Re: [PATCH] configure: silence 'shift' error message in version_ge()

[Eric Blake]

On 8/21/20 10:21 AM, Peter Maydell wrote:
> On Fri, 21 Aug 2020 at 16:00, Stefano Garzarella <sgarzare@redhat.com> wrote:
> > If there are less than 2 arguments in version_ge(), the second shift
> > prints this error:
> >      ../configure: line 232: shift: shift count out of range
> >
> > Let's shut it up, since we're expecting this situation.
> >
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > ---
> >   configure | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/configure b/configure
> > index 4e5fe33211..de4bd0df36 100755
> > --- a/configure
> > +++ b/configure
> > @@ -229,7 +229,7 @@ version_ge () {
> >           set x $local_ver1
> >           local_first=${2-0}
> >           # shift 2 does nothing if there are less than 2 arguments
> > -        shift; shift
> > +        shift; shift 2>/dev/null
>
> POSIX says
> https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#shift
>
> "If the n operand is invalid or is greater than "$#", this may be
> considered a syntax error and a non-interactive shell may exit"
>
> so I think that we need to actually avoid the excess shift,
> not just suppress any warning it might print. (I'm not sure
> Philippe's "shift || shift" patch can work for that, though,
> as the exit status doesn't distinguish "valid shift but don't
> do it again" from "valid shift and more args to come".)

Indeed.  'shift || shift' is not going to work.  'shift; shift 
2>/dev/null' is risky, as it can cause the shell to exit.  But this does 
exactly what you want:

shift ${2:+2}

which works out to 'shift 2' if $2 is set, or 'shift' (implicitly shift 
1) if $2 is not set.


--
Eric Blake, Principal Software Engineer
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