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.