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

From: Johannes Sixt <j.sixt@viscovery.net>
Date: Sep 5, 2011, 08:16 UTC
Message-ID: <4E648546.8060303@viscovery.net>
In-Reply-To: <7v39gbxwi6.fsf@alter.siamese.dyndns.org>

Am 9/5/2011 9:45, schrieb Junio C Hamano:

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> Johannes Sixt <j.sixt@viscovery.net> writes:
> 
>> 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.
> 
> That's strange...
> 
> I thought that VAR=<any string without $IFS character in it> would behave
> identically to VAR="<the same string as above>". You seem to be saying
> that they should act differently.

They are not the same.

First of all, the value of $IFS is irrelevant whether or not you need double-quotes on the RHS of an assignment, because it is purely a syntactic matter; $IFS plays no role during syntax analysis. It is only the presence of white-space that sometimes[*] requires quoting of some form.

The most visible difference is a backslash that is followed by a character that is not special:

$ foo="a\xb" env | grep foo; foo=a\xb env | grep foo foo=a\xb foo=axb

But it is the same elsewhere in a command:

$ echo "a\xb"; echo a\xb a\xb axb

The reason is that a backslash inside double-quotes remains as a literal character when it is not followed by a special character, whereas outside double-quotes an unquoted backslash is always removed.

[*] No quoting is required in cases like this: VAR=$(echo foo)

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>> [?] instead of \? is certainly also worth a try.
> 
> I obviously agree. Besides, [?] would sidestep the tricky backslash vs
> double quote issue entirely, so it would be a more robust solution to
> leave it around than "sometimes you need to avoid double-quote and some
> other times you would need double-quote" for other people to mimic writing
> tests later.

Good point, and I shall prefer this solution as well.

-- Hannes