avoid backing-up empty volumes to avoid typos

volume-backup.sh

@@ -6,6 +6,11 @@ usage() {
 }
 
 backup() {
+    if [ -z "$(ls -A /volume)" ]; then
+       >&2 echo "Volume is empty or missing, check if you specified a correct name"
+       exit 1
+    fi
+
     if ! [ "$ARCHIVE" == "-" ]; then
         mkdir -p `dirname /backup/$ARCHIVE`
     fi