Changing the default kernel in Red Hat Enterprise Linux 8 , 9 & 10

Solution Verified - Updated

Environment

  • Red Hat Enterprise Linux 8
  • Red Hat Enterprise Linux 9
  • Red Hat Enterprise Linux 10

Issue

  • How to change the default boot entry for kernel in Red Hat Enterprise Linux ?
  • Changing the default kernel of a Red Hat Enterprise Linux system when multiple kernel versions installed.

Resolution

Simple way common to RHEL8, RHEL9 and RHEL 10

1. Confirmation before the change

# grubby --default-kernel
/boot/vmlinuz-5.14.0-70.30.1.el9_0.x86_64

# uname -r
5.14.0-70.30.1.el9_0.x86_64

2. Show installed kernel list ( You can count 0,1,.. from the top of the title line)

# grubby --info=ALL | grep title
title="Red Hat Enterprise Linux (5.14.0-70.30.1.el9_0.x86_64) 9.0 (Plow)"    <---0
title="Red Hat Enterprise Linux (5.14.0-70.13.1.el9_0.x86_64) 9.0 (Plow)"    <---1

Note: The prefix of the BLSCFG (/boot/loader/entries/machine_id-kernel_version.conf) files should match the machine-id of the system

3. Set the default kernel

# grubby --set-default-index 1

4. Confirmation after the change

# grubby --default-kernel
/boot/vmlinuz-5.14.0-70.13.1.el9_0.x86_64

5. reboot the system

# reboot

6. Confirmation after system reboot

# uname -r 
5.14.0-70.13.1.el9_0.x86_64

# grubby --default-kernel
/boot/vmlinuz-5.14.0-70.13.1.el9_0.x86_64

More Information

There are two ways to change the default kernel.

Using grub2-set-default

Listing BLS entry files of Installed Kernels:

Each kernel information exists in a separated BLS file in /boot/loader/entries directory:

# cd /boot/loader/entries/
# ls -l
total 16
-rw-r--r--. 1 root root 540 Jan 31 10:55 d5e7db3976314d06a6e09451488125db-0-rescue.conf
-rw-r--r--. 1 root root 502 Jan 31 10:55 d5e7db3976314d06a6e09451488125db-5.14.0-362.13.1.el9_3.x86_64.conf
-rw-r--r--. 1 root root 502 Jan 31 10:55 d5e7db3976314d06a6e09451488125db-5.14.0-362.18.1.el9_3.x86_64.conf
-rw-r--r--. 1 root root 498 Jan 31 10:55 d5e7db3976314d06a6e09451488125db-5.14.0-70.22.1.el9_0.x86_64.conf

Changing Default Kernel Boot Entry:

To change the default kernel boot entry using the part of <XXX.conf>:

# grub2-set-default <XXX>          //note, delete the ".conf" suffix

like:

# grub2-set-default d5e7db3976314d06a6e09451488125db-5.14.0-70.22.1.el9_0.x86_64

Check grubenv File:

To confirm if the specified default kernel is reflected, check the saved_entry value in the output of the following command:

# grub2-editenv /boot/grub2/grubenv list

Otherwise, in case of UEFI enabled:

# grub2-editenv /boot/efi/EFI/redhat/grubenv list

Confirm GRUB_DEFAULT=saved in /etc/sysconfig/grub

Above steps to change the default kernel can work with GRUB_DEFAULT=saved in /etc/sysconfig/grub, which is the default configuration.

Confirm DEFAULTKERNEL value in /etc/sysconfig/kernel

To configure a kernel variant such as kernel, kernel-rt, kernel-rt-debug as the default boot kernel, ensure the DEFAULTKERNEL value is updated failing which system will boot with a wrong undesired kernel version.

To change from kernel-core i.e default kernel to kernel-rt user must edit /etc/sysconfig/kernel to reflect the correct variant:

# cat /etc/sysconfig/kernel | grep -i DEFAULTKERNEL
# DEFAULTKERNEL specifies the default kernel package type
DEFAULTKERNEL=kernel-core

# sed -i 's/DEFAULTKERNEL=.*/DEFAULTKERNEL=kernel-rt-core/g' /etc/sysconfig/kernel

# cat /etc/sysconfig/kernel | grep -i DEFAULTKERNEL
# DEFAULTKERNEL specifies the default kernel package type
DEFAULTKERNEL=kernel-rt-core

Using grubby

Listing the Default Kernel:

To get the 'kernel filename' 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

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"

Diagnostic Steps

  • 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
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