Re: Possible :q quoting bug

[Stephane Chazelas <stephane.chazelas@gmail.com>]

2019-08-08 20:36:00 +0430, Aryn Starr:
> I expect this function to be idempotent on the command line:
> ```
> Function reveal() {
> eval “${@:q}”
> }
> ```
> But it’s not. It loses empty args.
> I have written the following function to fix this ‘bug’:
> ```
> gquote () {
> 	local i
> 	for i in "$@"
> 	do
> 		test -z "$i" && print -rn "''" " " || print -rn -- "${i:q}" " "
> 	done
> }
> ```

You may want to use:

reveal() eval "${(qq)@}"

which is the zsh way to do it.

$var:q is reminiscent of tcsh's $var:q though works differently
(in tcsh, you do need cmd $var:q to pass the content of $var
literally to cmd as tcsh has a completly fucked-up way of
handling parameter expansions (hardly improved from the Thompson
shell's simplistic parameter expansion from the early 70s).

But like zsh's, tcsh's $var:q expands to nothing when $var is
empty or a list of empty elements.

Of the various ${(q)var}, ${(qq)var}, ${(qqq)var}, ${(qqqq)var}
${(q+)var}..., ${(qq)var} is the only one that is
localisation-safe as it always using single quotes. The other
ones (and also the quoting operators of other shells or of
various printf %q) are generally not safe as they may quote some
characters with \ (or use things like $'\n') and the encoding of
\ is found in the encoding of some multi-byte characters in some
locales.

Note that mksh added a ${var@Q} which was later copied by bash.

Initially, ${empty@Q} expanded to nothing (and it still does in
bash), but that changed at some point in mksh where it now
expands to ''. (possibly bash will follow).

I'd agree zsh's documentation is misleading though as it
currently says:

q
     Quote the substituted words, escaping further substitutions.  Works
     with history expansion and parameter expansion, though for
     parameters it is only useful if the resulting text is to be
     re-evaluated such as by eval.


-- 
Stephane