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= Updating the GRUB configuration file = | = Updating the GRUB configuration file = | ||
The GRUB configuration file is located at <code>/boot/grub2/grub.cfg</code> and is intended to be a static file that | The GRUB configuration file is located at <code>/boot/grub2/grub.cfg</code> and is intended to be a static file that does not need updating. In case of disk replacement, or installation of another Linux distribution, it should be updated. Use the following commands: | ||
<code>grub2-mkconfig -o /etc/grub2.cfg</code> | <code>grub2-mkconfig -o /etc/grub2.cfg</code> | ||
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These commands use information provided by the <code>os-prober</code> utility to add entries for other Linux distributions and Windows. | These commands use information provided by the <code>os-prober</code> utility to add entries for other Linux distributions and Windows. | ||
{{admon/note| | {{admon/note|Refrain from using `grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg` going forward. This is a valid location on Fedora 33 and older. But on Fedora 34 and newer, it is a small stub file that merely forwards to `/boot/grub2/grub.cfg`. See the reinstallation section below if you have accidentally overwritten this file.|}} | ||
= I have a `grub>` prompt! Now what? = | = I have a `grub>` prompt! Now what? = |
Revision as of 13:31, 2 July 2021
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. Upstream refers to GRUB 2 as just GRUB.
GRUB 2 is the bootloader used on x86_64 systems, and on aarch64 if UEFI firmware is also used.
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 the GRUB configuration file
The GRUB configuration file is located at /boot/grub2/grub.cfg
and is intended to be a static file that does not need updating. In case of disk replacement, or installation of another Linux distribution, it should be updated. Use the following commands:
grub2-mkconfig -o /etc/grub2.cfg
grub2-mkconfig -o /etc/grub2-efi.cfg
These commands use information provided by the os-prober
utility to add entries for other Linux distributions and Windows.
I have a grub>
prompt! Now what?
If you are stuck at a grub>
prompt, you can use any Fedora edition or spin Network Installer or DVD Installer, and use 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
Next, update the GRUB configuration file as described in the previous section. And then continue with the section below for firmware specific instructions on reinstalling GRUB.
Reinstalling GRUB
GRUB comes in two flavors, BIOS GRUB and UEFI GRUB. Reinstallation instructions depend on the firmware type. Systems with UEFI firmware have their GRUB RPM packages updated, which in turn updates the bootloader installed on the EFI System volume.
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.
Instructions for UEFI-based systems
- Learn what firmware your machine is running. See Discovering the firmware type section.
- Systems with UEFI firmware have the shim and GRUB RPM packages updated, which in turn updates the bootloader files found on the EFI System volume. Reasons for reinstallation include troubleshooting early boot problems, and following inadvertent use of
grub2-install
which results in an unsupported configuration on UEFI systems.
- Remove the following files:
# rm /boot/efi/EFI/fedora/grub.cfg # rm /boot/grub2/grub.cfg
- Reinstall the following packages:
# dnf reinstall shim-* grub2-efi-* grub2-common
Instructions for BIOS-based systems
- Learn what firmware your machine is running. See Discovering the firmware type section.
- Systems with BIOS firmware have the GRUB RPM packages updated, but the installed (or embedded) bootloader is never updated automatically. It's a good idea to update it between Fedora release versions.
- 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.
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.