Add an option to shrink the PDF file where it is

.github/workflows/test.yml

@@ -31,3 +31,12 @@ jobs:
         chmod +x shrinkpdf.sh
         ./shrinkpdf.sh -g -r 72 -o new.pdf orig.pdf
         ls -al orig.pdf new.pdf
+
+      # Shrink the PDF in place.
+    - name: Shrink the PDF in place
+      run: |
+        chmod +x shrinkpdf.sh
+        cp orig.pdf inplace.pdf
+        ls -al inplace.pdf
+        ./shrinkpdf.sh -g -r 72 -i inplace.pdf
+        ls -al inplace.pdf orig.pdf new.pdf

shrinkpdf.sh

@@ -29,6 +29,40 @@
 # ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 
+create_tempdir ()
+{
+	if command -v mktemp >/dev/null 2>&1; then
+		# Possibly unportable, but far more secure than the fallback below
+		mktemp -d
+	else
+		# Fallback method
+		temp_base="${TMPDIR:-/tmp}"
+		# $RANDOM might be undefined, but including a possibly empty string doesn't hurt
+		# shellcheck disable=SC3028
+		temp_dir="$temp_base/shrinkpdf.sh.$$.$RANDOM.$(date +%s)"
+
+		if mkdir "$temp_dir" 2>/dev/null; then
+			chmod 700 "$temp_dir"
+			echo "$temp_dir"
+		else
+			echo "could not create temporary directory $temp_dir." >&2
+			exit 1
+		fi
+	fi
+}
+
+cleanup_tempdir ()
+{
+	if [ -n "$1" ] ; then
+		rm -f "$1/output.pdf"
+		# ShellCheck complains about ls and weird filenames, but we only care about the number of newlines
+		# shellcheck disable=SC2012
+		if [ "$( ls -A "$1" 2>/dev/null | wc -l )" -eq 0 ] ; then
+			# only rm -fr if the directory is empty to avoid recursive deletion catastrophe bugs
+			rm -fr "$1"
+		fi
+	fi
+}
 
 shrink ()
 {
@@ -86,15 +120,17 @@ check_input_file ()
 check_smaller ()
 {
 	# If $1 and $2 are regular files, we can compare file sizes to
-	# see if we succeeded in shrinking. If not, we copy $1 over $2:
+	# see if we succeeded in shrinking.
 	if [ ! -f "$1" ] || [ ! -f "$2" ]; then
-		return 0;
+		echo 0;
+		return;
 	fi
 	ISIZE="$(wc -c "$1" | awk '{ print $1 }')"
 	OSIZE="$(wc -c "$2" | awk '{ print $1 }')"
 	if [ "$ISIZE" -lt "$OSIZE" ]; then
-		echo "Input smaller than output, doing straight copy" >&2
-		cp "$1" "$2"
+		echo 1;
+	else
+		echo 0;
 	fi
 }
 
@@ -105,7 +141,7 @@ check_overwrite ()
 	# Unfortunately the stronger `-ef` test is not in POSIX.
 	if [ "$1" = "$2" ]; then
 		echo "The output file is the same as the input file. This would truncate the file." >&2
-		echo "Use a temporary file as an intermediate step." >&2
+		echo "Use a temporary file as an intermediate step, or use the -i flag." >&2
 		return 1
 	fi
 }
@@ -123,16 +159,18 @@ usage ()
 	echo " -o  Output file, default is standard output."
 	echo " -r  Resolution in DPI, default is 72."
 	echo " -t  Threshold multiplier for an image to qualify for downsampling, default is 1.5"
+	echo " -i  Inplace operation: Overwrite the input file with the output if smaller. Use with caution."
 }
 
 # Set default option values.
 grayscale=""
 ofile="-"
 res="72"
 threshold="1.5"
+inplace=0
 
 # Parse command line options.
-while getopts ':hgo:r:t:' flag; do
+while getopts ':hgo:ir:t:' flag; do
   case $flag in
     h)
       usage "$0"
@@ -142,7 +180,25 @@ while getopts ':hgo:r:t:' flag; do
       grayscale="YES"
       ;;
     o)
-      ofile="${OPTARG}"
+      if [ "-" = "$ofile" ] ; then
+      	ofile="${OPTARG}"
+      else
+      	echo "invalid combination of options -o and -i."
+      	exit 1
+      fi
+      ;;
+    i)
+      if [ "-" = "$ofile" ] ; then
+      	temp_dir=$(create_tempdir)
+      	# I actually want this string expanded right here.
+      	# shellcheck disable=SC2064
+      	trap "cleanup_tempdir \"$temp_dir\"" EXIT INT TERM
+      	ofile="$temp_dir/output.pdf"
+      	inplace=1
+      else
+      	echo "invalid combination of options -o and -i."
+      	exit 1
+      fi
       ;;
     r)
       res="${OPTARG}"
@@ -179,4 +235,18 @@ get_pdf_version "$ifile" || pdf_version="1.5"
 shrink "$ifile" "$ofile" "$res" "$pdf_version" "$threshold" || exit $?
 
 # Check that the output is actually smaller.
-check_smaller "$ifile" "$ofile"
+if [ 1 -eq "$(check_smaller "$ifile" "$ofile")" ] ; then
+	# file got bigger, just overwrite with the smaller original
+	echo "Input smaller than output, doing straight copy" >&2
+	cp "$ifile" "$ofile"
+else
+	# file has shrunk
+	if [ 1 -eq "$inplace" ] ; then
+		# user requested inplace operation, so overwrite the original
+		# input file with the temporary and smaller output
+		cp "$ofile" "$ifile"
+
+		# if not inplace:
+		# noting to do, output has already been written to the right place
+	fi
+fi