File: sed.info,  Node: uniq,  Next: uniq -d,  Prev: tail,  Up: Examples

7.18 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 maintain a 2-line window using ‘P’ and ‘D’.  This
technique is often used in advanced ‘sed’ scripts.