It would be a big help if you describe the project you are trying to implement and post the Arduino program.
I wonder is your problem due to the fact that the nano resets when the serial port is opened by the PC program? Your PC program should open the serial port, allow time for the Arduino to reset before trying to send data and then keep the serial port open until the PC program is completely finished with the Arduino.
This may be easier to with Python rather than a bash script. This Simple Python - Arduino demo may give you some ideas.
...R