[fix] Check data file existence before sending #102

Closes #102

Author: stargieg

openwisp-monitoring/files/monitoring.agent

@@ -150,7 +150,14 @@ send_data() {
 				gzip -d "$file"
 			fi
 			filename="$TMP_DIR/$filename"
-			data=$(cat "$filename")
+			# check if the data file exist
+			if [ -f "$filename" ] ; then
+				data=$(cat "$filename")
+			else
+				[ "$VERBOSE_MODE" -eq "1" ] && logger -s "data file $filename not found." \
+					-p daemon.info
+				continue
+			fi
 			while true; do
 				if [ "$failures" -eq "$MAX_RETRIES" ]; then
 					[ -f "$RESPONSE_FILE" ] && error_message="\"$(cat "$RESPONSE_FILE")\"" || error_message='"".'