interleave pdf pages (parallel text extended)

#!/bin/sh
#script able to interleave the pages of two pdf files, saving the result in a new pdf file. Useful for any use, specially to mount parallel text books
echo "enter the name (with extension) of first PDF"
read filename1
echo "enter the name (with extension) of second PDF"
read filename2
pages1="`pdftk $filename1 dump_data output |grep Pages|cut -f2 -d :`"
pages2="`pdftk $filename2 dump_data output |grep Pages|cut -f2 -d :`"

if [ $pages1 -ge $pages2 ]
    then
    pagesincr="$(echo "scale=0; $pages2+1" |bc -l)"
echo "$filename1 has $pages1 pages"
echo "$filename2 has $pages2 pages"

rule="$(for x in $(seq 1 $pages2); do echo -n "A$x B$x ";  done; for x in $(seq $pagesincr $pages1); do echo -n "A$x ";done)"


    echo $rule

        elif
        [ $pages2 -ge $pages1 ]
            then
    pagesincr="$(echo "scale=0; $pages1+1" |bc -l)"

echo "$filename1 has $pages1 pages"
echo "$filename2 has $pages2 pages"

rule="$(for x in $(seq 1 $pages1); do echo -n "A$x B$x ";  done; for x in $(seq $pagesincr $pages2); do echo -n "B$x ";done)"


    echo $rule
                else
echo "$filename1 has $pages1 pages"
echo "$filename2 has $pages2 pages"

rule="$(for ((a=1, b=1; a <= $pages1, b <= $pages2 ; a++, b++)); do echo -n "A$a B$b "; done)"

echo $rule
fi

pdftk A=$filename1 B=$filename2 cat $rule output interleaved.pdf
echo "file created!"
exit 0