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Comment by neilk | Hacker News Reader

No, that just concatenates a file reading bytes from the end, backwards.

The real opposite of cat would be a putative 'trunc'. So if you typed

    $ cat somefile

It would print the file to your terminal. And then if you said

     $ trunc somefile

The printed lines would be removed from your terminal. :)

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In case you missed it, "tac" is literally the reverse of "cat". As in "cat".reverse() or something.

I get that tac exists and is useful for concatenating and printing files in reverse.

Neilk was just humorously pointing out that since cat stands for concatenate, its reverse should be truncate, and since cat is often abused to print a single file to a terminal this fictional trunc should "unprint" it.

Arguably you could view 'comm -3' as the reverse of cat.

  # output is all the lines of file1 and file2
  cat file1 file2

  # output is the lines in file1 not in file 2, and vice verse
  comm -3 file1 file2

> cat stands for concatenate

I almost resisted the urge, but no, the pedant in me :-) insists on pointing out that cat is short for catenate

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