swaygrab: Print usage and exit on unknown options

swaygrab/main.c

@@ -135,6 +135,15 @@ int main(int argc, char **argv) {
 		{0, 0, 0, 0}
 	};
 
+	const char *usage =
+		"Usage: swaygrab [options] <output> [file]\n"
+		"\n"
+		"  -c, --capture          Capture video.\n"
+		"  -v, --version          Show the version number and quit.\n"
+		"  -s, --socket <socket>  Use the specified socket.\n"
+		"  -R, --rate <rate>      Specify framerate (default: 30)\n"
+		"  -r, --raw              Write raw rgba data to stdout.\n";
+
 	int c;
 	while (1) {
 		int option_index = 0;
@@ -165,6 +174,9 @@ int main(int argc, char **argv) {
 #endif
 			exit(EXIT_SUCCESS);
 			break;
+		default:
+			fprintf(stderr, "%s", usage);
+			exit(EXIT_FAILURE);
 		}
 	}