@Maximilian-Gerhardt - this is awesome!!! Thank you.

Just wanted to thank everyone for helping me understand this. It's working great. I just need to get my head around the Linux side now. It works great with 'screen' - can send messages back and forth, but now I need the data from /dev/ttyS1 to get shoved into a curl POST as a line is captured. I'm thinking it'll look very similar to how I receive lines on the Arduino side (buffered char* terminated with \n\r etc). Anyone do this without pySerial?

@Maximilian-Gerhardt - Thanks! That explains it. Then I assume it's possible to push values / read from the Onion in the other direction?

I'm struggling to understand the relationship and interconnections between the Arduino Dock 2 and my Onion 2. Take a simple example of a gas sensor connected to Analog 0. I can write a program in the Arduino IDE to read this via Serial.read(A0) but how would access the value of A0 from inside OnionOS to lets say, push a value somewhere via curl. I was hoping someone can provide a high-level explanation or URL to some documentation that explains how to really wrangle the Arduino dock with the Onion? Maybe I should just be working with the onion dock if I want to control everything from inside linux with my familiar programming languages (Python, Node, etc). Thanks.