Using facter to describe a node

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.

How to do it...

We'll compare the output of facter with that of system utilities:

1. Use facter to find the current uptime of the system, the uptime fact:

t@cookbook ~$ facter uptime 0:12 hours

2. Compare this with the output of the Linux uptime command:

t@cookbook ~$ uptime
01:18:52 up 12 min, 1 user, load average: 0.00, 0.00, 0.00

How it works...

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.

There's more...

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

Variables in Puppet are marked with a $ character. Variables are immutable; once assigned a value, they cannot be changed. When using variables within a manifest, it is advisable to enclose the variable within braces, such as ${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.

Scope

In the variable example explained in the previous paragraph, the FQDN 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, as in ::, at the beginning of a variable identifier. To reduce namespace collisions, always use fully scoped variable identifiers in your manifests. A Unix user can think of top-scope variables such 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. A developer 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. In Puppet5, it is advisable to use the $facts fact, so the previous would be ${facts['fqdn']}. When referring to a variable, the braces ({}) are optional outside of a string, as shown in the following example:

$fqdn_ = $facts['fqdn']
notify {"::fqdn is ${::fqdn}": }
notify {"fqdn_ is ${fqdn_}": }
notify {"facts['fqdn'] is ${facts['fqdn']}": }

This produces the following output:

t@mylaptop ~ $ puppet apply fqdn.pp
Notice: Compiled catalog for mylaptop.example.com in environment production in 0.01 seconds
Notice: ::fqdn is mylaptop.example.com
Notice: /Stage[main]/Main/Notify[::fqdn is mylaptop.example.com]/message: defined 'message' as '::fqdn is mylaptop.example.com'
Notice: fqdn_ is mylaptop.example.com
Notice: /Stage[main]/Main/Notify[fqdn_ is mylaptop.example.com]/message: defined 'message' as 'fqdn_ is mylaptop.example.com'
Notice: facts['fqdn'] is mylaptop.example.com
Notice: /Stage[main]/Main/Notify[facts['fqdn'] is mylaptop.example.com]/message: defined 'message' as 'facts[\'fqdn\'] is mylaptop.example.com'
Notice: Applied catalog in 0.02 seconds

$fqdn_ is used to avoid a namespace collision with the top-scope ::fqdn.