Re: Is this a bug? Why not?

[Bart Schaefer <schaefer@brasslantern.com>]

Incidentally, this came up because of a discussion on the POSIX standards
mailing list (austin-group) in which David Korn just asserted that he'd
like to add the syntax ${"var"} which means to expand the value of var
as if it's quoted (what zsh's normal mode does all the time).

This differs from "${var}" because you can write ${"var"-*} and get the
value of $var quoted but the glob pattern unquoted.  Apparently the old
Bourne shell allows the abominable "${var"-*}  (note quote placement
overlapping with brace placement) to accomplish this.

I was hoping to be able to say "Oh, zsh already has syntax XYZ for that"
but in fact we don't -- zsh either always, or never, does it, depending
on the globsubst option; there's no way to flip globsubst on the fly.

On Mar 30, 11:06pm, Phil Pennock wrote:
} Subject: Re: Is this a bug? Why not?
}
} So the string ${var} when treated as a variable-name is empty but
} defined?  The four expansions with a nested ${var} are entirely
} consistent with the behaviour when expanding a variable which is
} defined with a value of length 0.
} 
} Seems unusual, but consistent.

Indeed.  OK, I can live with that, as it's been that way forever.

} In bash, ksh, bad substitution.  In ksh93, it gets weird:
}   $ echo ${var-???}
}   OSM SCM bin dbg doc etc lib man src tmp www
}   $ echo ${${var}-???}
}   ksh93: syntax error: `!' unexpected

That is weird, but AFAIK only zsh of all shells allows wrapping ${...}
in another ${...} as valid syntax, so this may just be an error ksh93
didn't catch.

} The ksh reference-name expansion made me think of the somewhat opposite
} expansion in zsh, (P)var ...
} 
} Hrm, zsh 4.3.10 1.4705:
} 
}   % print ${(P)-???}
}   zsh: 0239BCJPXZgiklms: ??
} 
} Strange that the first entry is somehow tying into expanding $- for the
} shell options ...

Actually that makes perfect sense; only the :- and :+ forms are doc'd
as working with a missing parameter name, so ${(P)-} really does mean
to grab the value of $- and try to use that as a parameter name.  It
otherwise has to behave like ${-?string}.