configure your boot menu with efibootmgr.

If not, shim can help you bootstrap. The EFI program /boot/efi/EFI/BOOT/fallback.efi will look for files called BOOT.CSV in your ESP and will add boot entries corresponding to them. The shim command provides its own BOOT.CSV file that will add an entry for grub2-efi.

During the boot process, you can use the EFI Shell to invoke the fallback.efi profile to boot the system:

  1. Enter the boot partition.

    > fs0:

  2. Navigate into the EFI\BOOT directory.

    > cd EFI\BOOT

  3. Invoke the fallback.efi profile.

    > fallback.efi
More information

  • If you have no boot entries at all, then just booting off your disk in UEFI mode should automatically invoke /boot/efi/EFI/BOOT/BOOTX64.EFI, which will, in turn, invoke fallback.efi.

  • If you already have incorrect boot entries, you’ll either need to delete them or to modify BOOT.CSV to create new entries with different names.

Adding other operating systems to the GRUB2 menu

Normally, GRUB2 is preset to boot multiple operating systems during the Fedora installation process. If you can, it is advisable to install non-Linux operating systems first. Then, during the installation process, all those operating systems and their locations will be discovered and properly set.

Adding other records into the GRUB2 menu only means to run grub2-mkconfig command to regenerate the configuration files. During this process, all operating systems known to the system will be added into the configuration. By reinstalling GRUB2, this configuration will be used for further boots.

Before you start

  • Make sure that the operating systems are on disks, connected to the system.

  • You have the os-prober package installed.

Procedure

  1. Recreate the GRUB2 configuration file.

    # grub2-mkconfig -o /boot/grub2/grub.cfg

  2. Install GRUB2.

    • On UEFI systems.

      # dnf reinstall shim-\* grub2-efi-\* grub2-common

    • On BIOS systems, specify the disk where the bootloader should be installed.

      # grub2-install /dev/sda
More information

  • The grub2-mkconfig command will add entries for all operating systems it can find.

  • When problems appear, see the GRUB manual to solve issues with booting secondary operating systems.

Setting default entry for GRUB2

Since grub2-mkconfig (and os-prober) cannot estimate which operating system, of those it finds, is to be marked as default, we usually are unable to predict the order of the entries in /boot/grub2/grub.cfg. To change the default layout, we need to set the default based on the name or title.

Before you start

  1. Open /etc/default/grub and make sure these lines exist in the file.

    GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=false

  2. If you needed to change the content of the /etc/default/grub, apply the changes to grub.cfg.

    # grub2-mkconfig -o /boot/grub2/grub.cfg
Procedure

  1. List all possible menu entries.

    # grep -P "^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2

  2. Select one of the displayed options and use it as an argument to set the default menu entry.

    # grub2-set-default <menuentry>

  3. Verify the default menu entry

    # grub2-editenv - list

  4. Regenerate the GRUB2 configuration file and reinstall the bootloader into the MBR, as described in Adding other operating systems to the GRUB2 menu.

More information

If you understand the risks involved, you can manually modify the /boot/grub2/grub.cfg file. In that case, set the number of the default operating system using the set default variable.

For example:

set default="5"

If you edit the configuration file manually, the settings will be overwritten each time the grub2-mkconfig command runs.

Restoring the bootloader using the Live disk

Sometimes, especially after a secondary operating systems has been installed, the master boot record gets damaged which then prevents the original Linux system from booting.

If this happens, it is necessary to reinstall GRUB2 to recreate the original settings. The process not only discovers all installed operating systems, but usually adds them to the GRUB2 configuration files, so they will all become bootable by GRUB2.

Before you start

  • Get the Fedora Live ISO from fedoraproject.org.

  • Prepare a bootable device using the downloaded ISO, either a CD or a USB.

Procedure

  1. Boot the Fedora live system from the bootable device you have created.

  2. Open the terminal.

  3. Examine the partition layout and identify the /boot and the /root partition.

    # lsblk -f -p

  4. If your /root partition is encrypted by LUKS, it must be decrypted.

    1. Make sure the crypt module is in use.

      # modprobe dm-crypt

    2. Decrypt the /root partition (e.g. /dev/sda3).

      # cryptsetup luksOpen /dev/sda3 myvolume

      The decrypted device (i.e. myvolume) will be accessible under /dev/mapper.

  5. Follow the BTRFS steps (used by default in Fedora 33 or newer).

    1. Mount the /root partition.

      • For LUKS.

        # mount /dev/mapper/myvolume /mnt -o subvol=root

      • For non-LUKS.

        # mount /dev/sda3 /mnt -o subvol=root

  6. Follow the LVM steps (used by default before Fedora 33).

    1. Scan the LVM volumes for the volume group corresponding to the /root partition.

      # vgscan

    2. Activate the volume group (e.g. fedora).

      # vgchange -ay fedora

    3. Find the logical volume corresponding to /root.

      # lvs

      The logical volume will be accessible under /dev/mapper.

    4. Mount the logical volume (e.g. /dev/mapper/fedora-root) corresponding to the /root partition.

      # mount /dev/mapper/fedora-root /mnt

  7. Mount the /boot partition (e.g. /dev/sda2).

    # mount /dev/sda2 /mnt/boot

  8. Mount system processes and devices into the /root filesystem.

    # mount -o bind /dev /mnt/dev
# mount -o bind /proc /mnt/proc
# mount -o bind /sys /mnt/sys
# mount -o bind /run /mnt/run

  9. On UEFI systems, bind the efivars directory and mount the EFI system partition (e.g. /dev/sda1).

    # mount -o bind /sys/firmware/efi/efivars /mnt/sys/firmware/efi/efivars
# mount /dev/sda1 /mnt/boot/efi

  10. Change your filesystem to the one mounted under /mnt.

    # chroot /mnt

  11. Re-install GRUB2.

    • On UEFI systems, several packages are required.

      # dnf reinstall shim-\* grub2-efi-\* grub2-common

    • On BIOS systems, specify the disk (e.g. /dev/sda) where GRUB2 should be installed.

      # grub2-install /dev/sda

  12. Re-generate the GRUB2 configuration file.

    # grub2-mkconfig -o /boot/grub2/grub.cfg

  13. Sync and exit the chroot.

    # sync && exit

  14. Reboot the system.

Using the GRUB2 boot prompt

If improperly configured, GRUB2 may fail to load and subsequently drop to a boot prompt. To boot into the system, follow the steps below.

Procedure

  1. Load the necessary modules to read your system’s partitions (you will also need to load part_msdos or part_gpt, depending on your partition table).

    • For BTRFS filesystems (Fedora 33 or newer).

      grub> insmod btrfs

    • For LVM filesystems (older than Fedora 33).

      grub> insmod xfs
grub> insmod lvm

  2. List the drives which GRUB2 sees.

    grub> ls

  3. Examine the output to understand the partition table of the /dev/sda device. The following example shows a DOS partition table with three partitions.

    (hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)

    A GPT partition table of the /dev/sda device with four partitions could look like this.

    (hd0) (hd0,gpt4) (hd0,gpt3)  (hd0,gpt2) (hd0,gpt1)

  4. Probe each partition of the drive and locate your vmlinuz and initramfs files.

    grub> ls (hd0,1)/

    The outcome of the previous command will list the files on /dev/sda1. The partition that contains the /boot directory is the correct one. There you will search for the full names of the vmlinuz and initramfs files.

  5. Follow the Pre-boot setup for BTRFS or the Pre-boot setup for LVM to recover your system.

  6. After the pre-boot setup, boot the system.

    grub> boot

  7. To restore the bootloader’s functionality, regenerate the GRUB2 configuration file and reinstall the bootloader, as described in Adding other operating systems to the GRUB2 menu.

Pre-boot setup for BTRFS filesystems

  • On BIOS systems.

    1. Set GRUB2 root to your /boot partition. If your /boot partition is (hd0,msdos1), the command will be.

      grub> set root=(hd0,msdos1)

    2. Next, select the desired kernel. Set the /root partition (e.g. /dev/sda2).

      grub> linux /vmlinuz-5.14.10-300.fc35.x86_64 root=/dev/sda2 ro rootflags=subvol=root

  • On UEFI systems.

    1. Set GRUB2 root to your EFI system partition. If your EFI system partition is (hd0,gpt1), use this command.

      grub> set root=(hd0,gpt1)

    2. Next, select the desired kernel. Find the path to vmlinuz and set the /root partition (e.g. /dev/sda3).

      grub> linux (hd0,gpt2)/vmlinuz-5.14.10-300.fc35.x86_64 root=/dev/sda3 ro rootflags=subvol=root

    3. Select the RAM filesystem that will be loaded.

      grub> initrd (hd0,gpt2)/initramfs-5.14.10-300.fc35.x86_64.img

Pre-boot setup for LVM filesystems

  • On BIOS systems.

    1. Set GRUB2 root to your /boot partition. If your /boot partition is (hd0,msdos1), use this command.

      grub> set root=(hd0,msdos1)

    2. Next, select the desired kernel. Set root to the logical volume that corresponds to the /root directory.

      grub> linux /vmlinuz-3.0.0-1.fc16.i686 root=/dev/mapper/fedora-root

    3. Select the RAM filesystem that will be loaded.

      grub> initrd /initramfs-3.0.0-1.fc16.i686.img

  • On UEFI systems.

    1. Set GRUB2 root to your EFI system partition. If your EFI system partition is (hd0,gpt1), use this command.

      set root=(hd0,gpt1)

    2. Next, select the desired kernel. Find the path to vmlinuz and set root to the logical volume that corresponds to the /root directory.

      linux (hd0,gpt2)/vmlinuz-3.0.0-1.fc16.i686 root=/dev/mapper/fedora-root

    3. Select the RAM filesystem that will be loaded.

      initrd (hd0,gpt2)/initramfs-3.0.0-1.fc16.i686.img

Booting the system using a configuration file on a different partition

If you end up in GRUB2 boot prompt, it is also possible to boot using a configfile that’s located on another partition, as is often the case with multi-boot systems containing Ubuntu and Fedora. Follow the below procedure if you need to boot from a configuration file on a different partition.

Procedure

  1. Load the necessary modules to read your system’s partitions (you will also need to load part_msdos or part_gpt, depending on your partition table).

    • For BTRFS filesystems.

      grub> insmod btrfs

    • For LVM filesystems.

      grub> insmod xfs
grub> insmod lvm

  2. Set GRUB2 root to your /boot partition. On UEFI systems, you should set GRUB2 root to the EFI system partition.

    grub> set root=(hd0,msdos1)

  3. Set the path to the configuration file.

    grub> configfile /grub2/grub.cfg
More information

  • The hd0,msdos1 line shows the pertinent /boot partition, which holds the grub.cfg file. The setting may be different on your system. See also Using the GRUB2 boot prompt for more information.

Setting a password for interactive edit mode

If you wish to protect the GRUB2 interactive edit mode with a password, but allow ordinary users to boot the computer, use the grub2-set-password command. You will be prompted for the password, and then will have to confirm it. The encrypted password will be stored in /boot/grub2/user.cfg.

To remove password protection, simply delete the user.cfg file.

Alternately, you can set this up manually:

Procedure

  1. Create the /etc/grub.d/01_users file and write the following lines into the file.

    cat << EOF
set superusers="root"
export superusers
password root <password>
EOF

  2. Regenerate the GRUB2 configuration file, as described in Adding other operating systems to the GRUB2 menu.

More information

You can encrypt the password by using pbkdf2. Use grub2-mkpasswd-pbkdf2 to encrypt the password, then replace the password line with:

password_pbkdf2 root grub.pbkdf2.sha512.10000.1B4BD9B60DE889A4C50AA9458C4044CBE129C9607B6231783F7E4E7191D8254C0732F4255178E2677BBE27D03186E44815EEFBAD82737D81C87F5D24313DDDE7.E9AEB53A46A16F30735E2558100D8340049A719474AEEE7E3F44C9C5201E2CA82221DCF2A12C39112A701292BF4AA071EB13E5EC8C8C84CC4B1A83304EA10F74

To remove password protection, simple remove the changes you made to the /etc/grub.d/01_users file and regenerate the GRUB2 configuration file, as before.

More details can be found at Ubuntu Help: GRUB2 Passwords.

Starting from Fedora 15, the --password=<encrypted_grub_passwd> --iscrypted kickstart option must be used if setting an encrypted GRUB2 password in the kickstart file.

Dealing with the "Absent Floppy Disk" Error

It has been reported by some users that GRUB2 may fail to install on a partition’s boot sector if the computer’s floppy controller is activated in BIOS without an actual floppy disk drive being present. Such situations resulted in an Absent Floppy Disk error.

To workaround this issue, go into the rescue mode and follow the procedure in Installing GRUB2 on a BIOS system GRUB2, but use the --no-floppy option with the grub2-install command.

# grub2-install <target device> --no-floppy

Using old graphics modes in bootloader

The terminal device is chosen with GRUB_TERMINAL. For more information, see the Grub manual.

Valid terminal output names depend on the platform, but may include console (PC BIOS and EFI consoles), serial (serial terminal), gfxterm (graphics-mode output), ofconsole (Open Firmware console), or vga_text (VGA text output, mainly useful with Coreboot).

The default is to use the platform’s native terminal output.

In Fedora, gfxterm is the default options. To get the legacy graphics modes:

Procedure

  1. Edit the /etc/default/grub file.

  2. Set the GRUB_TERMINAL variable to one of the above mentioned options.

  3. Regenerate the GRUB2 configuration file and reinstall the bootloader into the MBR, as described in Adding other operating systems to the GRUB2 menu.

Enabling Serial Console in GRUB2

To enable Serial console in grub:

Procedure

  1. Edit the /etc/default/grub file.

  2. Adjust baudrate, parity, bits, and flow controls to fit your environment and cables, see the example.

    GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,115200n8'
GRUB_TERMINAL=serial
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"

  3. Regenerate the GRUB2 configuration file and reinstall the bootloader into the MBR, as described in Adding other operating systems to the GRUB2 menu.

Additional resources

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