Re: Not to set PIPESTATUS unless necessary

[Andreas Kähäri]

On Sat, May 18, 2024 at 04:54:05AM -0500, Peng Yu wrote:
> It seems the problem is the definition of pipeline is wrong. If there is
> just one command, it can not be separated by control operators. Therefore,
> a pipeline must be made of two or more commands.
> 
> “ A pipeline is a sequence of one or more commands separated by one of the
> control operators ‘|’ or ‘|&’.”
> 
> The dictionary definition of pipeline is below. There must be transmission
> of information in a pipeline. With just one command, there can not be a
> pipeline.
> 
> https://www.merriam-webster.com/dictionary/pipeline
> 
> “ a direct channel for information”
> 
> Therefore, the current implementation appears to be a bug for following an
> error in the manual.

You would also need to take the POSIX standard's definiton of a pipeline
into account, which I belive would trumph any dictionary's definition
when it comes to implementing a Unix shell:

        A pipeline is a sequence of one or more commands separated by
        the control operator '|'.   [1]

This is more or less the same as what you found in the bash manual,
except for the extension that bash provides with |&
                
In the same document, you'll find the more formal grammar:

        pipeline         :      pipe_sequence
                         | Bang pipe_sequence
                         ;
        pipe_sequence    :                             command
                         | pipe_sequence '|' linebreak command
                         ;
        command          : simple_command
                         | compound_command
                         | compound_command redirect_list
                         | function_definition
                         ;

As you can see, a "pipeline" is made up from a "pipe sequence" or a "!"
followed by such a sequence.  A "pipe sequence" is a single "command" or
a "pipe sequence" followed by a "|" character, an (optional) linebreak
and a "command".  What a "command" is in the grammar should be self
evident.

On a higher level, a "program" is made up from "complete commands",
which are separate "lists" (AND-OR lists), which are single or multiple
"pipelines" separated by "||" or "&&".


[1]
https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/V3_chap02.html#tag_18_09_02


> 
> On Sat, May 11, 2024 at 1:28 PM Chet Ramey <chet.ramey@case.edu> wrote:
> 
> > On 5/10/24 5:13 PM, Peng Yu wrote:
> >
> > >> I don't
> > >> think an option to do this would be useful.
> > >
> > > I've already shown use cases where it may be useful. After all, it is
> > > not necessary to diligently set PIPESTATUS duplicating the info in $?
> > > for simple commands.
> >
> > I don't think it's generally useful enough to implement a special option
> > in the shell itself to enable it. It's easy enough to save $PIPESTATUS
> > if you want its value.
> >
> > --
> > ``The lyf so short, the craft so long to lerne.'' - Chaucer
> >                  ``Ars longa, vita brevis'' - Hippocrates
> > Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/
> >
> > --
> Regards,
> Peng

-- 
Andreas (Kusalananda) Kähäri
Uppsala, Sweden

.