4.14 Make Duplicate Lines Unique

This is an example of the art of using the N, P and D commands, probably the most difficult to master.

     #!/usr/bin/sed -f
     h
     
     :b
     # On the last line, print and exit
     $b
     N
     /^\(.*\)\n\1$/ {
         # The two lines are identical.  Undo the effect of
         # the n command.
         g
         bb
     }
     
     # If the N command had added the last line, print and exit
     $b
     
     # The lines are different; print the first and go
     # back working on the second.
     P
     D

As you can see, we mantain a 2-line window using P and D. This technique is often used in advanced sed scripts.