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.