Change default kernel for an AWS instance running Ubuntu 18.04

Question

1. Problem summary I have an instance on AWS running Ubuntu 18.04. The kernel version is 5.4.0. I have installed kernel 4.15.0 but I am failing to set it as the default.

2. What I have tried

A) I set the following values in /etc/default/grub

GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true
GRUB_DISABLE_SUBMENU=y

and made sure there is no value for GRUB_DEFAULT in /etc/default/grub.d/50-cloudimg-settings.cfg

Created an environment variable for GRUB_CONFIG using

export GRUB_CONFIG=`sudo find /boot -name "grub.cfg"

I rebuilt grub.cfg using sudo update-grub and rebooted the instances, but no change in the active kernel observed from running uname -r.

B) I also tried the solution of deleting the kernel that I no longer want described [here] (https://stackoverflow.com/questions/25993363/how-to-remove-a-newly-installed-kernel) but the instance then fails status checks and I cannot connect to it using SSH.

C) I set the value for GRUB_DEFAULT to the menu_string as GRUB_DEFAULT="gnulinux-advanced-0d5e1897-19aa-4121-b1f8-2c7e0f833c4d>gnulinux-4.15.0-20-generic-advanced-0d5e1897-19aa-4121-b1f8-2c7e0f833c4d", rebuilt grub.cfg using sudo update-grub, but still no change after rebooting.

Answer 1

Open the terminal and write:

uname -r

This is your kernel version before starting. Now, let's change it!

Write in the terminal:

grub-install --version

Remember the GRUB version it's saying. Now open the file:

vi /boot/grub/grub.cfg 

Check what is the version you want, and take its name. For example:

    menuentry 'Ubuntu, with Linux 4.15.0-188-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.15.0-188-generic-advanced-7e6f0fb2-8fc4-4e31-92ef-912be559e099' {
            recordfail
            load_video

if the GRUB version you had is < 2.00 the name is:

Advanced options for Ubuntu>Ubuntu, with Linux 4.15.0-188-generic

otherwise is:

gnulinux-advanced-7e6f0fb2-8fc4-4e31-92ef-912be559e099>gnulinux-4.15.0-188-generic-advanced-7e6f0fb2-8fc4-4e31-92ef-912be559e099

Now, in the terminal:

sudo vi /etc/default/grub

Press key "i" in the keyboard to modify the file. Go to the line where it says GRUB_DEFAULT and paste the previous name. It should be for grub >2 (instead of GRUB_DEFAULT=0):

GRUB_DEFAULT="gnulinux-4.15.0-188-generic-advanced-7e6f0fb2-8fc4-4e31-92ef-912be559e099>gnulinux-4.15.0-188-generic-advanced-7e6f0fb2-8fc4-4e31-92ef-912be559e099"

Press key "Esc" in the keyboard. Write :wq and press enter. File has been saved. Now redo the grub. If you put it wrongly it will tell you how it should be!

sudo update-grub

Now reboot:

sudo reboot

Check your kernel version!

uname -r

I only did this, and worked for me. It's an EC2 with Ubuntu 18.04 and kernel 5.4.