Re: Single quotes in ARG in "${v:+ARG}" are erroneously required to be paired

[Chet Ramey]

On 7/18/18 8:25 AM, Denys Vlasenko wrote:
> Unquoted ${v:+ARG} operator allows single-quoted strings
> in ARG:
> 
> $ x=a; echo ${x:+'b c'}
> b c
> 
> In this case, obviously single quotes must be paired.
> 
> If ${v:+ARG} is in double-quoted string, single quotes lose their special
> status:
> 
> $ x=a; echo "${x:+'b c'}"
> 'b c'
> 
> IOW: they work similarly to ordinary dquoted strings: echo "It's sunny"
> treats single quote as ordinary char.
> 
> However, bash errors out if they are not paired:
> 
> $ x=a; echo "${x:+'b c}"
>>          <------------- bash thinks 'str' did not end yet

It's ambiguous, and the standard behavior changed in early 2010, so bash
does it the way you want in posix mode. The history is complicated.

The POSIX standard says, in section 2.2.3:

"Within the string of characters from an enclosed "${" to the matching '}',
an even number of unescaped double-quotes or single-quotes, if any, shall
occur."

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02_03

That could mean that it's a restriction on the application, but the
standard doesn't say that when it does say it in other places.  The
word `unescaped' could be taken to mean that the outer double quotes
escape the single quote and remove the restriction, but that doesn't
make sense in the context of describing the effects of double quotes.

Shells, including bash and ksh93, used to interpret this to mean that
the single quotes had to be balanced inside double-quoted ${...}.

There was a posix interpretation on this issue

http://austingroupbugs.net/view.php?id=221

that comes closer to your interpretation, but it's explicitly
inconsistent: some expansions treat single quotes specially, and some
don't. Bash adheres to this interpretation when in posix mode and uses
its historical behavior when not.

Chet
-- 
``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/