#!/bin/bash

# caclurate the page counts of the two files
pagesA=$(pdftk abclager1.pdf dump_data | awk '/NumberOfPages/ {print $2}')
pagesB=$(pdftk abclager2_rev.pdf dump_data | awk '/NumberOfPages/ {print $2}')

# determine which file has more pages
if (( pagesA < pagesB )); then
    min=$pagesA
    max=$pagesB
    remainder="B"
else
    min=$pagesB
    max=$pagesA
    remainder="A"
fi

# generate a list of pages up to the common last page
for (( i = 1; i <= min; i++ )); do
    pages+=(A"$i" B"$i")
done

# append a list of remaining pages
for (( i = min+1; i <= max; i++ )); do
    pages+=("$remainder$i")
done

pdftk A=abclager1.pdf B=abclager2_rev.pdf cat "${pages[@]}" output output.pdf