Ubuntu -bash: do-release-upgrade: command not found

Ubuntu -bash: do-release-upgrade: command not found

Here is a sample session:
$ uname -mrs
$ lsb_release -a
$ sudo do-release-upgrade -d

How to fix do-release-upgrade: command not found error

You need to install package named ubuntu-release-upgrader-core that manages release upgrades. It is the core of the Ubuntu Release Upgrader. To install it, type the following apt command or apt-get command:
$ sudo apt install ubuntu-release-upgrader-core

How to upgrade Ubuntu 16.04 to 18.04

First update your current system by running the following command:
$ sudo apt update
$ sudo apt upgrade
Reboot the Linux system if a new Linux kernel was installed using the reboot command or shutdown command:
$ sudo reboot

Warning: Make sure you keep backups before you start upgrade procedure.

The do-release-upgrade command used to upgrade the operating system to the latest release from the command-line. This is the preferred command if the machine has no graphic environment or if the machine is to be upgraded over a remote connection such as ssh command. Now start the upgrade procedure:
$ sudo do-release-upgrade -d

Follow on-screen instructions. To finish the upgrade, a restart is required. At the end, you should be promoted to reboot the box as follow:

How to verify updated Ubuntu Linux 18.04 system

Find out the Linux kernel version, type:
$ uname -a
$ uname -mrs
Next find out Ubuntu Linux distro version, run:
$ lsb_release -a

Conclusion

You learned how to install do-release-upgrade tool and upgrade the operating system to the latest release. And there you have it Ubuntu Linux 16.04 LTS updated to Ubuntu 18.04 LTS successfully. See this page for more info.