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RHCSA - Deploy, Configure, & Maintain Systems: Modify the System Bootloader
GRUB2 (GRand Unified Bootloader 2) serves as the initial software that loads the operating system into memory during the computer's startup process. Offering a user-friendly interface and support for multiple filesystems, GRUB2 allows users to select the desired operating system or kernel to boot, configure system parameters, and even troubleshoot boot issues.
Modifying the Bootloader
The main configuration file for GRUB2 is /etc/default/grub
. In the configuration file, you can modify various parameters to customize the bootloader behavior. Some common modifications include changing the default boot entry, setting the timeout for the boot menu, and configuring kernel parameters.
Modifying the bootloader:
To change the default boot entry you modify the value in /etc/default/grub
for GRUB_DEFAULT
. The default entry is saved
which remembers the last kernel booted into, however amending this to a numerical value will ensure the system boots into a specified kernel. This value is an index starting at 0
for the first (latest installed) kernel.
Update the GRUB_DEFAULT
setting to 0
:
sudo vim /etc/default/grub
Amend variable to display as follows and leave the file open for further modifications:
GRUB_DEFAULT=0
When the system boots, by default you get 5 seconds at the GRUB2 boot menu to interrupt the boot process. This may not be long enough so the value can be updated to give more time. This is done with setting GRUB_TIMEOUT
.
Update the GRUB_TIMEOUT
variable to give yourself 10 seconds to interrupt the boot process and leave the file open for further modifications:
GRUB_TIMEOUT=10
There may situations that you want the boot process to be more verbose, or just want to tweak other kernel parameters. This is done via the GRUB_CMDLINE_LINUX
variable.
Update the GRUB_CMDLINE_LINUX
variable, removing rhgb
and quiet
from the end of the line, which will make the boot process more verbose:
The below variable is how my system looks after removing rhgb
& quiet
. DO NOT COPY THIS. Instead remove the settings yourself:
GRUB_CMDLINE_LINUX="crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/os_vg-swap rd.lvm.lv=os_vg/root rd.lvm.lv=os_vg/swap"
You can now save and exit the configuration file.
After making the modifications to the /etc/default/grub
you will need to run the grub2-mkconfig
to recreate the grub2 boot configuration file, which may be a different file depending on whether the system is BIOS
or UEFI
.
To determine if your system is BIOS
or UEFI
you can run the following, which will echo to screen what your system is using:
Determine if system is BIOS
or UEFI
:
[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS
For BIOS
systems, the configuration file is /boot/grub2/grub.cfg
.
For UEFI
systems, the configuration file is /boot/efi/EFI/redhat/grub.cfg
.
Update the grub.cfg
file with amended settings from /etc/default/grub
For BIOS
systems:
sudo grub2-mkconfig -o /boot/grub2/grub.cfg
For UEFI
systems:
sudo grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
Now the grub.cfg
file has been updated, you can reboot your system and observe that the changes have taken effect.
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