Sometime in processing, you will need to listen to the serial port that arduino attached to. Finding out the portIndex is easy on windows by simply checking in Control Panel -> Device Manager. However when you are in linux (ubuntu), you will need to use command to do it.

1. First plugin your arduino to usb
2. type lsusb to see list of attached usb devices
3. type dmesg | tail and it show you which serial port your usb device bind to. In my case, it is something like: [3486.xxxxxx] usb 2-2: FTDI USB Serial Device Converter now attached to ttyUSB0
4. or, you may also type dmesg | grep tty
5. Then you need to figure out the index of this ttyUSB0. Ubuntu reserved ttyS0 to terminal and 1 until 31. normally ttyUSB0 means 32.