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7.10 Reverse Lines of Files
===========================

This one begins a series of totally useless (yet interesting) scripts
emulating various Unix commands.  This, in particular, is a ‘tac’
workalike.

   Note that on implementations other than GNU ‘sed’ this script might
easily overflow internal buffers.

     #!/usr/bin/sed -nf

     # reverse all lines of input, i.e. first line became last, ...

     # from the second line, the buffer (which contains all previous lines)
     # is *appended* to current line, so, the order will be reversed
     1! G

     # on the last line we're done -- print everything
     $ p

     # store everything on the buffer again
     h