convert wrapper to use pdftoppm in pdfsandwich

convert.sh

#!/bin/bash

# Convert wrapper for pdfsandwich, to avoid using imagemagick's convert for PDF to PBM conversion, as it relies on GS under the hood, 
# and GS security settings disallow such conversion on most distros. 
# Instead of ImageMagick's convert, we then rely on poppler-utils' pdftoppm
# To be invoked with 'pdfsandwich -convert .../convert.sh'

# By using this I figured out the options passed
# convert -verbose $*
# which are of the form
# -units PixelsPerInch  -type Bilevel -density 300x300 -verbose "/tmp/pdfsandwich_tmp743704/pdfsandwich_inputfile86bb3e.pdf[1]" /tmp/pdfsandwich_tmp743704/pdfsandwichcdbaf6.pbm

# Extract first "file + page" arg
pdffile="${@: -2:1}"

# Calculate the page
page=$(echo $pdffile | sed -e 's/[^\[]*\[//' -e 's/\]//')
page=$(expr $page + 1)
# cleanup the PDF filename
pdffile=$(echo $pdffile | sed -e 's/\[.*//')

# The script is invoked twice : for PDF to PBM conversion and later. Only the first one is diverted
if [ "${pdffile: -4}" == ".pdf" ]; then

    pdftoppm -singlefile -f $page -l $page "$pdffile" "${@: -1}"
    mv "${@: -1}".ppm "${@: -1}"
else
    convert $*
fi