Re: [PATCH] Add shell-quasiquote.

[David Kastrup <dak@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Random832 <random832@fastmail.com>
>> Date: Wed, 21 Oct 2015 13:06:46 -0400
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> >> From: Random832 <random832@fastmail.com>
>> >> It doesn't have a documented way for the caller to insist that the
>> >> string be quoted for a POSIX shell rather than the user's shell.
>> >
>> > On what OS would that distinction be important, and why?
>> 
>> Any OS which may have both a POSIX shell that a script may want to
>> execute and a non-POSIX shell that is the user's shell. So basically all
>> of them, especially if support for more non-POSIX shells such as csh,
>> rc, scsh, fish, tclsh, is added in the future - or if a user's
>> configuration supports them in the present by replacing or advising the
>> function.
>
> First, do csh and the rest really non-Posix?  I wonder.  I always
> understood "Posix shells" as a short for "any shell on a Posix host".

I think it's more like "any shell trying to obey an (ex-)POSIX
standard".  Which would be
<URL:http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html>
and would not cover a csh.  Users may set a different login and/or
interactive shell.  That's strictly speaking no longer "POSIX" if it
does not try to be Bourne shell compatible in the main regards.

> Is that incorrect?  In what way are those "non-Posix"?

Not trying to meet an (ex-)POSIX standard I think.

>> It would mainly be useful in the presence of a broader mechanism,
>> which doesn't exist yet, for executing POSIX shell scripts regardless
>> of the user's interactive shell.
>
> On Posix hosts?  I thought that was automatic, since each script says
> what interpreter should run it in its "shebang" line.  Right?

Not necessarily.  You can leave off the shebang line and scripts will be
directly called by a fork (or something programmed to be equivalent) of
your shell which might be more efficient than going through shebang.  To
avoid a csh executing a Bourne-only script, you can place a single : in
the first line.  That's a noop for a Bourne shell, and it tells csh and
its ilk that it should not even try interpreting that script itself but
rather leave that to /bin/sh or similar.

These days, there are very few shell scripts _not_ started with a
shebang line.  But it's a definite possibility, even though then calling
such scripts _not_ from a shell command line might fail.  According to
the man pages for GNU/Linux I have here, system(3) uses /bin/sh and
should likely work whereas execve(2) requires the shebang line in order
to pick an interpreter rather than loading a binary.

Haven't tried this for a long time.

-- 
David Kastrup