fp (FilterPath) is the filter equivalent for the Unix `test` util.
```sh
$ ls
file-foo dir-read-write dir-read-only
For help, run `fp` with `-?` or `--help`.
The accepted filter flags are the same ones used by Unix `test`.
See [POSIX documentation](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html)
or [`man test`](https://manned.org/man/test).