Re: Add support for '<&n' file descriptor filenames

[W. T. K. <wk...@tr...>]

On Sun, Feb 10, 2013 at 07:30:32PM -0800, sfeam (Ethan Merritt) wrote:
> Could you explain the motivation for the patch?
> The example you give
> 
>   cat data-1 | ./src/gnuplot -p -e "plot '<&0', '<&3', '<&4'" 3<data-2 4<data-3
> 
> seems entirely artificial since you could just as easily say
> 
>     gnuplot -p -e "plot data-1, data-2, data-3"
> 
> Alternatively you could use named pipes, which also work already.
> So what, really, is the use case for this?

I often have data-generating programs that I want to run several times
with different parameters.  I'd like to plot these with the same
gnuplot script.  The changing parameters make '<myprogram -a 1 -b 2'
impossible without script edits, and the:

  $ (cat plot.gp; myprogram -a 1 -b 2) | gnuplot

idiom scales poorly:

  $ (cat plot.gp; myprogram -a 1 -b 2; cat plot-2.gp; myprogram -a 3 -b 4) | gnuplot

where the plotting script has to be split between plot.gp and
plot-2.gp to grab the subshell-inlined data.

With named pipes, you'd have something like:

  $ mkfifo data-1
  $ mkfifo data-2
  $ gnuplot plot.gp
  $ myprogram -a 1 -b 2 > data-1
  $ myprogram -a 3 -b 4 > data-2
  $ rm -f data-1 data-2

which is also not particularly elegant.

I'm used to file descriptor redirection in the shell, and it seemed
like a natural fit:

  $ gnuplot plot.gp 3< <(myprogram -a 1 -b 2) 4< <(myprogram -a 3 -b 4)

This is basically the same as the named-pipe case, except that there
are no setup/teardown steps.

Thanks for taking a look :).

Trevor

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