add sequential number at the beginning of files in Bash : I have 5 files I wan to add sequential numbers and tabulation at the beginning of each file but the second file should start with the last number from the first file and so on here's an example:
 

    file1
    line1
    line2
    ....
    line13
    
    file2
    line1
    line2
  

    file5
    line1
    line2
    


    output file1
    1  line1
    ........
    13 line13
   
    output file2
    14 line1
    15 line2 

    and so on 

   