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3.18 groups/classes

The groups action (equivalently referred to as classes as of version 1.4.0) is used to define classes which stand for groups of hosts. If you use the NIS (network information service) facility for defining netgroups then this idea will already be familiar to you and you can probably use your already-defined netgroups in cfengine.

To define a group, you simply make a list and assign it a name. Here is an example of the syntax:

 
groups:
 
   science = ( saga tor odin )

   packages = ( saga ) 

   AllHomeServers   = ( saga )
   AllBinaryServers = ( saga )

   OIH_servers = ( saga )
   OIH_clients = ( tor odin )

   notthis = ( !this )

   ip_in_range = ( IPRange(129.0.0.1-15) )  # host is in ip address range
   ip_in_range = ( IPRange(129.0.0.1/24) )  # host is in ip address range (CIDR notation)

To include a list of hosts from a NIS netgroup, you use the `+' symbol, or the `+@' construction. For example:

 
groups:
 
   science = ( +science-allhosts )

   physics = ( +physics-allhosts )

   physics_theory = ( +@physics-theory-sun4 dirac feynman schwinger )

Using an enormous netgroup does not use up any space. A group declaration results in the storage of only the class name regardless of how many hosts are in the list. The rule is that the left hand side of the assignment becomes defined (true) if the list on the right hand side includes the host which is parsing the file -- i.e. $(host).

In some cases your netgroups will not correspond exactly to the list you want, but it might be more convenient to use a netgroup except for certain hosts. You can `undefine' or remove hosts from the netgroup list by using the minus `-' symbol. For example:

 
group = ( +mynetgroup -specialhost -otherhost )

which means, of course, all hosts in netgroup mynetgroup except for specialhost and otherhost. Finally, you may also subtract two netgroups in the following manner.

 
group = ( +bignetgroup -smallnetgroup )

The `minus' command effectively eliminates its members from bignetgroup if they exist within that group. If none of the hosts in smallnetgroup exist in bignetgroup then the command has no effect.

Groups may now contain previously defined cfengine groups too. This allows one class to inherit the attributes of another class, for instance:

 
  AllSun4Hosts   = ( sonny sunny solar stella )
  AllUltrixHosts = ( ully olly wally golly )

  AllBSD = ( AllSun4Hosts AllUltrixHosts )

The classes on the right hand side are effectively ORed together into the left hand side. This enables complex classes to be constructed from several other basic classes, e.g.

 
  SpecialTimes = ( Hr00 Monday Day1 )

which evaluates to true every day when it between 00:00 hours and 00:59, all day Monday and all day on the first day of every month.

Finally, you can define groups (strictly classes) by the result of a shell command. A shell command or program is deemed to be `true' if it exits with a status of zero, i.e. it calls exit(0). Any other value is taken to be false. You can include shell commands as the members of groups in order to define classes based on the outcomes of your own scripts by enclosing the script in single or double quotes:

 
   have_cc = ( '/bin/test -f /usr/ucb/cc' )

The class have_cc will then be defined if the shell command returns true. Of course, you can put any script or program in the single quotes as long as they adhere to the convention that zero exit status means true. If you have several members which are shell commands, then the effect is to make the class the logical OR of the scripts' results.

As of version 1.4.0, you may use the synonym classes for groups.


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