We can do this as follows:
awk '1;!(NR%<Number after which the line has to be insserted>){print "String to be inserted";}' origionalfiletoprocess >outfilewithinsertedstring
Eg:
awk '1;!(NR%100){print "Shashwat Shriparv";}' filecontainingtxt>outputfilewithnewinsertedlines
This command will read filecontainingtxt and will insert string Shashwat Shriparv after 100 lines the the output fill will be outputfilewithnewinsertedlines
