Enhance backtunnel-share with improved desktop integration, port collision checks, and pre-flight validations

scripts/backtunnel-share

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 
 # Copyright (c) 2025. LUXIM d.o.o., Slovenia - Matjaž Mozetič.
+# Licensed under the GNU GPL v3.0
 
 # backtunnel-share: Share a folder using reverse SSH for a limited duration
 # Syntax: backtunnel-share /path/to/folder with remoteuser:remotehost for 2h
@@ -163,8 +164,23 @@ fi
 
 echo "Tip: On the remote side, mount with:"
 echo "     backtunnel-access '${FOLDER}' from ${REMOTE_USER}@${REMOTE_HOST} -p ${TUNNEL_PORT}"
+echo "To stop sharing early: press Ctrl+C here."
 
+# --- optional pre-flight: warn if remote loopback port is already in use (best-effort) ---
+if ssh -o BatchMode=yes -o ConnectTimeout=5 "${REMOTE_USER}@${REMOTE_HOST}" \
+     "command -v nc >/dev/null 2>&1 && nc -z 127.0.0.1 ${TUNNEL_PORT}"; then
+  echo "⚠️  Port ${TUNNEL_PORT} on remote 127.0.0.1 appears in use; choose another with -p." >&2
+  # Uncomment the next line to make it a hard failure:
+  # exit 1
+fi
+
+# Bind reverse to 127.0.0.1 on the REMOTE host (private-by-default).
+# Keepalive so the tunnel drops on dead links.
+# Exit fast if the reverse bind fails (e.g., port in use).
 timeout "$DURATION" ssh -N \
+  -o ExitOnForwardFailure=yes \
+  -o ServerAliveInterval=15 \
+  -o ServerAliveCountMax=3 \
   -R "${TUNNEL_PORT}:localhost:${LOCAL_SSH_PORT}" \
   -- "${REMOTE_USER}@${REMOTE_HOST}"