Starting shellcheck cleanup

borg.backup.functions.close.sh

@@ -16,7 +16,7 @@ if [ $# -ge 1 ] && [ "$1" = "1" ]; then
 	printf "${PRINTF_MASTER_BLOCK}" "ERROR" "$(date +'%F %T')" "${MODULE}";
 else
 	# running time calculation
-	DURATION=$[ $(date +'%s')-$START ];
+	DURATION=$(( $(date +'%s')-START ));
 	echo "=== [Run time: $(convert_time ${DURATION})]";
 	printf "${PRINTF_MASTER_BLOCK}" "END" "$(date +'%F %T')" "${MODULE}";
 fi;