delete backup file if it is the same as the last one to avoid writing a file

when no files have changed on a daily backup

backup_cosmicvoyage

@@ -7,15 +7,40 @@ test -d $SHIPS || (echo "ships does not exit" ; exit 1)
 
 test -d $BACKUP || mkdir $BACKUP
 
+OLDBACKUP=`ls -1 $BACKUP/ships-*.tar.gz 2>/dev/null|tail -1`
+
 cd $SHIPS
 
 TS=`date -u +%Y-%m-%dZ%H:%M`
 
 TGZ="$BACKUP/ships-$TS.tar.gz"
 
-echo backing up $SHIPS to $TGZ
+# if you run the script more than once within 1 minute it gets
+# confused
+
+if [ -f $TGZ ]
+then
+    echo backup file $TGZ already exists, not overwriting
+    exit 1
+fi
+
+
+echo "backing up $SHIPS/* to $TGZ"
 
 tar cvfz $TGZ */*.txt */LICENSE
 
+# don't keep backup file if nothing has changed
+
+if [ -f "$OLDBACKUP" ]
+then
+    if cmp -s "$OLDACKUP" $TGZ
+    then
+	echo "file is different, keeping new file"
+    else
+	echo "nothing has changed, no new backup"
+	rm -f $TGZ
+    fi
+fi
+
 test -f $TGZ && echo backup of file $TGZ was successful