Find the process that is currently using the serial port

Run as root


$ fuser /dev/ttyUSB0 [or /dev/ttyS0]
> /dev/ttyUSB0:         1761


$ ps 1761
>  PID TTY      STAT   TIME COMMAND
   1761 pts/0    S+     0:00 minicom


This process can also be killed directly with fuser as root
$ fuser -k /dev/ttyUSB0


To quickly connect to board using GNU screen:
$ screen /dev/ttyUSB0 115200