The file specification must be a complete file path specification and the file must have the same protections as the primary aliases file; for example, it must be world readable.
The contents of the included file are inserted into the aliases file at its point of reference. The same effect can be achieved by replacing the reference to the included file with the file's actual contents. The format of include files is identical to that of the primary aliases file itself. Indeed, include files may themselves include other files. Up to three levels of include file nesting are allowed.