Re: nofork command substitution

[Chet Ramey]

On 5/25/23 6:29 AM, Koichi Murase wrote:
> 2023年5月24日(水) 22:27 Chet Ramey <chet.ramey@case.edu>:
> > > [...]
> >
> > Thanks for testing it.
>
> Thank you for all the explanations and clarifications.
>
> > Both are valid, but don't get too fond of ${(command)} -- I'm going to
> > remove it in favor of people using ${ (command); } if they want a subshell.
>
> Does that mean ${(command)} would be explicitly unsupported even
> though {(command)} is allowed for the brace grouping?

It means that left paren is no longer one of the valid characters that
can follow `${'. If you want that, use a subshell command after a space.

> > > Then, can I understand the grammar in the following way?  First, there
> > > are two types of nofork command substitutions:
> > >
> > >     ${ compound_list }
> > >     ${| compound_list }
> > >
> > > where `compound_list' is what is defined by EBNF in POSIX XCU 2.10.2.
> >
> > You could always look at the shell grammar:
> >
> > [...]
> >
> > funsub:         DOLBRACE compound_list '}'
> >                           {
> >                             $$ = $2;
> >                           }
> >           |       DOLBRACE newline_list '}'
> >                           {
> >                             $$ = (COMMAND *)NULL;
> >                           }
>
> OK, I haven't checked the grammar definition in the source code. I now
> looked into parse.y, but it doesn't seem to be so clear to me what
> would be the distinction among the variations of C = <space>, <tab>,
> <newline>, `(', and `|' for `${C command; }' just by looking at the
> definition. It seems to be processed on subst.c:6880 semantically.

It is, but that's not the code I'm talking about. Look at parse_comsub()
in parse.y and the places that call it.

The different characters that follow the open brace don't really have
any effect on the compound_list that gets parsed. They determine how the
parsed command is executed.

--
``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/