Change default kernel using grubby Tool

There are several ways to fulfill the same task, I am providing one of them.

1. Check the information about currently loaded kernel:

# uname -r
5.4.17-2036.101.2.el7uek.x86_64

2. Find all available kernels in your system and locate their index number:

# grubby --info=ALL
index=0
kernel=/boot/vmlinuz-5.4.17-2036.101.2.el7uek.x86_64
args="ro console=tty1 console=ttyS0,115200n8 earlyprintk=ttyS0,115200 rootdelay=300 numa=off transparent_hugepage=never net.ifnames=0"
root=/dev/mapper/rootvg-rootlv
initrd=/boot/initramfs-5.4.17-2036.101.2.el7uek.x86_64.img
title=Oracle Linux Server 7.9, with Unbreakable Enterprise Kernel 5.4.17-2036.101.2.el7uek.x86_64

index=1
kernel=/boot/vmlinuz-3.10.0-1160.42.2.el7.x86_64
args="ro console=tty1 console=ttyS0,115200n8 earlyprintk=ttyS0,115200 rootdelay=300 numa=off transparent_hugepage=never net.ifnames=0"
root=/dev/mapper/rootvg-rootlv
initrd=/boot/initramfs-3.10.0-1160.42.2.el7.x86_64.img
title=Oracle Linux Server 7.9, with Linux 3.10.0-1160.42.2.el7.x86_64

index=2
kernel=/boot/vmlinuz-0-rescue-d3dd3af16fd242cebb997c6041d68ad3
args="ro console=tty1 console=ttyS0,115200n8 earlyprintk=ttyS0,115200 rootdelay=300 numa=off transparent_hugepage=never net.ifnames=0"
root=/dev/mapper/rootvg-rootlv
initrd=/boot/initramfs-0-rescue-d3dd3af16fd242cebb997c6041d68ad3.img

3. Check currently loaded kernel index using grubby tool (actually, we could find the same from 1st and 2nd steps, but let’s do one more time):

# grubby --default-index
0

4. Change the default kernel, in my case I want to set it to vmlinuz-3.10.0-1160.42.2.el7.x86_64 and it’s index number is 1:

# grubby --set-default 1

5. Reboot the system and check the kernel again:

# reboot
# uname -r
3.10.0-1160.42.2.el7.x86_64