full and diff backup with 7zip at linux

7zback.sh

#!/bin/bash                                                                                                                
# daily backup                                                                                                             
# last update 2017-12-25                                                                                                   
#                                                                                                                          
                                                                                                                           
function usage {                                                                                                           
  echo "$1 not set"                                                                                                        
  echo "usage: "                                                                                                           
  echo "7zback <BackToPath> <BackfromPath>" && exit 1                                                                      
}                                                                                                                          
                                                                                                                           
DIRR=$1                                                                                                                    
BACK=$2                                                                                                                    
                                                                                                                           
if [ -z $DIRR ]; then usage BackTo; fi                                                                                     
                                                                                                                           
if [ -z $BACK ]; then usage Backfrom; fi                                                                                   
                                                                                                                           
TODAY=$(date +"%Y%m%d%H%M")                                                                                                
MONTH=$(date +"%Y%m")                                                                                                      
                                                                                                                           
echo "Starting backup...." > log.                                                                                          
echo " "                                                                                                                   
                                                                                                                           
# www backup                                                                                                               
if [ ! -f $DIRR/back-$MONTH.7z ];                                                                                          
then                                                                                                                       
echo "www - creating monthly backup."                                                                                      
7za a $DIRR/back-$MONTH.7z "$BACK"                                                                                         
else                                                                                                                       
echo "www - creating nightly backup."                                                                                      
7za u $DIRR/back-$MONTH.7z "$BACK" -ms=off -t7z -u- -up0q3r2x2y2z0w2!$DIRR/back-$TODAY.7z                                  
fi                                                                                                                         
                                                                                                                           
echo                                                                                                                       
echo "Done!"