#!/bin/sh

# the open is done right away, reading however, is done only after a
# connection is established to the unix domain socket. This keeps the file
# around, and also "waits" for the last write from the producer process to
# happen. (Tested with socat version 1.7.3.2(-1 debian testing)
socat open:/proc/$PID/fd/$FD unix-listen:/tmp/$PID.$FD.sock &

# Wait until the producer process exits, then "retrieve" the file.
socat unix-connect:/tmp/$PID.$FD.sock -> /tmp/the-file  # -> it is!