Re: M4sh tests 77 and 78 vs /bin/sh -> dash

[Bob Friesenhahn]

On Thu, 12 Mar 2020, Zack Weinberg wrote:
> I agree in principle.
>
> On further investigation, there is a concrete reason why autoconf 2.69
> prefers SHELL=/bin/bash to SHELL=/bin/dash, namely that dash doesn't
> support $LINENO, necessitating a complicated and fragile workaround
> (see _AS_LINENO_PREPARE in lib/m4sugar/m4sh.m4).  This has been the
> behavior for a very long time (since roughly 2008).  Moreover, the
> changed behavior of autoconf trunk was not intentional; it's because
> commit 2b59b6f8a79b8bf77e178ff4e5aa0ede433d39cf missed a direct use of
> $as_echo in _AS_DETECT_BETTER_SHELL, causing _AS_DETECT_BETTER_SHELL
> *not to work at all*!

This is a good find.  It explains a lot.

> I'm still looking into why tests/testsuite malfunctions when run by
> dash, but right now I am inclined to say that the bug in
> _AS_DETECT_BETTER_SHELL should be fixed and then we should
> independently discuss whether it makes sense to prefer dash to bash.
> It *would* discourage people from writing bashisms in their
> configure.ac's and it *is* 30% faster than bash (on my computer,
> according to "time make check"), but not supporting $LINENO is a big
> loss.

Years ago when we were investigating slowness of configure it was 
determined that Bash is the "world's slowest shell" since it was 
considerably slower than all other shells tested.  The difference was 
often more significant outside of GNU/Linux.  This likely has not 
changed.

One reason why some projects are discarding use of Autoconf (and 
moving to CMake and Meson/Ninja) is due to how long it takes to 
execute a configure script.  Given modern multicore CPUs, it often 
takes as much (or more) time to run the configure script than to 
compile the software.  If configure can select the fastest shell which 
works reliably, then Autoconf's reputation will improve.

Bob
--
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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