#!/bin/bash 
#
BackUpDIR="/mnt/backup/daily/";
SubDIRs="databases websites";

for SubDIR in $SubDIRs; 
do

 if [ $(find $BackUpDIR$SubDIR/ -name '*.xz' -mtime -6 | wc -l) -eq 0 ]; then
   # no fresh backups for last 6 days
   continue
 else
   # delete backups greater than 7 days old 
   find $BackUpDIR$SubDIR/ -name '*.xz' -mtime +7 -delete
 fi

done