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

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

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