The user may override the hidden option, causing hidden links to be listed. Note that it is also possible to hide a link by specifying its protection as non-listable. Such links will only be returned by the directory server when the actual name of the link has been specified. The filter attribute is a pointer to a program that can be used to filter the contents of directories to which links are made. In addition to the linked directory and arguments, the filter has access to all other information available from the current directory.
Filters come in several types.
By default, a filter is a directory filter, and is applied when searching directories. A hierarchy filter is similar to a directory filter. Directory and hierarchy filters come in two types. The default is post-expansion. The filter is applied after all union links have been expanded.