Output Variables

Terraform output variables are a powerful feature that allow you to store the results of expressions from your configuration files for later use. In previous lessons, we covered input variables and reference expressions; output variables complement these by enabling you to retrieve and present important output information after your infrastructure has been provisioned.

Capturing Resource Attributes

Consider a configuration that creates a random pet name using Terraform's resource definitions. In this example, a resource called random_pet generates a pet name, and an output variable called pet-name captures the generated id. This is especially useful for passing data to other tools or for quick resource verification.

Below is an example configuration:

resource "local_file" "pet" {
  filename = var.filename
  content  = "My favorite pet is ${random_pet.my-pet.id}"
}

resource "random_pet" "my-pet" {
  prefix    = var.prefix
  separator = var.separator
  length    = var.length
}

output "pet-name" {
  value = random_pet.my-pet.id
  desc  = ""
}

The output block starts with the keyword output followed by the variable name. Inside the block, the value argument is required and uses a reference expression (random_pet.my-pet.id). Although the desc argument is optional, it is a good practice to include a brief description of the output's purpose.

Configuration of Related Variables

To support the resources defined above, ensure the following variables are declared in your configuration:

variable "filename" {
  default = "/root/pets.txt"
}

variable "content" {
  default = "I love pets!"
}

variable "prefix" {
  default = "Mrs"
}

variable "separator" {
  default = "."
}

variable "length" {
  default = "1"
}

These variable definitions set the necessary defaults and ensure the configuration runs smoothly during execution.

Displaying Outputs with Terraform

When you run terraform apply, Terraform automatically displays the output variables after resource creation. For example:

$ terraform apply
...
Outputs:
  pet-name = Mrs.gibbon

You can also retrieve the value of output variables at any time using the terraform output command. Running the command without any arguments lists all outputs:

$ terraform output
pet-name = Mrs.gibbon

To display an individual output variable, simply specify its name:

$ terraform output pet-name
Mrs.gibbon

Further Reading

For more comprehensive information on Terraform output variables and other configuration concepts, please refer to the following resources:

• Terraform Official Documentation
• Terraform CLI Documentation

By incorporating output variables into your Terraform configurations, you can streamline the process of accessing essential resource details and improve the overall management of your infrastructure.