mkdir -p in

for i in $(seq 5); do
  echo $i >in/$i
done

(seq 6 && echo 6) >exp
echo 6 >out
(exec 3<&- 4<&- 5<&- 6<&- 7</dev/null 8<&7 9<&7 &&
 ulimit -n 10 &&
 sort -n -m --batch-size=7 -o out out in/1 in/2 in/3 in/4 in/5 out
)