#!/bin/sh
[ $# = 1 ] || exit 1

PID="$(netstat -pan | fgrep ":$1 " | sed -n 's,^.*ESTABLISHED \([0-9]\+\).*$,\1,p')"

# vsftpd behaviour hardwired, 
readlink /proc/$PID/fd/7

# or do like this:
#ls -l /proc/$PID/fd/* | fgrep /var/ftp | cut -f12- -d" "