What I'd like therefore is a rationale: if -a, -o and parentheses are
not in POSIX 1990, please say so; if there's some other reason, then
please give it, and it had better be a good one, because if one cannot
program to nearly 20-year-old standards, then I begin to wonder what
the point of prescriptive standards is at all; we may as well give up
and write descriptive standards instead.
If you write a script with
#! /bin/sh
then yes, you cannot program to _any_ standard. Autoconf 2.64 is now
expecting that all systems have _somewhere in the PATH_ a POSIX shell,
but that need not be /bin/sh.
If you use M4sh, then you have access to almost all useful functionality
(e.g. shell functions). You also have wrappers to sanitize incompletely
specified parts (e.g. AS_ECHO), to work around widely deployed bugs
(e.g. AS_UNSET, or even in modern shells $LINENO which the Debian dash
lacks), and to speed up execution when commonly found extensions enable
it (e.g. AS_VAR_APPEND).
Paolo
