O sinal -W é uma opção que a maioria dos usuários de BSD/OSX nunca precisará usar, pois só se aplica a " montagens sindicais ". Esse tipo de montagem simplesmente permite que vários sistemas de arquivos sejam montados, aparecendo como apenas um sistema de arquivos. Para explicar isso ainda mais, aqui está o resumo de um artigo, intitulado " Acumulações da União em 4.4BSD- Lite ", dos autores, Jan-Simon Pendry e Marshall Kirk McKusick:
Unlike a traditional mount that hides the contents of the directory on which it is placed, a union mount presents a view of a merger of the two directories. Although only the filesystem at the top of the union stack can be modified, the union filesystem gives the appearance of allowing anything to be deleted or modified. Files in the lower layer may be deleted with whiteout in the top layer. Files to be modified are automatically copied to the top layer. This new functionality makes possible several new applications including the ability to apply patches to a CD-ROM and eliminate symbolic links generated by an automounter. Also possible is the provision of per-user views of the filesystem, allowing private views of a shared work area, or local builds from a centrally shared read-only source tree.
Este artigo também discute as operações de whiteout:
If a name is being removed from the lower layer's namespace a whiteout is created in the upper layer. A whiteout has the effect of masking out the name in the lower layer. Operations that cause a whiteout to be created include unlink, rmdir and rename.
Colocando-o completamente, rm com o sinalizador -W usa o "whiteout" criado na camada superior para restaurar os arquivos "excluídos" da camada inferior. Para uso, consulte o manual .