Re: Optimising cat in pipelines

[Dave Jennings]

On 22/11/24 04:42, Lawrence Velázquez wrote:
> On Thu, Nov 21, 2024, at 12:55 AM, Dave Jennings wrote:
> > [[ $filter_option=reverse ]] && filter_fn=rev
> > [[ $output_option=start-only ]] && output_fn=head
>
> As an aside, these [[...]] commands always succeed because they
> treat their contents as a single nonnull operand.  They don't
> recognize the "=" operand unless it is surrounded by whitespace:
>
>         $ var=foo
>         $ [[ $var=bar ]]; echo "$?"
>         0
>         $ [[ $var = bar ]]; echo "$?"
>         1

Thanks. That was a typo. Definitely have the spaces in scripts.

> > My question is, does bash do any optimisation if it sees:
> >
> > input | cat | output
> >
> > Does it optimise the cat (and one of the pipes) away?
>
> As Greg already said, this is not done.
>
>
> > I'm guessing not but it would be very nice if it did.
>
> Such an optimization would be unsafe because bash can't know for
> sure that cat merely passes data through.  It could be a wrapper
> script, a loadable builtin, a function, or literally anything.

Yes. That makes sense.

So I'll use this technique in low bandwidth pipelines where the extra 
cat doesn't matter but in high bandwidth cases I'll craft a different 
path for each combination (it can explode a bit for longer pipelines but 
should be do-able).

Thanks, Dave.