Re: Design question(s), re: why use of tmp-files or named-pipes(/dev/fd/N) instead of plain pipes?

[Ken Irving]

On Sun, Oct 18, 2015 at 07:36:49PM -0400, Chet Ramey wrote:
> On 10/17/15 8:43 PM, Linda Walsh wrote:
> > 
> > Chet Ramey wrote:
...
> >> I think you're missing that process substitution is a word expansion
> >> that is defined to expand to a filename.  When it uses /dev/fd, it
> >> uses pipes and exposes that pipe to the process as a filename in
> >> /dev/fd.  Named pipes are an alternative for systems that don't support
> >> /dev/fd.
> > -----
> >     ??? I've never seen a usage where it expands to a filename and
> > is treated as such.
> 
> Every example of process substitution ever given expands to a filename,
> and the result is treated as a filename.

The manpage section on process substitution could perhaps present the
concept more clearly by starting with something like the sentence just
above, e.g., very roughly:

    Process Substitution, taking the form of <(list) or >(list),
    expands the process list to a filename, allowing the construct to
    be used in place of a filename for output or input to a command.
    It is supported on systems that support named pipes (FIFOs) or the
    /dev/fd method of naming open files. ...

The section goes right into what seems like implementation details, and
the use of it is only mentioned in the fourth sentence or so.

Ken