Remove blank pages from PDF from command line

Question

I have a PDF that has some blank pages inserted. These pages are the background colour (grey in this case). I would like to remove these pages using a bash script.

It has been suggested that we can to scan for text using e.g. pdftotext, but in my case this does not find text even on the non-blank pages.

Answer 1

There does not appear to be a utility to remove blank pages from PDFs, but we can create a histogram of colours using the convert command from imagemagick. Blank slides will only have one entry which can be detected with wc. Once we have a list of non-blank pages we can feed this into pdftk.

Note that imagemagick numbers pages starting from 0, so we need to adjust for this. We can use a low value in the -density flag improve performance (though too low seems to result in imagemagick segfaulting).

If we call the following script pdf_rm_blank.sh, running pdf_rm_blank.sh A will create A.rm.pdf from A.pdf

#!/bin/sh
IN="$1"
PAGES=$(pdfinfo $IN.pdf | grep ^Pages: | tr -dc '0-9')

non_blank() {
    for i in $(seq 1 $PAGES)
    do
        if [ $(convert -density 35 "$IN.pdf[$((i-1))]" -define histogram:unique-colors=true -format %c histogram:info:- | wc -l) -ne 1 ]
        then
            echo $i
            #echo $i 1>&2
        fi
        echo -n . 1>&2
    done | tee out.tmp
    echo 1>&2
}

set +x
pdftk $IN.pdf cat $(non_blank) output $IN.rm.pdf

Answer 2

Thank you for the code, gmatht. I have modified it to check for page coverage using GhostScript and delete pages with coverage less than a threshold (0.1%).

#!/bin/sh
IN="$1"
filename=$(basename "${IN}")
filename="${filename%.*}"
PAGES=$(pdfinfo "$IN" | grep ^Pages: | tr -dc '0-9')

non_blank() {
    for i in $(seq 1 $PAGES)
    do
        PERCENT=$(gs -o -  -dFirstPage=${i} -dLastPage=${i} -sDEVICE=inkcov "$IN" | grep CMYK | nawk 'BEGIN { sum=0; } {sum += $1 + $2 + $3 + $4;} END { printf "%.5f\n", sum } ')
        if [ $(echo "$PERCENT > 0.001" | bc) -eq 1 ]
        then
            echo $i
            #echo $i 1>&2
        fi
        echo -n . 1>&2
    done | tee "$filename.tmp"
    echo 1>&2
}

set +x
pdftk "${IN}" cat $(non_blank) output "${filename}.pdf"