The call

The value for the user attribute is passed into the class explicitly in the node definition site.pp:

node default {
  class { 'example' :
    user => 'pierre',
  }
}

The definition

The module, as defined in modules/example/manifests/init.pp creates a variable using that value, which we can access further down in the class definition:

class example (
  $user,
) {

  # notice how the double quotes allow for variable substitution
  debug("The value of the variable \$user is ${user}, double-quoted")

  # single quoted strings are literal
  debug('Single quoted strings are interpreted literally (no substitution).  The value of the variable \$user is ${user}')

}

Executing locally

When executed with puppet apply -d ./manifests/site.pp --modulepath=./modules/, we can see those debug lines in amongst the output:

...
Debug: Scope(Class[Example]): The value of the variable $user is pierre, double-quoted
Debug: Scope(Class[Example]): Single quoted strings are interpreted literally (no substitution).  The value of the variable \$user is ${user}
...

No defaults

There are currently no defaults set in the class, so if we run the script without explictly defining the ‘user’ variable:

node default {
  class { 'example' :
  }
}

we get an error:

...
Debug: Facter: fact "fips_enabled" has resolved to false.
Debug: Reset text domain to :production
Debug: importing '/home/alexs/devops-workstream/puppet/tutorial/03-class-variables/modules/example/manifests/init.pp' in environment production
Debug: Automatically imported example from example into production
Error: Evaluation Error: Error while evaluating a Resource Statement, Class[Example]: expects a value for parameter 'user' (file: /home/alexs/devops-workstream/puppet/tutorial/03-class-variables/manifests/site.pp, line: 11, column: 3) on node puppetnode

Adding class defaults:

By adding a default value to the user variable, we can still explicitly define it in the call, or omit it and use the default.

class example (
  $user = 'centos',
) {
...

Here’s the output when left undefined to fall back to the default:

...
Debug: Scope(Class[Example]): The value of the variable $user is centos, double-quoted
Debug: Scope(Class[Example]): Single quoted strings are interpreted literally (no substitution).  The value of the variable \$user is ${user}
...

Open-source

All the code for these tutorials is available as part of DevOps-Workstream. The segments from this tutorial specifically make up puppet/tutorial/03-class-variables and puppet/tutorial/04-class-defaults here on GitHub.

If you’d like to see all the previous and future installments of this tutorial, they’re available under the puppet tag.

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