File: sed.info,  Node: uniq -d,  Next: uniq_-u.php">uniq -u,  Prev: uniq,  Up: Examples

7.19 Print Duplicated Lines of Input
====================================

This script prints only duplicated lines, like ‘uniq -d’.

     #!/usr/bin/sed -nf

     $b
     N
     /^\(.*\)\n\1$/ {
         # Print the first of the duplicated lines
         s/.*\n//
         p

         # Loop until we get a different line
         :b
         $b
         N
         /^\(.*\)\n\1$/ {
             s/.*\n//
             bb
         }
     }

     # The last line cannot be followed by duplicates
     $b

     # Found a different one.  Leave it alone in the pattern space
     # and go back to the top, hunting its duplicates
     D