GNU ln has this ( https://man.archlinux.org/man/ln.1#d https://man.archlinux.org/man/ln.1#d ): -d, -F, --directory allow the superuser to attempt to hard

GNU ln has this ( https://man.archlinux.org/man/ln.1#d https://man.archlinux.org/man/ln.1#d ): -d, -F, --directory allow the superuser to attempt to hard link directories (this will probably fail due to system restrictions, even for the superuser) which implies that that's not quite an absolute limit. I don't see any comment either way on https://illumos.org/man/1/ln https://illumos.org/man/1/ln , but it's plausible that some version of Solaris had wiggle room; it's a terrible idea for obvious reasons, but there's really no hard technical reason why a system couldn't allow you to create hard links to directories.