arduino --verify / --upload not working (command line)
It's quite possible that this "version 2.1" doesn't support command line processes. I've never heard of an Arduino IDE version 2.1. My understanding is the version you get from apt-get install arduino is something like 2:1.0.5+dfsg2 so maybe you interpreted that as 2.1. That version is based on Arduino IDE 1.0.5, though apparently it has some non-standard modifications. Command line operation was first added in Arduino IDE 1.5.2, though some additional functionalities were added in newer versions.

You should always install the official Arduino IDE downloaded from:
https://www.arduino.cc/en/Main/Software