If hostname doesn't have a domain part (e.g.: localhost) the coreutils binary prints hostname while GNU hostname prints nothing
GNU hostname:
$ hostname -f
debianpc
$ hostname -s
debianpc
$ hostname -d
$ hostname -sd
$
uutils hostname:
$ cargo run -r --all-features -- hostname -f
debianpc
$ cargo run -r --all-features -- hostname -s
debianpc
$ cargo run -r --all-features -- hostname -d
debianpc
$ cargo run -r --all-features -- hostname -sd
debianpc