Notes on updating Ubuntu from the command line (16.04 → 18.04, 18.04 → 20.04 both)

Basically, if you execute the following command, you can update (it takes a lot of time)

sudo apt-get -y update; sudo apt-get -y upgrade;
do-release-upgrade 

If you are asked to have a dialogue on the way, basically enter "y"

Would you like to remove a package that is no longer supported (or does not exist in the repository)?
186 packages will be removed.
It may take several hours to remove the package.
continue[yN]Details[d]

After completion, you can confirm the update completion from the following

$ cat /etc/os-release 
NAME="Ubuntu"
VERSION="20.04.1 LTS (Focal Fossa)"
…

Error Please install all available updates for your release before upgrading.

$ do-release-upgrade 
Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.

Resolved by apt-get -y update and sudo apt-get -y upgrade

sudo apt-get -y update; sudo apt-get -y upgrade;

Error You have not rebooted after updating a package which requires a reboot. Please reboot before upgrading.

$ do-release-upgrade
Checking for a new Ubuntu release
You have not rebooted after updating a package which requires a reboot. Please reboot before upgrading.

Reboot to solve

reboot

Error Package version cannot be upgraded

It may be good to uninstall the corresponding package

#(Example) When uninstalling ros kinetic
sudo apt-get remove ros-*

reference

Please install all available updates for your release before upgrading｜Ubuntu