Conditional Expressions

If you are used to traditional programming languages such as C#, Python, Java, etc., you’ll be familiar with the concept of if / else statements. Terraform has no if or else statement but instead uses ternary conditional operators.

The syntax of a conditional expression is as follows:

condition ? true_val : false_val

A conditional expression uses the value of a boolean expression to select one of two values. This expression evaluates to true_val if the value of conditionis true, and otherwise, to false_val. This is the equivalent of an If-statement.

In Terraform, this logic is particularly useful when fed into the count statement to deploy multiple of resources. In Terraform, deploying 0 resources is also fine if the condition is not met.

Example 1

For example, the statement below checks if the variable var.serveris set to “UbuntuServer”. If it is true, then count = 0 and will be deployed zero times. If it is set to anything else, then count = 1, and the resource will be deployed 1 time.

Note that Terraform does support traditional logical, equality, and comparison operators such as == (equal to) or != (not equal to) && (and), etc. These operators can be added together to make more complex conditionals.

count = var.server == "UbuntuServer" ? 0 : 1

Example 2

Another common use of conditional expressions is to define defaults to replace invalid values. The example below checks if the variable var.server is an empty string. If it is, then the value is “MicrosoftWindowsServer”. If not, then it is the actual value of var.server .

var.server != "" ? var.server : "MicrosoftWindowsServer"

Example 3

When creating a conditional expression, the two result types can be of any type. In the example below, we have an integer of 100 if the condition is true, and a string “UbuntuServer” if the condition is false.

var.server ? 100 : "UbuntuServer"

However, this can cause confusion as Terraform will attempt to find a type that they can both convert to and make those conversions automatically if so. In the above case, both can be converted to a String.

To avoid this, writing the condition with a specific conversion function is recommended (see below using the toString function):

var.server ? tostring(100) : "UbuntuServer"