On Mon, Nov 18, 2024 at 11:31:25 +0000, Klüver, Tibor wrote:
> Built-in `[ -a ... ]` returns true when false, negation with `!` is
> also very strange.
> `[[` is not affected, neither is `/bin/[`.
>
> Repeat-By:
> * touch file
> * [ -a file ] && echo exists => exists
> * [ ! -a file ] && echo exists => exists
The -a operator has two separate meanings in the test command.
It can either be "unary" (taking one argument), in which case it's
a synonym for -e. Or it can be "binary" (taking two arguments),
in which case it's meant to sit in between two other expressions,
and is treated as a Logical AND.
The more complex your test expression becomes, the more likely it is
that the parser will fail to guess your intention, and will use the
other meaning of -a.
Your best bet is simply to *never* use it. It's a daft synonym for -e,
so if you want to test for file existence, use -e. If you want a
Logical AND between two test expressions, simply call test twice:
[ "$x" ] && [ "$y" ]
This avoids all of the problems with -a.
