The second item in the specification gives the user much finer control on the lookup process. Action items are placed between two service names and are written within brackets. The general form is
status => success | notfound | unavail | tryagain action => return | continue
The case of the keywords is insignificant. The status values are the results of a call to a lookup function of a specific service. They mean
If we have a line like
ethers: nisplus [NOTFOUND=return] db files
this is equivalent to
ethers: nisplus [SUCCESS=return NOTFOUND=return UNAVAIL=continue TRYAGAIN=continue] db [SUCCESS=return NOTFOUND=continue UNAVAIL=continue TRYAGAIN=continue] files
(except that it would have to be written on one line). The default value for the actions are normally what you want, and only need to be changed in exceptional cases.
If the optional
! is placed before the status this means
the following action is used for all statuses but status itself.
! is negation as in the C language (and others).
Before we explain the exception which makes this action item necessary
one more remark: obviously it makes no sense to add another action
item after the
files service. Since there is no other service
following the action always is
Now, why is this
[NOTFOUND=return] action useful? To understand
this we should know that the
nisplus service is often
complete; i.e., if an entry is not available in the NIS+ tables it is
not available anywhere else. This is what is expressed by this action
item: it is useless to examine further services since they will not give
us a result.
The situation would be different if the NIS+ service is not available
because the machine is booting. In this case the return value of the
lookup function is not
notfound but instead
as you can see in the complete form above: in this situation the
files services are used. Neat, isn’t it? The
system administrator need not pay special care for the time the system
is not completely ready to work (while booting or shutdown or