How to remove old kernels from Ubuntu

This method should work for both Ubuntu server OS (instances) and desktops that we use.

Use case: Automatic or manual updates to the OS may result in multiple kernels installed in your system. This is usually alright, but it can become a problem if your the OS has a small /boot partition, and it runs out of space because of a large number of old kernels installed there.

You could follow these simple steps to delete the older kernels

1. Verify the kernel that is currently booted and is in use:

uname -r
4.4.0-79-generic

 
Whatever you do, don't delete this kernel.

2. Finding out what kernels are currently installed in the system:

dpkg —list | grep linux-image

linux-image-4.4.0-72-generic 4.4.0-72.93  amd64 Linux kernel image for version 4.4.0 on 64 bit x86 SMP
linux-image-4.4.0-75-generic 4.4.0-75.96  amd64 Linux kernel image for version 4.4.0 on 64 bit x86 SMP
linux-image-4.4.0-77-generic 4.4.0-77.98  amd64 Linux kernel image for version 4.4.0 on 64 bit x86 SMP
linux-image-4.4.0-78-generic 4.4.0-78.99  amd64 Linux kernel image for version 4.4.0 on 64 bit x86 SMP
linux-image-4.4.0-79-generic 4.4.0-79.100 amd64 Linux kernel image for version 4.4.0 on 64 bit x86 SMP

3. Deleting the kernels that you don't need using apt-get:

sudo apt-get purge linux-image-x.x.x-xx-generic

4. Updating Grub:
Although this should be done automatically, manually updating grub shouldn't hurt.

sudo update-grub2

5. Reboot the system

Hope this helps.