$ ssh -fTNF './config' -MS './sockd5/ctrl_socket' -i './keys/id_rsa' -l 'root' -p '22' 'example.com' 2>&1 | cat -A; echo OK
ControlSocket ./sockd5/ctrl_socket already exists, disabling multiplexing^M$
^C

$ ssh -fTNF './config' -MS './sockd/ctrl_socket' -i './keys/id_rsa' -l 'root' -p '22' 'example.com'; echo OK
ControlSocket ./sockd/ctrl_socket already exists, disabling multiplexing
OK