Check if a variable is undef in puppet template

Question

What is the proper way to check if a variable is undef in a puppet template?

In the manifest the variable is defined as follows

$myvar = undef

How is this checked in the template?

Is saw the following two variants

<% if @myvar -%>
<% end -%>

and

<% if not @myvar.nil? and @myvar -%>
<% end -%>

They both seem to work in my case, but I wonder if the first approach fails in on certain cases?

Answer 1

The Puppet documentation (at the time of writing this answer) explains it very well: https://puppet.com/docs/puppet/latest/lang_template_erb.html#concept-5365

Since undef is not the same as false, just using an if is not a good way to check for it. Also when a variable is defined, but has a value of false or nil it is also impossible to check with a simple if.

This is why you want to use scope.lookupvar(‘variable’) and check its return value for :undef or :undefined (or nil) to know if it was set to undef, or never set at all.