Changing the Default Kernel

Use grubby to set the default kernel that GRUB2 boots into after a timeout period is reached when displaying the GRUB2 boot menu.

You might change the default kernel from RHCK to UEK, from UEK to RHCK, or to switch to a specific kernel version.

You can follow one of two options to set the default kernel in GRUB 2, by using the grubby command, choose either of the following:

Use the grubby --set-default command to set the default kernel.
To switch to a different default kernel, run the following command making sure to specify the full path to the selected default kernel:
```
sudo grubby --set-default /boot/vmlinuz-5.15.0-208.159.3.2.el9uek.x86_64
```
The change takes effect immediately and persists across system reboots.
Use the grubby --set-default-index command to set the default kernel to match the kernel at a particular index point in the kernel boot list. The index values are displayed when you run the grubby --info command
For example, you can use the --set-default-index=0 option to set the default kernel to the first kernel listed in the kernel boot index by running:
```
sudo grubby --set-default-index=0
```

Example 5-1 Switch to the Most Recent Available RHCK or UEK Kernel

By using the naming convention to identify UEK kernels and RHCK kernels that are available in the /boot directory, you can easily switch the default kernel to use the most recent version of either kernel type.

To switch to the most recent version of UEK on the system, run:

sudo grubby --set-default $(ls /boot/vmlinuz-* | grep 'uek' | sort -V | tail -1)

To switch to the most recent version of RHCK on the system, run:

sudo grubby --set-default $(ls /boot/vmlinuz-* | grep -v 'uek' | sort -V | tail -1)

Reboot the system after setting the default kernel to switch to that kernel type.