I am quite new to Linux/Gnu myself. However, I have had my fair share of installing Ubuntu. My question for you is are you new to Linux? why would you run the command shell? Also, do you even know what developer mode is used for?
You can download Ubuntu from ubuntu.com, download the iso image, copy the image using Chromebook Recovery Utility. From there you may want to duckduckgo or google on how to enable usb booting.