Create a virtual TTY device to log traffic on a physical serial port

virtual_tty

#!/bin/bash

# Default device path
DEFAULT_DEVICE="/dev/ttyUSB0"
DEFAULT_VIRT_DEVICE="/tmp/virtual_tty"

# Function to display usage information
usage() {
    echo "Usage: $0 [OPTIONS]"
    echo "Creates a virtual TTY connected to a real serial device with bidirectional logging."
    echo ""
    echo "Options:"
    echo "  -d, --device PATH    Serial device to connect to (default: $DEFAULT_DEVICE)"
    echo "  -v, --virtdev PATH   Virtual TTY device path (default: $DEFAULT_VIRT_DEVICE)"
    echo "  -b, --baud RATE      Baud rate (default: auto-detect from device)"
    echo "  -l, --logdir DIR     Directory to store logs (default: /tmp/tty_logs)"
    echo "  -h, --help           Display this help and exit"
    echo ""
    echo "Example: $0 --device /dev/ttyACM0 --baud 115200"
    exit 1
}

# Parse command line arguments
DEVICE=$DEFAULT_DEVICE
VIRT_DEVICE=$DEFAULT_VIRT_DEVICE
BAUD_RATE=115200
LOG_DIR="/tmp/tty_logs"

while [[ $# -gt 0 ]]; do
    case $1 in
        -d|--device)
            DEVICE="$2"
            shift 2
            ;;
        -v|--virtdev)
            VIRT_DEVICE="$2"
            shift 2
            ;;
        -b|--baud)
            BAUD_RATE="$2"
            shift 2
            ;;
        -l|--logdir)
            LOG_DIR="$2"
            shift 2
            ;;
        -h|--help)
            usage
            ;;
        *)
            echo "Unknown option: $1"
            usage
            ;;
    esac
done

# Check if device exists
if [ ! -c "$DEVICE" ]; then
    echo "Error: $DEVICE does not exist or is not a character device"
    exit 1
fi

# Create log directory
mkdir -p "$LOG_DIR"
timestamp=$(date +"%Y%m%d_%H%M%S")

# Auto-detect baud rate if not specified
if [ -z "$BAUD_RATE" ]; then
    BAUD_RATE=$(stty -F "$DEVICE" speed)
    echo "Detected baud rate: $BAUD_RATE"
else
    echo "Using specified baud rate: $BAUD_RATE"
fi

echo "Taking ownership of $DEVICE..."
sudo chown $USER $DEVICE

# Create FIFOs for logging
TX_FIFO="/tmp/tx_fifo_$$"
RX_FIFO="/tmp/rx_fifo_$$"
mkfifo $TX_FIFO
mkfifo $RX_FIFO

# Function to clean up on exit
cleanup() {
    echo -e "\nCleaning up and exiting..."
    # Kill background processes
    [[ -n $SOCAT_PID ]] && kill $SOCAT_PID 2>/dev/null
    [[ -n $TX_LOGGER_PID ]] && kill $TX_LOGGER_PID 2>/dev/null
    [[ -n $RX_LOGGER_PID ]] && kill $RX_LOGGER_PID 2>/dev/null
    # Remove temporary files
    rm -f $TX_FIFO $RX_FIFO
    echo "Logs saved to:"
    echo "TX log: $LOG_DIR/${timestamp}_tx.log"
    echo "RX log: $LOG_DIR/${timestamp}_rx.log"
    exit 0
}

# Set up trap for Ctrl+C and other signals
trap cleanup SIGINT SIGTERM EXIT

# Start TX and RX loggers
echo "Setting up bidirectional TTY with logging..."

# Use a simpler socat approach - one instance for the virtual TTY
socat -d PTY,raw,echo=0,link=$VIRT_DEVICE,mode=777 \
    OPEN:$DEVICE,raw,echo=0,crnl,b$BAUD_RATE &
SOCAT_PID=$!

# Wait for the virtual device to be ready
sleep 1

# Check if socat started successfully
if ! ps -p $SOCAT_PID > /dev/null; then
    echo "Error: Failed to create virtual TTY. Check permissions and device availability."
    cleanup
    exit 1
fi

# Set up TX logging (data to device)
socat -u OPEN:$VIRT_DEVICE,rdonly OPEN:$TX_FIFO,creat,append &
TX_SOCAT_PID=$!

# Set up RX logging (data from device) 
socat -u OPEN:$DEVICE,rdonly OPEN:$RX_FIFO,creat,append &
RX_SOCAT_PID=$!

# Start log file writers
cat $TX_FIFO > "$LOG_DIR/${timestamp}_tx.log" &
cat $RX_FIFO > "$LOG_DIR/${timestamp}_rx.log" &

echo -e "\nSetup complete!"
echo "Virtual TTY: $VIRT_DEVICE → $DEVICE"
echo "Baud rate: $BAUD_RATE"
echo "TX log (to device): $LOG_DIR/${timestamp}_tx.log"
echo "RX log (from device): $LOG_DIR/${timestamp}_rx.log"
echo -e "\nWaiting for communication (press Ctrl+C to exit)...\n"

# Keep script running until Ctrl+C
while true; do
    sleep 1
    # Check if socat is still running
    if ! ps -p $SOCAT_PID > /dev/null; then
        echo "Error: socat process terminated unexpectedly"
        exit 1
    fi
done