Re: FYI [PATCH] libtool: minimise forks per invocation under bash.

[Stefano Lattarini]

On 12/18/2011 11:07 AM, Gary V. Vaughan wrote:
> Hi Stefano,
>
> On 18 Dec 2011, at 17:02, Stefano Lattarini wrote:
>
> > On 12/18/2011 10:52 AM, Stefano Lattarini wrote:
> > > On 12/18/2011 06:15 AM, Gary V. Vaughan wrote:
> > > > +# We should try to minimise forks, especially on Windows where they are
> > > > +# unreasonably slow, so skip the feature probes when bash is being used:
> > > > +if test set = "${BASH_VERSION+set}"; then
> > > > +    : ${lt_HAVE_ARITH_OP="yes"}
> > > > +    : ${lt_HAVE_XSI_OPS="yes"}
> > > > +    # The += operator was introduced in bash 3.1
> > > > +    test -z "$lt_HAVE_PLUSEQ_OP" \
> > > > +&&   test 3000 -lt "$((${BASH_VERSINFO[0]}*1000 + ${BASH_VERSINFO[1]}))" \
> > > > +&&   lt_HAVE_PLUSEQ_OP=yes
> > > > +fi
> > > This will likely break with dash 0.5.2:
> > >
> > >   $ cat foo.sh
> > >   #!/bin/sh
> > >   if test set = "${BASH_VERSION+set}"; then
> > >     : ${lt_HAVE_ARITH_OP="yes"}
> > >     : ${lt_HAVE_XSI_OPS="yes"}
> > >     # The += operator was introduced in bash 3.1
> > >     test -z "$lt_HAVE_PLUSEQ_OP" \
> > > &&  test 3000 -lt "$((${BASH_VERSINFO[0]}*1000 + ${BASH_VERSINFO[1]}))" \
> > > &&  lt_HAVE_PLUSEQ_OP=yes
> > >   fi
> > >   $ dash foo.sh
> > >   foo.sh: 7: Syntax error: Bad substitution
> > Update: the same happens with NetBSD 5.1 /bin/sh (which is probably
> > an ash-derivative).
> Thanks for the report.
>
> Unfortunately, I'm out of ideas on how to portably detect the bash version 
> without
> spending a fork, in which case it seems easiest to spend that fork actually 
> testing
> for += support rather than poking at the bash version.
>
> Can anyone think of something better than just removing the whole 
> lt_HAVE_PLUSEQ_OP
> clause from the patch quoted above, and letting the shell figure it by itself 
> later
> on?
Adding an extra eval seems to do the trick:

  eval 'test 3000 -lt "$((${BASH_VERSINFO[0]}*1000 + 
${BASH_VERSINFO[1]}))"'

Of course, a comment about why this eval is needed would be definitely
warranted.

Or, to be even safer, you could directly poke at $BASH_VERSION instead:

  case $BASH_VERSION in
    [12].*|3.0.*) ;;
    *) lt_HAVE_PLUSEQ_OP=yes;;
  esac

Regards,
  Stefano