Check validity of arguments

griddle

@@ -1,18 +1,30 @@
 #!/bin/sh
 
+set -e
+
 main() {
 	if test "$#" -ne 2; then
-		echo "Usage: griddle <inputfile> <outputfile>"
+		printf "Usage: griddle <inputfile> <outputfile>\n"
 		return 1
 	fi
 
 	inputfile="$1"
 	if ! test -e "$inputfile"; then
-		echo "Input file $inputfile does not exist."
+		printf "Input file %s does not exist.\n" "$inputfile"
 		return 2
 	fi
 
 	outputfile="$2"
+	if test -e "$outputfile"; then
+		printf "Output file $outputfile already exists. Overwrite? (y/N) "
+		read response
+		if ! test "$response" = "y" -o "$response" = "Y"; then
+			printf "Not overwriting $outputfile.\n"
+			return 0
+		else
+			printf "Overwriting $outputfile.\n"
+		fi
+	fi
 
 	echo "$inputfile" "$outputfile"
 }