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 Printing the Last Lines
 =======================
 
    Printing the last N lines rather than the first is more complex but
 indeed possible.  N is encoded in the second line, before the bang
 character.
 
    This script is similar to the `tac' script in that it keeps the
 final output in the hold space and prints it at the end:
 
      #!/usr/bin/sed -nf
      
      1! {; H; g; }
      1,10 !s/[^\n]*\n//
      $p
      h
 
    Mainly, the scripts keeps a window of 10 lines and slides it by
 adding a line and deleting the oldest (the substitution command on the
 second line works like a `D' command but does not restart the loop).
 
    The "sliding window" technique is a very powerful way to write
 efficient and complex `sed' scripts, because commands like `P' would
 require a lot of work if implemented manually.
 
    To introduce the technique, which is fully demonstrated in the rest
 of this chapter and is based on the `N', `P' and `D' commands, here is
 an implementation of `tail' using a simple "sliding window."
 
    This looks complicated but in fact the working is the same as the
 last script: after we have kicked in the appropriate number of lines,
 however, we stop using the hold space to keep inter-line state, and
 instead use `N' and `D' to slide pattern space by one line:
 
      #!/usr/bin/sed -f
      
      1h
      2,10 {; H; g; }
      $q
      1,9d
      N
      D
 
    Note how the first, second and fourth line are inactive after the
 first ten lines of input.  After that, all the script does is: exiting
 on the last line of input, appending the next input line to pattern
 space, and removing the first line.
 

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