copy a pdf by keeping the N first pages

pdfkeepn.sh

#! /bin/bash

if [ $# -ne 3 ]; then
	echo "Usage: $0 <n> <orig.pdf> <new.pdf>"
	exit 1
fi

command -v pdfseparate >/dev/null || { echo >&2 "require pdfseparate"; exit 1; }
command -v pdfunite    >/dev/null || { echo >&2 "require pdfunite"   ; exit 1; }

TMPDIR="$(mktemp -d -- /tmp/pdfkeepn.XXXXXXXX)" || exit 1
trap 'rm -rf -- "$TMPDIR"' EXIT

N="$1" ; shift
origpdf="$1" ; shift
finalpdf="$1" ; shift

if [ ! -f "$origpdf" ]; then
	echo "No such original file $origpdf"
	exit 1
fi

[ -d "$finalpdf" ] && finalpdf="$finalpdf/$(basename -- "$origpdf")"
if [ -e "$finalpdf" ]; then
	echo "destination $finalpdf already exists"
	exit 1
fi
if [ "$origpdf" = "$finalpdf" ]; then
	echo "source and destination are the same file!"
	exit 1
fi

listpages() { ( cd -- "$TMPDIR" && ls -1 -- *.pdf | sort -g; ); }

pdfseparate -f 1 -l $N "$origpdf" "$TMPDIR/%d.pdf" || echo "pdfseparate (returns $?)"

listpages

if [ $N -eq 1 ]; then
	mv -f -- "$TMPDIR/1.pdf" "$TMPDIR/merged.pdf"
else
	( cd -- "$TMPDIR" && pdfunite $(listpages) "merged.pdf"; ) || echo "pdfunite (returns $?)"
fi
mv -f -- "$TMPDIR/merged.pdf" "$finalpdf"