Fast forward-backward (write-read)

[Tim Chase]

On 10/23/12 12:17, Virgil Stokes wrote:
> On 23-Oct-2012 18:09, Tim Chase wrote:
>>> Finally, to my question --- What is a fast way to write these
>>> variables to an external file and then read them in
>>> backwards?
>> Am I missing something, or would the fairly-standard "tac"
>> utility do the reversal you want?  It should[*] be optimized to
>> handle on-disk files in a smart manner.
> Not sure about "tac" --- could you provide more details on this
> and/or a simple example of how it could be used for fast reversed
> "reading" of a data file?

Well, if you're reading input.txt (and assuming it's one record per
line, separated by newlines), you can just use

  tac < input.txt > backwards.txt

which will create a secondary file that is the first file in reverse
order.  Your program can then process this secondary file in-order
(which would be backwards from your source).

I might have misunderstood your difficulty, but it _sounded_ like
you just want to inverse the order of a file.

-tkc