I usually used tac | tac for a stupid way of pausing in a pipeline. Though it doesn’t work in this case. A typical use is if you want to watch the output of something slow-to-run changing but watch doesn’t work for some reason, eg:
while : ; do
( tput reset
run-slow-command ) | tac | tac
done
`cat` will begin output immediately. `tac` buffers the entire input in order to reverse the line order before printing it. Piping through tac ensures EOF is reached, while piping through tac a second time puts the lines back in order
