From Fedora Project Wiki

What is GRUB 2

GRUB 2 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 - Linux in the case of Fedora. The kernel, in turn, initializes the rest of the operating system.

GRUB 2 has replaced GRUB version 0.9x, which has become GRUB Legacy.

GRUB 2 is the default bootloader on AMD64 and Intel 64 BIOS systems. For upgrades of BIOS systems the default is also to install GRUB 2, but you can opt to skip bootloader configuration entirely.

Changing kernel command-line parameters with grubby

The grubby utility updates the bootloader-specific configuration files. The utility is a recommended way for making routine changes to the kernel boot parameters and setting a default kernel.

Following are some of the selected illustrations of grubby usage:

  • To add one kernel parameter to a single boot entry:
# grubby --args=<NEW_PARAMETER> --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64
  • To add multiple kernel parameters to a single boot entry:
# grubby --args="<NEW_PARAMETER1> <NEW_PARAMETER2 <NEW_PARAMETER_n>" --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64
  • To add one kernel parameter to all currently existing and future boot entries:
# grubby --args=<NEW_PARAMETER> --update-kernel=ALL
  • To remove one kernel parameter from all currently existing and future boot entries:
# grubby --remove-args=<PARAMETER_TO_REMOVE> --update-kernel=ALL
  • To set the default kernel:
# grubby --set-default=/boot/vmlinuz-5.11.12-300.fc34.x86_64

Updating and repairing the GRUB 2 main configuration file

The /boot/grub2/grub.cfg file is the main GRUB 2 configuration file. It is a static file that you rarely need to modify. Except in cases of disk replacement or installation of another Linux distribution.

Discovering computer firmware

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.

The grub2-mkconfig -o /etc/grub2.cfg and grub2-mkconfig -o /etc/grub2-efi.cfg commands add entries for other detected operating systems. That will be done based on the output of the os-prober utility.

To update the GRUB 2 configuration file on Fedora 33 and earlier use grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg on UEFI-based systems, and grub2-mkconfig -o /etc/grub2-efi.cfg on BIOS-based systems.

Repairing GRUB 2

If your machine running Fedora 34 is not working because of the broken GRUB 2 bootloader, the first possible step to take is that you boot into the rescue mode to repair the already installed operating system.

For more details see Booting Your Computer in Rescue Mode.

After completing steps specified in the previous link, run the following command to mount the root partition:

# chroot /mnt/sysimage

Reinstalling GRUB 2

The GRUB 2 packages contain commands for installing the bootloader, and for creating the grub.cfg configuration file.

The grub2-install command embeds two stages of the bootloader - one at LBA 0, and one in either the master boot record (MBR) gap, or the GUID Partition Table (GPT) BIOS Boot partition. The bootloader files are placed in the /boot/ directory.

Instructions for UEFI-based systems

  • Reinstall the following packages:
# dnf reinstall shim-* grub2-efi-*

Instructions for BIOS-based systems

  • Find the device node the /boot/ directory is located on:
# mount | grep "/boot "
/dev/sda4 on /boot type ext4 (rw,relatime,seclabel)

The device node is /dev/sda4.

  • Reinstall the bootloader while specifying the device node without the number:
# grub2-install /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.
Do not use the grub2-install command on UEFI systems. On those systems, bootloaders are in the shim and grub-efi packages. By reinstalling those packages, the bootloaders are reinstalled to their proper location in /boot/efi/ on the EFI system partition.

Fixing a damaged GRUB 2 configuration file using a plaintext stub file on UEFI-based systems

On Fedora 34 and later, you can repair a malfunctioning grub.cfg configuration file by creating a stub.

  1. Discover the Universally Unique Identifier (UUID) for the /boot/ mount point:
# lsblk --fs
NAME   FSTYPE  FSVER            LABEL                 UUID                                 FSAVAIL FSUSE% MOUNTPOINT
sr0    iso9660 Joliet Extension Fedora-WS-Live-34-1-2 2021-04-23-11-17-40-00                     0   100% /run/media/jdoe/Fedora-WS-Live-34-1-2
zram0                                                                                                     [SWAP]
├─vda1 ext4    1.0                                    dc29837b-22dc-4469-be85-fc9acf3009fd  699.8M    21% /boot
└─vda2 btrfs                    fedora_localhost-live c58f3698-5587-40f2-b920-64d46c43161d   23.7G    14% /home

The UUID of /boot/ is dc29837b-22dc-4469-be85-fc9acf3009fd.

  1. Create a custom grub.cfg file with the following content:
search --no-floppy --fs-uuid --set=dev dc29837b-22dc-4469-be85-fc9acf3009fd
set prefix=($dev)/grub2

export $prefix
configfile $prefix/grub.cfg

At the end of first line (--set=dev), there is the UUID value of the /boot/ mount point.

The above example assumes default partitioning, where a separate ext4 file system is mounted on the /boot/ directory. In case of other configurations, you need to insert /boot/ into line 2. For example:

set prefix=($dev)/boot/grub2
  1. Move or copy the custom grub.cfg file you created in the previous step to /boot/efi/EFI/fedora/grub.cfg

Enabling serial console in GRUB 2

On Fedora 34 and later, you can enable serial console for usage on virtual environments. The following procedure explains how to achieve this goal.

# grubby --args="systemd.journald.forward_to_console=1 console=ttyS0,38400 console=tty1" --update-kernel=/boot/vmlinuz-5.11.16-300.fc34.x86_64

The first command specifies the baud rate, console forwarding for systemd, what console to use (tty1) and on what kernel such changes should be applied.

# grubby --set-default=/boot/vmlinuz-5.11.16-300.fc34.x86_64

The second command ensures the specified kernel is going to be loaded by default on next reboot.

For instructions on how to enable serial consol in GRUB 2 for baremetal machines, see Using GRUB via a serial line.