Nowhere above did I describe how to actually "root" the phone. That's all just unlocking the bootloader and installing TWRP.

Adding the "root" app (I highly recommend "SuperSU") is quite simple though. You'll have to download the .ZIP version of the app from the developer, rather than the Play Store (Android security won't allow it via normal Play Store). Then reboot into TWRP ("recovery") and "Install" the .ZIP from there. Done.

"Un-rooting" just means "uninstall the app (SuperSU)", best done from the in-app menu.

EDIT: By the way, I just checked here, and on my Linux box (Mint, but same for Ubuntu) the package required for fastboot can be installed thusly:

sudo apt-get install android-tools-fastboot

Same for adb:

sudo apt-get install android-tools-adb

Cheers