Change default kernel to boot from in RHEL8

Issue

  • How to change the default boot entry for kernel in RHEL 8?

  • Changing the default kernel of an RHEL 8 system with multiple kernel versions installed.

Resolution

Listing the Default Kernel:

  • To get the file name of the current default kernel.

# grubby --default-kernel

Eg:
# grubby --default-kernel
/boot/vmlinuz-4.18.0-80.7.1.el8_0.x86_64
  • To get the index number of the current default kernel.

# grubby --default-index

Eg:
# grubby --default-index
0

Listing Available Kernels:

  • To get the list of file names of available kernels.

# ls -l /boot/vmlinuz-*

Eg:
# ls -l /boot/vmlinuz-*
-rwxr-xr-x. 1 root root 7872864 Jul 12 05:27 /boot/vmlinuz-0-rescue-31c619b4ae7b4a29b0cdd151ca2a7ca6
-rwxr-xr-x. 1 root root 7876960 Jun 24 06:52 /boot/vmlinuz-4.18.0-80.7.1.el8_0.x86_64
-rwxr-xr-x. 1 root root 7872864 Mar 13 08:34 /boot/vmlinuz-4.18.0-80.el8.x86_64

Changing Default Kernel Boot Entry:

  • To change the default kernel boot entry using kernel filename.

# grubby --set-default <kernel-filename>

Eg:
# grubby --set-default /boot/vmlinuz-4.18.0-80.el8.x86_64
OR
# grubby --set-default vmlinuz-4.18.0-80.el8.x86_64
  • To change the default kernel boot entry using kernel entry-index.

# grubby --set-default-index=<kernel-entry-index>

Eg:
# grubby --set-default-index=1
  • To determine the order of the kernel entry-index, the latest kernel installed will have the entry-index of 0 and the older kernel version will get 1. In the mentioned example older kernel kernel-4.18.0-80.el8.x86_64 will have entry-index 1 and the latest kernel kernel-4.18.0-80.7.1.el8_0.x86_64 will have entry-index 0

  • The detailed information of a certain kernel version can be collected by using --info, it will show the index as well.

# grubby --info <kernel-filename>

Eg:
# grubby --info /boot/vmlinuz-4.18.0-80.7.1.el8_0.x86_64
index=0
kernel="/boot/vmlinuz-4.18.0-80.7.1.el8_0.x86_64"
args="ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet $tuned_params"
root="/dev/mapper/rhel-root"
initrd="/boot/initramfs-4.18.0-80.7.1.el8_0.x86_64.img $tuned_initrd"
title="Red Hat Enterprise Linux (4.18.0-80.7.1.el8_0.x86_64) 8.0 (Ootpa)"
id="31c619b4ae7b4a29b0cdd151ca2a7ca6-4.18.0-80.7.1.el8_0.x86_64"

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