socat tricks - relaying serial ports, sniffing/monitoring/data logging traffic

shell commands

This first example assumes two usb serial port adapters are connected to the host machine.  They enumerate as ttyUSB0 and ttyUSB1.
We want data coming into ttyUSB0 to be piped right back out of ttyUSB1 (so our host sits as a man in the middle).
We want an application running on the host to be able to connect to a com port and monitor the traffic.

# create the fifo
sudo mkfifo -m 777 /tmp/fifo

# stream serial data on /dev/ttyUSB0 to the fifo AND a virtual com port /tmp/relay
sudo socat /dev/ttyUSB0,raw,echo=0 system:'sudo tee /tmp/fifo | sudo socat - "pty,raw,echo=0,link=/tmp/relay"' &

# read from the fifo and stream it back out /dev/ttyUSB1
sudo cat /tmp/fifo | sudo socat - "/dev/ttyUSB1,raw,echo=0" &

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This example takes data in on ttyUSB0 and records the input and output to log files as well as relaying it to a virtual com 
port so that an application running on the host machine can connect to the virtual com port.

touch in.log
touch out.log
sudo socat /dev/ttyUSB0,raw,echo=0 system:'tee in.log | socat - "pty,raw,echo=0,link=/tmp/relay" | out.log' &