Manage Nodes with Chef Client - Learning Chef - page 121

ohai output is in JavaScript Simple Object Notation (JSON) form. JSON is a commonly
used format for machine-readable output, as it can be easily parsed into the object-represent-
ation used for programming languages like Ruby. Although you can run ohai as a standalone
application, this is not very common. Instead, the output of ohai is intended to be read by
machines, specifically by chef-client and associated tools, so JSON is the perfect format.
chef-client reads the JSON output from ohai and converts it into a node object, which is
accessible by your Chef code.
You can refer to the node's IP address in your code with the following attribute . An attribute
is a variable maintained by Chef. In your code, you specify a quoted string in a pair of brack-
ets with the name used as an index in the collection, then Chef will return the value. In this
case, we want to know the IP address. By referring to the prior ohai output, Chef knows the
index name is ipaddress :
node [ 'ipaddress' ]
We used an attribute variable in our Chef code back in Recipes Specify Desired Configura-
tion . node is another attribute Chef makes available to your code. It contains all the informa-
tion generated by running ohai on the node. Similar to the ENV attribute we used in Recipes
Specify Desired Configuration , the node attribute is a collection of name/value pairs.
Name/value pair collections can be nested. This is what is indicated in the multiple levels of
indentation in the ohai output. So to access the kind of virtualization software we are using
(the “virtualization system”), use the following nested set of name/value pair references, be-
cause system is a name/value pair within the virtualization collection:
node [ 'virtualization' ][ 'system' ]