On Sun, Oct 10, 2021 at 1:48 PM bm1les <bm1...@protonmail.com> wrote:
> Exactly my point. Even if the circumstances were contrived, I think it
> would good to fix it just for the sake of correctness.
>
Sure, knowing what circumstances could cause a problem assists in
achieving correctness.
> The issue is actually a pattern I found not only in /etc/netstart but also
> in /etc/rc. (( )) cannot deal with an empty result yet it sometimes
> includes calls to sysctl which apparently can return an empty string in
> some cases.
>
> So options are:
>
> 1. ensure that sysctl always returns a number where it is expected
> 2. work around the issue by using /bin/[ in place of or in addition to,
> the arithmetic expression (depending on the expression being tested), so
> that whatever returns empty string can be tested instead of blowing up.
>
3. use syntax with works when the expansion is empty, such as
(( $(sysctl blah) + 0 != 0 ))
which is unary plus when it's empty, or
(( $(sysctl blah)0 != 0 ))
which multiplies the value by 10 when not empty and is zero when it's
empty.
Philip Guenther
(Per POSIX rules, any arithmetic expression is effectively evaluated in
double-quotes. If you follow the spec from there, you can work out that if
an arithmetic expression that directly** contains double-quotes parses
correctly, then it must also parse correctly with the double-quotes
removed; adding double-quotes can't fix an arithmetic expression. The
reverse is not true: adding double-quotes can break the parsing. ** i.e.,
not counting quotes inside a nested parameter expansion or command
substitution.)
