How to install Ubuntu from within other Linux Distributions without reboot

Yes, there is a procedure to install Ubuntu offline (without rebooting) from your current Linux system. I do not think, It is better to include all the details here.

You can check the exact procedure for 14.04 / for 16.04 from Ubuntu Installation Guide

I have recently tried that and installed Kubuntu 12.04. It worked flawlessly.

It requires at least one reboot, to open your newly installed system. You can't switch from one OS to another without rebooting

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Reference: Ubuntu Installation Guide