Re: [PATCH] shell portability: Use sed instead of non-portable variable expansion

From: Johannes Sixt <j.sixt@viscovery.net>
Date: Sep 5, 2011, 07:35 UTC
Message-ID: <4E647BD5.8060609@viscovery.net>
In-Reply-To: <7v7h5nxxwf.fsf@alter.siamese.dyndns.org>

Am 9/5/2011 9:15, schrieb Junio C Hamano:

Show 17 quoted lines

> Johannes Sixt <j.sixt@viscovery.net> writes:
> 
>>>  run_backend() {
>>>  	echo "$2" |
>>> -	QUERY_STRING="${1#*\?}" \
>>> -	PATH_TRANSLATED="$HTTPD_DOCUMENT_ROOT_PATH/${1%%\?*}" \
>>
>> What happens if you write these as
>>
>> 	QUERY_STRING=${1#*\?} \
>> 	PATH_TRANSLATED=$HTTPD_DOCUMENT_ROOT_PATH/${1%%\?*} \
>>
>> i.e., drop the double-quotes?
> 
> Interesting. Your conjecture is that the shell may be dropping the
> backslash inside dq context when it does not understand what follows the
> backslash, i.e. "\?"  -> "?", losing the quote. I find it very plausible.

Actually, it's the opposite: Within double-quotes, a backslash is only removed when the next character has a special meaning (essentially $, `, ", \), otherwise, it remains and loses its quoting ability. This means, that the backslash would remain as a literal character in our patterns on the right of % or #, and they would not work anymore as intended.

Other shells seem to parse the pattern following % and # in a different mode, which keeps the quoting ability of the backslash even inside double-quotes... (And to me it looks like those shells are wrong.)

Without double-quotes, backslashes (that are not themselves quoted) are always removed and give the subsequent character its literal meaning. Hence, in my version, the question mark would unambiguously (I think) act as a literal rather than a wildcard.

> If that is the case, either the above or my [?] would work it around, I
> would think.

[?] instead of \? is certainly also worth a try.

-- Hannes