cat  > a4scanned_to_a5booklet.sh <<EOF
#!/bin/bash
#
# a4scanned_to_a5booklet.sh scanned_front_pages.pdf scanned_back_pages.pdf booklet.pdf

# create a base filename that does not exist in the current directory
bfn="a4s2a5tmp"
while [ -f $bfn ]
do
  bfn=${bfn}x
done

# check page numbers
fnp=$(pdfinfo "$1" | awk '/^Pages:/ {print $2}')
bnp=$(pdfinfo "$2" | awk '/^Pages:/ {print $2}')
n=$fnp
if [ $fnp -ne $bnp ]
then	
  echo "Error: Page number mismatch"
  exit
fi

# split PDFs into images
echo "Splitting $1"
pdfimages $1 ${bfn}front
echo "Splitting $2"
pdfimages $2 ${bfn}back

# split each image into two and re-order pages
i=0
for f in ${bfn}front*
do
  echo "Processing $f"
  convert -rotate 270 -crop '50%x100%' +repage $f ${bfn}-%d
  nn=$(printf "%05d" $((4*n-2*i)))
  mv -i ${bfn}-0 ${bfn}-$nn
  nn=$(printf "%05d" $((1+2*i)))
  mv -i ${bfn}-1 ${bfn}-$nn
  i=$((i+1))
done
i=0
for f in ${bfn}back*
do
  echo "Processing $f"
  convert -rotate 270 -crop '50%x100%' +repage $f ${bfn}-%d
  nn=$(printf "%05d" $((2*n-2*i)))
  mv -i ${bfn}-0 ${bfn}-$nn
  nn=$(printf "%05d" $((2*n+1+2*i)))
  mv -i ${bfn}-1 ${bfn}-$nn
  i=$((i+1))
done

# combine pages to a single PDF
echo "Combining pages"
convert -compress jpeg -quality 80 ${bfn}-* $3

# delete temporary files
read -p "Are you sure you want to delete ${bfn}* (temporary files)? " -n 1 -r
echo
if [[ $REPLY =~ ^[Yy]$ ]]
then
  echo "Deleting temporary files..."
  rm ${bfn}*
fi
EOF
chmod +x a4scanned_to_a5booklet.sh

a4scanned_to_a5booklet.sh scanned_front_pages.pdf scanned_back_pages.pdf booklet.pdf