adding suffix yyyymmdd (filename_20151028.csv) in format through shell script.

Adding date suffix in a file by using shell script mentioned  syntax...
 
       current_time=`date '+%Y%m%d'`  // get date in yyyymmdd format
        index=`echo "${#i}-4" | bc`        // here i is variable contains file name
        fileName=${i:0:$index}             // adding defined index
        extension=${i:$index}     
        echo $extension                             // printing extension