Frequently I find myself in need of a serial console which I can use to troubleshoot / interact with various embedded hardware devices. I recently found a hint on how to use the program 'screen' to do this.
All you need to do is start screen with the arguments
screen /dev/tty.usbserial-A200294u 9600
Once you are finished, type Ctrl+A followed by Ctrl+\ (confirm exit with 'y').
To dump the data to file, add the -L flag to the command:
screen -L /dev/tty.usbserial-A200294u 9600
This will write all output from the serial port to screen.0 in the current directory.
I have not yet figured out a generic way to send a sequence of bytes as found in an arbitrary file over a serial link. While I have seen some programs such as kermit or xmodem, they seem to assume certain file transfer protocols are being used, and I can't see how to just send the generic bytes. If you are aware of how to do this, please let me know.