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I was really happy about tac when I found out about it, same with paste and comm as already mentioned.

Gotten a lot of mileage out of the various seldom used options of uniq, diff and cut too.

I have a friend who uses `tac | tac` in pipelines to make the pipeline wait for all the data before continuing, which is an interesting hack.

If you install the "moreutils", there's sponge doing the same.

tac, but also rev. I can't think of an example right now, but that's been useful as well.

I often use rev if I need something from the end of log lines. Rev, cut field by delimiter, rev back