Using inline templates

Templates are a powerful way of using Embedded Puppet (EPP) or Embedded Ruby (ERB) to help build config files dynamically. You can also use EPP or ERB syntax directly without having to use a separate file by calling the inline_epp or inline_template function. EPP and ERB allow you to use conditional logic, iterate over arrays, and include variables. EPP is the replacement of ERB; EPP uses native Puppet language. ERB uses Ruby language. ERB allows for using native Ruby functions which may not be available in EPP, so unless you need something Ruby specific, it is better to go with the native EPP templates. In the following example, we'll use a Ruby construct, so we'll use an ERB inline template.

How to do it...

Here's an example of how to use inline_template.

Pass your Ruby code to inline_template within the Puppet manifest, as follows:

cron { 'chkrootkit':
  command => '/usr/sbin/chkrootkit > /var/log/chkrootkit.log 2>&1',
  hour    => inline_template('<%= @hostname.sum % 24 %>'),
  minute  => '00',
}

How it works...

Anything inside the string passed to inline_template is executed as if it were an ERB template. That is, anything inside the <%= and %> delimiters will be executed as Ruby code, and the rest will be treated as a string.

In this example, we use inline_template to compute a different hour for this cron resource (a scheduled job) for each machine, so that the same job does not run at the same time on all machines. For more on this technique, see the Efficiently distributing cron jobs recipe in Chapter 5, Users and Virtual Resources.

There's more...

In ERB code, whether inside a template file or an inline_template string, you can access your Puppet variables directly by name using an @ prefix, if they are in the current scope or the top scope (facts):

<%= @fqdn %>

To reference variables in another scope, use scope.lookupvar, as follows:

<%= "The value of something from otherclass is " +
scope.lookupvar('otherclass::something') %>

You should use inline templates sparingly. If you really need to use some complicated logic in your manifest, consider using a custom function instead (see the Creating custom functions recipe in Chapter 8, External Tools and the Puppet Ecosystem). As we'll see later, EPP templates use global scope for their variables; you always refer to variables with their full scope.

See also

• The Using ERB templates recipe in Chapter 4, Working with Files and Packages
• The Using array iteration in templates recipe in Chapter 4, Working with Files and Packages