Re: Built-in `test -a` returns incorrectly

[Chet Ramey]

On 11/18/24 6:31 AM, Klüver, Tibor wrote:

> Bash Version: 5.2
> Patch Level: 15
> Release Status: release
>
> Description:
>          Built-in `[ -a ... ]` returns true when false, negation with `!` is 
> also very strange.
>          `[[` is not affected, neither is `/bin/[`.

`test' uses an algorithm based on the number of arguments supplied. The
man page specifies the precedence and behavior for each number of
arguments.

> Repeat-By:
>          * touch file
>          * [ -a file ] && echo exists         => exists

Two arguments: -a is a unary primary with two arguments, and file exists.

>          * [ ! -a file ] && echo exists       => exists

Three arguments: -a is a binary primary, both arguments are non-empty
strings; the test succeeds.

>          * [ ! ! -a file ] && echo exists     => (nothing)

Four arguments: the first argument is `!' so this is the negation of
the three argument test; the test fails.

>          * [ ! ! ! -a file ] && echo exists   => (nothing)

Five arguments: operator-precedence parsing using the historical test
algorithm. It's basically `not-not-not' -a file, which is the same as
`not' -a file, which fails.

>          * [ ! ! ! ! -a file ] && echo exists => exists

Six arguments: operator-precedence parsing again.

Same as previous, just add another `not' at the beginning, so it's
the result of the binary `-a file' test, which succeeds.

This is the reason POSIX recommends not using more than two or three
argument tests and using the shell's && and || operators to compose
more complicated tests.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/

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