arduino --verify / --upload not working (command line) I installed Arduino IDE version 2.1 on Ubuntu via apt-get. Then I typed the following command in terminal to see if the Arduino IDE compiles my sketch. 

 arduino --verify xx.ino  


However, this command launches the GUI IDE. Is the CLI not supported anymore, or is the cause in apt-get the package?
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