Re: Portability of shell parameter expansions (${word%pattern} and friends)?

[Ralf Wildenhues]

* Benoit Sigoure wrote on Tue, Nov 06, 2007 at 03:33:49PM CET:
>
> By the way, while we're at it, how portable is $((math expression))?  It 
> also saves lots of forks compared to `expr whatever`.

Same issue.  You should be able to factorize likewise with shell
functions, similar to Libtool's approach:

# ac_func_arith
# -------------
# Arithmetic evaluation, avoids expr if the shell is sane.
#
# subshell and eval are needed to keep Solaris sh from bailing out:
if ( eval 'test $(( 1 + 1 )) = 2' ) 2>/dev/null; then
  # With "$@", bash does not split positional parameters:
  eval 'ac_func_arith ()
  {
    ac_func_arith_result=$(( $* ))
  }'
else
  ac_func_arith ()
  {
    ac_func_arith_result=`expr "$@"`
  }
fi

# usage: avoid result=`ac_func...` to avoid fork
ac_func_arith 3 + 5
echo $ac_func_arith_result

Of course that limits you to the set of expressions common to both
methods, so instead of `3+5', use `3 + 5', and so on.

The above could help save a few forks in Autotest (plus one per test)
and some in _AC_FEATURE_CHECK_LENGTH, _AC_COMPUTE_INT_COMPILE (when
Autoconf starts to use shell functions).

Cheers,
Ralf