Terraform Variables and Attributes Explained

In this blog, we will understand the input/output variables and resource attributes in the Terraform.

HCL Basics

The HCL file consists of Blocks and Arguments.

   <block> <parameter> {
       key1 = value1
       key2 = value2
   }

A block is defined within curly braces & it contains a set of arguments in a key-value pair.
In Terraform: A block contains information about the infrastructure platform & set of resources within that platform, that we want to create.

Basic command

terraform init: Terraform initializes the project & identifies the provider to be used for the target environment.
terraform plan: Terraform drafts a plan to get a preview of the changes.
terraform apply: It makes the necessary changes required on the target environment to bring it to the desired state.

Example: Create the local file using Terraform

config file: main.tf

resource "local_file" "testing" {
  filename = "/opt/test.txt"
  content = "This test is created by Terraform."
}

Here,
- Block-type: resource
- Resource-type: local_file, It provides two bits of info, local → provider & file → resource type
- Resource-name: testing
- filename & content are the arguments

Execute,
terraform init
terraform plan
terraform apply

Note: We can have multiple configuration files within the configuration directory, saying that the file should have a .tf extension.

Input variable

Input variables are used to make your configurations more flexible and reusable. They allow you to parameterize the values in your configuration files instead of hardcoding them.

Syntax:

variable "variable_name" {
  type        = <type>       # Optional, e.g., string, number, bool, list, map
  default     = <default>    # sets a default value
}

Types of Input Variables

String: A single line of text.

 variable "environment" {
   type    = string
   default = "production"
 }

Number: A numeric value, which can be an integer or a float.

 variable "length" {
   type    = number
   default = 1
 }

Bool: A boolean value (true or false).

 variable "enable_monitoring" {
   type    = bool
   default = true
 }

List: A sequence of values of the same type and can have duplicate elements.

 variable "salutation" {
   type = list
   default = ["Mr", "Mrs", "Sir", "Maam"]
 }

We can also define the type of element in the list, it could be string, number, or bool.

 variable "salutation" {
   type = list(string)
   default = ["Mr", "Mrs", "Sir", "Maam"]
 }

Set: A sequence of values of the same type, but no duplicate element.

 variable "length" {
   type    = set(number)
   default = [1, 2, 5]
 }

Map: A collection of key-value pairs, where keys are strings, and values can be any type.

 variable "content" {
   type = map(string)
   default = {
     "stat1" = "Hello world"
     "stat2" = "How are you"
   }
 }

Object: A custom structure with named attributes of specific types.

 variable "car_config" {
   type = object({
     make      = string
     model     = string
     year      = number
     hybrid    = bool
     features  = list(string)
   })
   default = {
     make      = "Toyota"
     model     = "Camry"
     year      = 2023
     hybrid    = true
     features  = ["Sunroof", "Bluetooth", "Backup Camera"]
   }
 }

Tuple: A sequence of values of different types.

 variable "car_details" {
   type = tuple([string, number, bool]) # Car name, manufacturing year, and hybrid status
   default = ["Toyota", 2023, true]
 }

Example: Use a different input variable and create the local file using Terraform.

Config file: main.tf, variable.tf

git clone https://github.com/minex970/terraform-overview/
cd basic
terraform init
terraform plan
terraform apply

Verify:

-rwxr-xr-x  1 root root   97 Dec  1 19:13 car-1.txt
-rwxr-xr-x  1 root root   82 Dec  1 19:13 car-2.txt
-rwxr-xr-x  1 root root   87 Dec  1 19:26 car-3.txt

Resource attributes

The attributes refer to the properties or outputs of a resource that can be accessed by other resources, variables, outputs, or modules. These attributes allow you to connect resources and pass information between them, enabling dynamic configurations.

# Using resource attributes in the resource block
resource "random_pet" "my_pet" {
  prefix = "Mr"
  separator = "."
  length = 1
}

resource "local_file" "pet" {
  filename = "/opt/pets.txt"
  content = "My favorite pet is ${random_pet.my_pet.id} \n"
}

Here, we use the following expression to get the output of the my_pet resource name and use them in the pet resource name.
${random_pet.my_pet.id}, this expression is also known as an interpolation sequence.

Output variable

Like the input variable, Terraform also supports the Output variable.
This is used to store the output of any resource or say the attributes of that, into an output variable.

Example: Use output variable to store the value of resource attribute.

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

Command: terraform output, to list the value of all output variables w/o running terraform apply command.

Github URL: https://github.com/minex970/terraform-overview/tree/master/basic

Terraform Essentials: Using Input/Output Variables and Resource Attributes

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