It can be useful to override site defaults for a specific mount point on a client system. For example, assuming that the automounter maps are stored in NIS and the /etc/nsswitch.conf file has the following directive:

automount:  files nis

and the NIS auto.master map file contains the following:

/home auto.home

Also assume the NIS auto.home map contains the following:

beth      fileserver.example.com:/export/home/beth
joe      fileserver.example.com:/export/home/joe
*      fileserver.example.com:/export/home/&

and the file map /etc/auto.home does not exist.

For the above example, lets assume that the client system needs to mount home directories from a different server. In this case, the client will need to use the following /etc/auto.master map:

/home /etc/auto.home2
+auto.master

And the /etc/auto.home2 map contains the entry:

*   labserver.example.com:/export/home/&

Because only the first occurrence of a mount point is processed, /home will contain the contents of /etc/auto.home2 instead of the NIS auto.home map.

Alternatively, if you just want to augment the site-wide

auto.home

map with a few entries, create a /etc/auto.home file map, and in it put your new entries and at the end, include the NIS auto.home map. Then the /etc/auto.home file map might look similar to:

mydir someserver:/export/mydir
+auto.home

Given the NIS auto.home map listed above, an ls of /home would now give:

$ ls /home
beth	joe	mydir

This last example works as expected because autofs knows not to include the contents of a file map of the same name as the one it is reading and so moves on to the next map source in the nsswitch configuration.