these two messages appear, even though I have a file called "java.opts" in Matlab's startup folder.
>> a = arduino('/dev/ttyACM0','Uno')
Failed to open serial port /dev/ttyACM0 to communicate with board Uno. Make sure there is no other MATLAB arduino object for this board. For troubleshooting, see Arduino Hardware Troubleshooting.
>> a = arduino('/dev/serial/by-id/ttyACM0','Uno')
Cannot find Arduino hardware on port /dev/serial/by-id/ttyACM0. Make sure Arduino hardware is properly plugged in. Otherwise, please specify both port and board type. For more information, see arduino function reference page.