Facter is a separate utility upon which Puppet depends. It is the system used by Puppet to gather information about the target system (node); facter calls the nuggets of information facts. You may run facter from the command line to obtain real-time information from the system.
When facter is installed (as a dependency for puppet), several fact definitions are installed by default. You can reference each of these facts by name from the command line.
Running facter without any arguments causes facter to print all the facts known about the system. We will see in later chapters that facter can be extended with your own custom facts. All facts are available for you to use as variables; variables are discussed in the next section.
Variables in Puppet are marked with a dollar sign ($) character. When using variables within a manifest, it is preferred to enclose the variable within braces "${myvariable}" instead of "$myvariable". All of the facts from facter can be referenced as top scope variables (we will discuss scope in the next section). For example, the fully qualified domain name (FQDN) of the node may be referenced by "${::fqdn}". Variables can only contain alphabetic characters, numerals, and the underscore character (_). As a matter of style, variables should start with an alphabetic character. Never use dashes in variable names.
In the variable example explained in the There's more… section, the fully qualified domain name was referred to as ${::fqdn} rather than ${fqdn}; the double colons are how Puppet differentiates scope. The highest level scope, top scope or global, is referred to by two colons (::) at the beginning of a variable identifier. To reduce namespace collisions, always use fully scoped variable identifiers in your manifests. For a Unix user, think of top scope variables as the / (root) level. You can refer to variables using the double colon syntax similar to how you would refer to a directory by its full path. For the developer, you can think of top scope variables as global variables; however, unlike global variables, you must always refer to them with the double colon notation to guarantee that a local variable isn't obscuring the top scope variable.