4.2 Run a command-aware TCP simulator

Extend the echo server so that GetID? returns the handler’s process ID and GetTemp? returns an integer between 0 and 100. Other lines are echoed.

Use the simulator/ directory created in Chapter 3. Keep connection_handler.sh for the default echo mode.

Replace the launcher and add a handler

1. In simulator/, replace tcpserver.bash with this version:

   #!/usr/bin/env bash
   # SPDX-License-Identifier: GPL-2.0-or-later
   # Run a line-based device handler for each local TCP connection.
   set -euo pipefail
   
   PORT=9399
   HANDLER=connection_handler.sh
   [[ $# -le 2 ]] || { printf 'Usage: %s [PORT] [HANDLER]\n' "$0" >&2; exit 2; }
   if [[ $# -gt 0 ]]; then
       if [[ "$1" =~ ^[0-9]+$ ]]; then
           PORT="$1"
           shift
       fi
       if [[ $# -gt 0 ]]; then
           HANDLER="$1"
           shift
       fi
   fi
   [[ $# -eq 0 ]] || { printf 'Too many arguments\n' >&2; exit 2; }
   [[ "$PORT" =~ ^[0-9]{1,5}$ ]] || { printf 'Invalid port\n' >&2; exit 2; }
   PORT="$((10#$PORT))"
   (( PORT >= 1 && PORT <= 65535 )) || { printf 'Invalid port\n' >&2; exit 2; }
   
   SCRIPT_DIR="$(cd -- "$(dirname -- "$0")" && pwd)"
   case "$HANDLER" in
       /*) HANDLER_SCRIPT="$HANDLER" ;;
       *) HANDLER_SCRIPT="$SCRIPT_DIR/$HANDLER" ;;
   esac
   [[ -x "$HANDLER_SCRIPT" ]] || { printf 'Handler is not executable: %s\n' "$HANDLER_SCRIPT" >&2; exit 1; }
   
   if command -v socat >/dev/null; then
       printf 'Attempting to start socat echo server on 127.0.0.1:%s\n' "$PORT"
       exec socat "TCP-LISTEN:$PORT,bind=127.0.0.1,reuseaddr,fork" "EXEC:'$HANDLER_SCRIPT'"
   elif command -v tcpsvd >/dev/null; then
       printf 'Attempting to start tcpsvd echo server on 127.0.0.1:%s\n' "$PORT"
       exec tcpsvd -l localhost -c 1 -vvE 127.0.0.1 "$PORT" "$HANDLER_SCRIPT"
   fi
   printf 'Install socat to run this simulator.\n' >&2
   exit 1
   

2. In the same directory, save advanced_connection_handler.sh:

   #!/usr/bin/env bash
   # SPDX-License-Identifier: GPL-2.0-or-later
   # Reply with a handler ID, a temperature, or the original input.
   while IFS= read -r received_text; do
       case "$received_text" in
           "GetID?") printf '%s\n' "$$" ;;
           "GetTemp?") printf '%s\n' "$((RANDOM % 101))" ;;
           *) printf '%s\n' "$received_text" ;;
       esac
   done
   

3. Make both scripts executable:

   chmod +x tcpserver.bash advanced_connection_handler.sh
   

Choose a port and handler

Start one of these commands in a simulator terminal:

Stop each server with Ctrl+C before trying another command on the same port.

Test the command-aware replies

1. Start the advanced handler on 9399:

   ./tcpserver.bash 9399 advanced_connection_handler.sh
   

2. In another terminal, connect a client:

   socat - TCP:127.0.0.1:9399
   

3. Enter these commands, one per line:

   GetID?
   GetTemp?
   WhatIsThis?
   

   The replies must be a numeric process ID, a temperature from 0 through 100, and the literal string WhatIsThis?.

Run three independent simulators

In three terminals, start:

./tcpserver.bash 9399 advanced_connection_handler.sh

./tcpserver.bash 9400 advanced_connection_handler.sh

./tcpserver.bash 9401 advanced_connection_handler.sh

If GNU Parallel is installed, the equivalent command is:

parallel ./tcpserver.bash {} advanced_connection_handler.sh ::: 9399 9400 9401

Each connection has its own handler process. This lets the next lesson check that the three IOC connections reach different devices.

Verification

Connect a client to each port and repeat the three commands above. The three handler IDs must differ. Stop all clients and servers with Ctrl+C; their listening ports must be released.