Types of variables

In Terraform, there are several types of variables that you can define, including:

1. String: A sequence of characters. Example: “hello”.
2. Number: A numeric value. Example: 42.
3. Boolean: A logical value that is either true or false.
4. List: A sequence of values of the same type. Example: [ “apples”, “oranges”, “bananas” ].
5. Map: A collection of key-value pairs, where the keys and values can be of any type. Example: { “name” = “John”, “age” = 30 }.
6. Object: A complex data type that can contain multiple attributes. Example: { name = “John”, age = 30, address = { street = “123 Main St”, city = “Anytown”, state = “CA” } }.
7. Tuple: A sequence of values of different types. Example: [ “John”, 30, true ].

How to declare variables in Terraform?

As per below image we can declare variables :

• variables.tf – Good for providing non-sensitive values, reasonable defaults
• environmental variables – Easy to set , Use TF_VAR_<Variable> pattern
• Command line – Assign with runtime flag such as -var= ‘my_var=value’

For more details please go through below link – https://www.devopsschool.com/blog/terraform-variables-complete-reference-guide/

Where we can declare the value?

We can also declare value in any of below:

• variables.tf
• environmental variables
• Command line

And below files are only specific to value , we can declare here also.

• .auto.tfvars
• terraform.tfvars.json
• terraform.tfvars

Mostly we prefer terraform.tfvars only for value declaration

Note: If we declare the variables but not set the values , terraform will prompt for the value, terraform will ask for required values not otherwise provided

Precedence Order

How variables are assigned and used in Terraform, along with their precedence.

When multiple methods are used to assign values to variables, Terraform uses the following precedence order, from highest to lowest:

1. Command Line
2. .auto.tfvars files
3. terraform.tfvars.json file
4. terraform.tfvars file
5. Environment Variables
6. variables.tf

1. Command Line

Highest Precedence

You can assign variable values directly from the command line using the -var flag.

Example:

terraform apply -var="instance_type=t2.micro"

2. .auto.tfvars Files

Terraform automatically loads any files ending with .auto.tfvars or .auto.tfvars.json in the current directory.

Example:

default.auto.tfvars:

instance_type = "t2.micro"

3. terraform.tfvars.json File

You can define variables in a JSON file named terraform.tfvars.json.

Example:

terraform.tfvars.json:

{
  "instance_type": "t2.micro"
}

4. terraform.tfvars File

This is a common method to assign values to variables by creating a file named terraform.tfvars.

Example:

terraform.tfvars:

instance_type = "t2.micro"

5. Environment Variables

You can set environment variables using the TF_VAR_ prefix. This method is useful for setting sensitive values like API keys.

Example:

export TF_VAR_instance_type="t2.micro"

6. variables.tf

Lowest Precedence

This file contains the default values for variables. If no other values are provided, Terraform uses these defaults.

Example:

variables.tf:

variable "instance_type" {
  description = "The type of instance to use"
  type        = string
  default     = "t2.micro"
}

Interactive

If a required variable is not provided by any of the methods above, Terraform will prompt you to enter the value interactively during the plan or apply phase.

Example:

terraform apply
var.instance_type
  The type of instance to use

  Enter a value: t2.micro

Module Invocation

When invoking a module, you can pass variables to the module directly. This method provides another opportunity to assign values to variables.

module "my_module" {
  source         = "./path_to_module"
  instance_type  = "t2.micro"
}

Summary

• Command Line: Highest precedence, overrides all other methods.
• .auto.tfvars Files: Automatically loaded, useful for shared configurations.
• terraform.tfvars.json and terraform.tfvars Files: Common methods for defining variable values in separate files.
• Environment Variables: Convenient for setting sensitive or default values.
• variables.tf: Contains default values for variables, lowest precedence.
• Interactive: Prompts for values if not provided.
• Module Invocation: Passes values directly to modules when called.

Code Example

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

variable "instance_name" {
  type = string
  default = "Rajesh"
}

resource "aws_instance" "example-number" {
  count = var.instance_count
  ami           = "ami-053b0d53c279acc90"
  instance_type = "t2.micro"
  
  tags = {
    Name = var.instance_name
  }
}


variable "security_groups" {
  type = list(string)
  default = ["sg-0541801a7a059ba17"]
}


resource "aws_instance" "example-list" {
  ami           = "ami-053b0d53c279acc90"
  instance_type = "t2.micro"
  vpc_security_group_ids = var.security_groups
}

variable "instance_tags" {
  type = map(string)
  default = {
    Name = "my-instance4mMap-Rajesh"
  }
}

resource "aws_instance" "example-map" {
  ami           = "ami-053b0d53c279acc90"
  instance_type = "t2.micro"
  tags = var.instance_tags
}

variable "create_vm" {
  description = "If set to true, it will create vm"
   type   = bool
}

resource "aws_instance" "example-bool" {
  count   = var.create_vm ? 1 : 0
  ami           = "ami-053b0d53c279acc90"
  instance_type = "t2.micro"
  tags = var.instance_tags
}

Break down Description for above steps:

Let’s break down each part to understand how these variables work and are used in the resource configurations.

Variable Declarations

1. Number Variable

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

• Usage: This variable determines the number of instances to create. It has a default value of 1.

2. String Variable

variable "instance_name" {
  type = string
  default = "Rajesh"
}

• Usage: This variable sets the name tag for the instance. It has a default value of “Rajesh”.

3. List Variable

variable "security_groups" {
  type = list(string)
  default = ["sg-0541801a7a059ba17"]
}

• Usage: This variable holds a list of security group IDs. It has a default list containing one security group ID.

4. Map Variable

variable "instance_tags" {
  type = map(string)
  default = {
    Name = "my-instance4mMap-Rajesh"
  }
}

• Usage: This variable holds a map of tags for the instance. It has a default map with a single key-value pair.

5. Boolean Variable

variable "create_vm" {
  description = "If set to true, it will create vm"
  type = bool
}

• Usage: This variable is a boolean that determines whether to create an instance. It’s typically used for conditional resource creation.

Resource Configurations

1. Using a Number Variable

resource "aws_instance" "example-number" {
  count = var.instance_count
  ami = "ami-053b0d53c279acc90"
  instance_type = "t2.micro"
  
  tags = {
    Name = var.instance_name
  }
}

• Explanation: The count meta-argument is set to the value of var.instance_count. The tags argument uses var.instance_name for the Name tag.

2. Using a List Variable

resource "aws_instance" "example-list" {
  ami = "ami-053b0d53c279acc90"
  instance_type = "t2.micro"
  vpc_security_group_ids = var.security_groups
}

• Explanation: The vpc_security_group_ids argument is set to the value of var.security_groups, which is a list of security group IDs.

3. Using a Map Variable

resource "aws_instance" "example-map" {
  ami = "ami-053b0d53c279acc90"
  instance_type = "t2.micro"
  tags = var.instance_tags
}

• Explanation: The tags argument is set to the value of var.instance_tags, which is a map of tag key-value pairs.

4. Using a Boolean Variable

resource "aws_instance" "example-bool" {
  count = var.create_vm ? 1 : 0
  ami = "ami-053b0d53c279acc90"
  instance_type = "t2.micro"
  tags = var.instance_tags
}

Count can use global in terraform which we call meta argument , from above examples

if variable var.create_vm is true than set count is 1 if variable is false than it will create 0

• Explanation: The count meta-argument is set to 1 if var.create_vm is true, otherwise it’s set to 0. This allows for conditional creation of the instance.

Summary

• Number Variable: Controls the number of instances.
• String Variable: Sets the name tag for instances.
• List Variable: Provides a list of security group IDs.
• Map Variable: Provides a map of tags for instances.
• Boolean Variable: Conditionally creates an instance based on the boolean value.

Terraform plan will give below result