system ("sort -n -t , -k 1,3 input_file.txt > ss.txt");
       rename (ss.txt,  input_file.txt);

##</code><code>##

       -k, --key=POS1[,POS2]
         start a key at POS1, end it at POS 2 (origin 1)


-t, --field-separator=SEP use SEP instead of non- to whitespace transition

 -n, --numeric-sort
              compare according to string numerical value