Scan two sided documents with a one-side only scanner

rearrange_double_sided_scan.sh

#!/bin/bash

readonly TMP_DIR="/tmp/rearrange_double_sided_scan.$$"

pages_rearranged() {
    local dir="$1"
    local count
    count=$(( $(ls "$dir" | wc -l) / 2 ))

    paste -d '\n' \
          <(ls "$TMP_DIR" | sort -n | head -n "$count") \
          <(ls "$TMP_DIR" | sort -n | tail -r -n "$count")
}

main() {
    local source_file="$1"

    mkdir "$TMP_DIR"
    pushd "$TMP_DIR" > /dev/null
    pdfseparate "$source_file" "%d"
    mv "$source_file" "${source_file/.pdf/}.original.pdf"
    pdfunite $(pages_rearranged .) "$source_file"
    popd > /dev/null
    rm "$TMP_DIR"/*
    rmdir "$TMP_DIR"
}

main "$@"