Ibuilt a tiny Unix‑like 'OS' with shell and filesystem for Arduino UNO (2KB RAM)

Your IO points can be individual files so you can 'echo 127 >/n/arduino/pwm0' to set and reading is just as easy 'cat /n/arduino/pwm0' If you want to save all the pin states you 'tar -c /n/arduino | gzip >arduinostate.tgz' then reverse that process to write the values back.

To change pin config you can create a ctl file which you can use to control and configure things using key=value pairs, e.g. echo 'pinmode pin=3 mode=output' >/n/arduino/ctl. Errors are communicated through the file system so a failed write() will return -1 along with an errstr(2) message to your calling process.

Want a cli in addition? No problem, just make a file called e.g. con (console) and hang a REPL off it. Now open that file using a console emulator like con(1) on Plan 9: 'con -C /n/arduino/con' and type your heart away.

I have toyed with the linked library on 9front and tinkered with robotfs and the RNG demos. This is one of the cleanest ways to interact with software IMO.