Re: Avoid unportable nested double-quotes and backquotes.

[Paul Eggert]

On 05/23/2010 04:37 AM, Bruno Haible wrote:

> In summary, I did find a shell that does not understand "`...\"...\"...`"
> but I did not find a shell that does not understand "`..."..."...`". That
> paragraph in autoconf.texi dates back to 2000-05-19 12:17:29. It may have
> referred to shells that were in common use between 1995 and 2000 and have
> disappeared by now.

There is a similar warning in the POSIX-2008 rationale.  The rationale
for quoting
<http://www.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap02.html#tag_23_02_02_03>
says:

  Some implementations have allowed the end of the word to terminate the 
backquoted command substitution, such as in:

    "`echo hello"

  This usage is undefined....

I interpret this to mean that:

  ac_dir_suffix="/`echo "$ac_dir"|sed 's%^\./%%'`"

results in undefined behavior, since a POSIX-conforming implementation
is allowed to interpret this as if it were:

  ac_dir_suffix="/`echo `"$ac_dir"|sed 's%^\./%%'``"

which obviously is not wanted.

It could well be that no widely-used shells currently take advantage
of this misfeature of POSIX; still, it's safer to conform to POSIX
when it is easy to do so, as is the case here.