Hi,
> OTOH, on Linux, after assigning the empty string, exists('$FOOBAR')
> still returns 1, while ":unlet $FOOBAR" gives error E488: Trailing
> characters.
>
> The classical way to test for "empty or undefined" is
>
> if ("X" . $FOOBAR) == "X"
>
> which, AFAICT, returns true even if $FOOBAR is undefined (i.e. has
> never been set in this environment).
We can also test for "is set" with
[[ -n ${FOOBAR+x} ]] && echo 1
or more simply with bash 4.2+
[[ -v FOOBAR ]] && echo 1
Anyway, the distinction becomes interesting as a few programs interpret the
unset state as "use a default value", while an empty string really means an
empty string.
Given foo.c, and a properly configured gnumake (i.e. not MinGW make), the
following will yield different results
$> CC=clang make foo
# prints: clang foo.c -o foo
$> CC= make foo
# prints: foo.c -o foo
# and can't compile anything
$> unset CC # just to assert the situation
$> make foo
# prints: cc foo.c -o foo
This is the recurrent use case I run into that's behind the issue I've opened a
while back.
Regards,
--
Luc Hermitte
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