if statements

An if statement takes a Boolean condition and an arbitrary block of Puppet code, and executes the code block only if the condition is true. Optionally, an if statement can include elsif and else clauses.

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Behavior

Puppet's if statements behave much like those in any other language. The if condition is evaluated first and, if it is true, the if code block is executed. If it is false, each elsif condition (if present) is tested in order, and if all conditions fail, the else code block (if present) is executed. If none of the conditions in the statement match and there is no else block, Puppet does nothing and moves on. If statements executes a maximum of one code block.

In addition to executing the code in a block, an if statement also produces a value, so the if statement can be used wherever a value is allowed.The value of an if expression is the value of the last expression in the executed block, or undef if no block was executed.

Syntax

An if statement consists of:

The if keyword.
A condition (any expression resolving to a Boolean value).
A pair of curly braces containing any Puppet code.
Optionally: any number of elsif clauses, which are processed in order.
Optionally: the else keyword and a pair of curly braces containing Puppet code.

An elsif clause consists of:

The elsif keyword.
A condition.
A pair of curly braces containing any Puppet code.

if $facts['is_virtual'] {
  # Our NTP module is not supported on virtual machines:
  warning('Tried to include class ntp on virtual machine; this node might be misclassified.')
} elsif $facts['os']['name'] == 'Darwin' {
  warning('This NTP module does not yet work on our Mac laptops.')
} else {
  # Normal node, include the class.
  include ntp
}

Conditions

The condition of an if statement can be any expression that resolves to a Boolean value. This includes:

Variables
Expressions, including arbitrarily nested and and or expressions
Functions that return values

Expressions that resolve to non-Boolean values are automatically converted to Booleans. For more information, see the Booleans documentation.

Regex capture variables

If you use the regular expression match operator in a condition, any captures from parentheses in the pattern are available inside the associated code block as numbered variables (for example, $1, $2), and the entire match is available as $0. This example captures any digits from a hostname such as www01 and www02, and stores them in the $1 variable:

if $trusted['certname'] =~ /^www(\d+)\./ {
  notice("Welcome to web server number $1.")
}

Regex capture variables are different from other variables in a couple of ways:

The values of the numbered variables do not persist outside the code block associated with the pattern that set them.
In nested conditionals, each conditional has its own set of values for the set of numbered variables. At the end of an interior statement, the numbered variables are reset to their previous values for the remainder of the outside statement. This causes conditional statements to act like local scopes, but only with regard to the numbered variables.