Introduction
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 what was formerly known as GRUB (i.e. version 0.9x), which has, in turn, become GRUB Legacy.
Starting with Fedora 16, GRUB 2 is the default bootloader on x86 BIOS systems. For upgrades of BIOS systems the default is also to install GRUB 2, but you can opt to skip bootloader configuration entirely.
Adding and removing kernel command-line parameters using grubby
The GRUB 2 packages contain commands for installing a bootloader and for creating a bootloader configuration file.
The grub2-install command installs the GRUB 2 bootloader usually in the master boot record (MBR), in free and unpartioned space. The bootloader files are placed in the /boot/ directory. You can install the GRUB 2 bootloader with:
# grub2-install /dev/sda
Grubby is a utility that 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 paramters 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 is the main GRUB 2 configuration file. It is a static file that you rarely modify. Except in cases of disk replacement or installation of another Linux distribution.
To discover what firmware your machine is using, run the following command:
- On UEFI system:
# ls -ld /sys/firmware/efi
- On BIOS system:
# ls -lrt /etc/grub2.cfg
A directory listing of either of these commands indicate that you are running the corresponding firmware.
The grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg command adds entries for other detected operating systems. That will be done based on the output of the os-prober tool.
grub2-mkconfig -o /boot/grub2/grub.cfg. Reinstalling GRUB
Appendix
Enabling serial console in GRUB 2
To enable serial console for usage on virtual environments you need to run the following command:
# grubby --args="systemd.journald.forward_to_console=1 console=ttyS0,38400 console=tty1" --update-kernel=/boot/vmlinuz-5.11.16-300.fc34.x86_64 # grubby --set-default=/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. 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
Setting default entry
if file
/etc/sysconfig/kernel have UPDATEDEFAULT=yesin every kernel update the grub entry is update to last entry, if you don't want that please set:
UPDATEDEFAULT=no(write "no" in lower case)
Due to grub2-mkconfig (and os-prober) we cannot predict the order of the entries in /boot/grub2/grub.cfg, so we set the default by name/title instead.
Open /etc/default/grub and ensure this line exists:
GRUB_DEFAULT=saved
and ensure this line not exists:
GRUB_SAVEDEFAULT=true
or ensure this line exists:
GRUB_SAVEDEFAULT=false
If GRUB_SAVEDEFAULT is set to true, then, when an entry is selected, save it as a new default entry for use by future runs of GRUB. So, maybe, you need be sure that GRUB_SAVEDEFAULT is not set to true. GRUB_SAVEDEFAULT is only useful if GRUB_DEFAULT is saved.
Apply the change to grub.cfg by running:
grub2-mkconfig -o /boot/grub2/grub.cfg
Now list all possible menu entries
grep -P "submenu|^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2
Now set the desired default menu entry
grub2-set-default "<submenu title><menu entry title>"
Verify the default menu entry
grub2-editenv list
The above method fails to work on some F20 (and newer) systems due to a missing or improperly linked /boot/grub2/grubenv file. The /boot/grub2/grubenv is symbolic linked to /boot/efi/EFI/fedora/grubenv but /boot is not mounted at the time of booting. So grub2 does not have access to the environment variables. To fix this, change /boot/grub2/grubenv to point to ../efi/EFI/fedora/grubenv instead and your chosen default OS will boot without any problems.
There are other, simpler, ways of setting the default entry, but they are prone to error if/when grub2-mkconfig is re-run. These include directly setting the default in /boot/grub2/grub.cfg or setting GRUB_DEFAULT to either a number or an entry title in /etc/default/grub. Neither of these methods is recommended.
If you understand the risks involved and still want to directly modify /boot/grub2/grub.cfg, here's how you can do it:
Edit /boot/grub2/grub.cfg, and change the line
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This will not survive grub2-mkconfig. It might not even survive a kernel update.
set default="0"
to
set default="5"