Turn a scanned duplex pdf [1,2,3,5,6,4] into the correct order [1,2,3,4,5,6]

reorder-duplex-scan.sh

#!/bin/sh

FILENAME="$1"
COUNT=$(pdfinfo "$FILENAME" |  awk '$1 == "Pages:" {print $2}')
HALF=$((COUNT/2))
ARGUMENTS=""
for i in $(seq 1 $HALF)
do
  ARGUMENTS="$ARGUMENTS Reorder-Page-$i.pdf"
done

for i in $(seq $COUNT -1 $((HALF+1)))
do
  ARGUMENTS="$ARGUMENTS Reorder-Page-$i.pdf"
done

for i in $(seq 1 $COUNT)
do
  pdfseparate -f $i -l $i  "$FILENAME" "Reorder-Page-$i.pdf"
done
pdfunite $ARGUMENTS "$2"
rm Reorder-Page-*