Re: PIPESTATUS differs from $? for compound command

[Mike Jonkmans]

On Thu, Dec 12, 2024 at 03:56:52AM -0500, Lawrence Velázquez wrote:
> On Thu, Dec 12, 2024, at 2:01 AM, Mike Jonkmans wrote:
> > Yes, the posix grammar doesn't allow it.
> > But it is not clear, from the bash documentation,
> > that a pipeline is not a command.
> Sure, the documentation doesn't explicitly say that.

AFAIAC the documentation is what this is about.
Posix or yacc grammar terms do not correspond exactly to the notions
in the documentation of bash.
I.e. 'command' is used but not defined in the doc's SHELL GRAMMAR.

> But it also doesn't say that pipelines are composed of pipelines,
> the way it says that lists are composed of pipelines.
> So why would you make that assumption?

Given the doc's definition of a pipeline:
        [time [-p]] [ ! ] command [ [|⎪|&] command2 ... ]

and note that nowhere else is a '!' found in the doc's grammar.

So the only way - given the doc - to parse '! ! false'
is by having a pipeline as command.

> > Moreover, bash accepts e.g.:
> >     ! ! false
> > How can you parse that, without a pipeline being a command?
> The yacc grammar uses two separate non-terminals to realize the
> colloquial notion of a "pipeline":

Yes, I am familiar with yacc. But it is about the documentation's grammar.

> > And, as said, ksh allows the construct (! true | ! true).
> > It is not impossible, it just is not posix.
> Of course it's not impossible, it's just goofy.  A pipeline like:
>       ! true | ! true | ! true
> looks like it's composed of several segments that are each being
> individually "negated", but it's logically equivalent [*] to:
>       ! ! ! true | true | true
> so why allow the confusion?  (POSIX doesn't permit the latter either,
> which is consistent at least.)
> [*]: Yes, I know that in bash ! ! just cancels out, so it probably
>      wouldn't be *exactly* the same.

PIPESTATUS and pipefail would make it different.
Useful? Maybe. But one can easily workaround it.
Note that I am not advocating a change in code in this thread.

-- 
Regards, Mike Jonkmans