Managing Input Variables

1. Declaring Input Variables

Variable Name	Type	Description	Default Value
instance_type	`string`	EC2 instance type to launch	`"t2.micro"`
ami_id	`string`	AMI ID for the EC2 instance	`"ami-12345678"`
count	`number`	Number of instances to launch	`1`

Example: Declaring Variables in Terraform

variable &#34;instance_type&#34; {
  description = &#34;EC2 instance type to launch&#34;
  type        = string
  default     = &#34;t2.micro&#34;
}

variable &#34;ami_id&#34; {
  description = &#34;AMI ID for the EC2 instance&#34;
  type        = string
  default     = &#34;ami-12345678&#34;
}

variable &#34;count&#34; {
  description = &#34;Number of instances to launch&#34;
  type        = number
  default     = 1
}

2. Setting Variable Types

Variable Type	Description	Example	Notes
string	Textual value	`name = "John"`	Default type if not specified
number	Numeric value (integer or float)	`port = 8080`	Must be a valid number
bool	Boolean value (true or false)	`enabled = true`	Valid values: true, false
list	Ordered collection of values	`tags = ["web", "prod"]`	Values can be of any type
map	Collection of key-value pairs	`labels = { env = "prod", team = "backend" }`	Keys must be strings, values can be any type
set	Ordered collection of unique values	`allowed_ips = ["10.0.0.1", "10.0.0.2"]`	Introduced in Terraform 1.6
tuple	Ordered collection of values with specific types	`coordinates = [10.0, 20.0]`	Introduced in Terraform 1.6
object	Collection of attributes with specific types	`server = { name = "web", port = 80 }`	Keys are strings, values can be any type

Example: Declaring a variable with type

variable &#34;instance_type&#34; {
  type = string
  description = &#34;EC2 instance type&#34;
  default = &#34;t3.micro&#34;
}

Example: Using a list variable

variable &#34;allowed_ports&#34; {
  type = list(number)
  description = &#34;List of allowed ports&#34;
  default = [22, 80, 443]
}

Example: Using a map variable

variable &#34;environment_vars&#34; {
  type = map(string)
  description = &#34;Environment variables for the app&#34;
  default = {
    NODE_ENV = &#34;production&#34;
    LOG_LEVEL = &#34;info&#34;
  }
}

3. Defining Default Values

Variable	Default Value	Description
variable "instance_type"	`"t2.micro"`	Instance type for EC2 launch template
variable "ami_id"	`"ami-0c55b159cbfafe1d0"`	Amazon Machine Image ID for the instance
variable "vpc_cidr"	`"10.0.0.0/16"`	Primary CIDR block for VPC

Example: Using Default Values in a Variable Definition

variable &#34;instance_type&#34; {
  description = &#34;Type of EC2 instance to launch&#34;
  type        = string
  default     = &#34;t2.micro&#34;
}

Example: Default Value in a Resource Block

resource &#34;aws_instance&#34; &#34;web_server&#34; {
  ami           = var.ami_id
  instance_type = var.instance_type
  vpc_security_group_ids = [
    aws_security_group.web_sg.id,
  ]
}

4. Adding Variable Descriptions

Property	Description	Example
description	Human-readable description of the variable	`"Description of the variable"`

Example: Variable with Description

variable "instance_type" {
  type        = string
  description = "The EC2 instance type to use"
  default     = "t3.micro"
}

Note: Descriptions are displayed in the Terraform console and documentation tools to help users understand the purpose of each variable.

5. Validating Variable Values

Property Description Example

validation

Defines a validation rule for a variable

Property	Description	Example
validation	Defines a validation rule for a variable	`variable "instance_type" { type = string validation { condition = contains(["t2.micro", "t2.small", "t2.medium"], var.instance_type) error_message = "The instance_type must be one of t2.micro, t2.small, or t2.medium." } }`

variable "instance_type" {
  type = string
  validation {
    condition     = contains(["t2.micro", "t2.small", "t2.medium"], var.instance_type)
    error_message = "The instance_type must be one of t2.micro, t2.small, or t2.medium."
  }
}

Example: Validating Instance Type

variable "instance_type" {
  type = string
  validation {
    condition     = contains(["t2.micro", "t2.small", "t2.medium"], var.instance_type)
    error_message = "The instance_type must be one of t2.micro, t2.small, or t2.medium."
  }
}

Example: Validating Numeric Range

variable "cpu_count" {
  type = number
  validation {
    condition     = var.cpu_count >= 1 && var.cpu_count <= 64
    error_message = "The cpu_count must be between 1 and 64."
  }
}

Note: Validation rules are evaluated during terraform plan and terraform apply. If a validation fails, Terraform will display the error message and halt execution.

6. Marking Variables Sensitive

Variable	Type	Sensitivity	Description
sensitive_var	`string`	NEW	Variable marked as sensitive to prevent logging

Example: Declaring a Sensitive Variable

sensitive = true

Note: Sensitive variables are not shown in logs or output unless explicitly requested.

7. Setting Variables via CLI

Variable Source	Command Flag	Description
CLI Flags	`-var`	Set a variable from the command line. Example: `-var="instance_type=t3.micro"`
CLI File	`-var-file`	Load variables from a file (HCL or JSON format). Example: `-var-file="prod.tfvars"`
Environment Variables	`TERRAFORM_` prefix	Use environment variables with the `TERRAFORM_` prefix. Example: `TERRAFORM_instance_type=t3.micro`

Example: Setting Variables via CLI

terraform apply -var="instance_type=t3.micro" -var-file="prod.tfvars"

Note: CLI flags override variables set in files or environment variables. Use -var for one-time overrides and -var-file for configuration files.

8. Using Variable Files

File Type	Usage	Notes
.tfvars	`terraform apply -var-file=prod.tfvars`	Standard variable file format for defining values
.tfvars.json	`terraform apply -var-file=prod.json`	JSON format for variable files, useful for automation

Example: Using a .tfvars file

instance_type = "t3.medium"
ami_id        = "ami-0c55b159cbfafe1d0"

Example: Using a JSON variable file

{
  "instance_type": "t3.medium",
  "ami_id": "ami-0c55b159cbfafe1d0"
}

Note: Variable files are loaded in order of precedence, with -var-file taking priority over default files like terraform.tfvars.

9. Using Environment Variables

Variable	Source	Description
TF_VAR_<name>	Environment Variable	Set Terraform variables via environment variables prefixed with `TF_VAR_`

Example: Setting Environment Variables

export TF_VAR_instance_type="t3.micro"
export TF_VAR_ami_id="ami-12345678"
terraform plan

Note: Environment variables take precedence over variables defined in .tfvars files or terraform.tfvars.

10. Referencing Variables

Variable Type	Reference Syntax	Description
Local Variables	`local.variable_name`	References a variable defined in the `locals` block
Input Variables	`var.variable_name`	References an input variable defined in the `variable` block
Output Variables	`output.output_name`	References an output variable defined in the `output` block
Module Variables	`module.module_name.variable_name`	References a variable from a child module
Resource Attributes	`resource_type.resource_name.attribute`	References an attribute from a resource
Data Attributes	`data.data_type.data_name.attribute`	References an attribute from a data source

Example: Referencing Variables in Expressions

resource "aws_instance" "web" {
  ami           = var.ami_id
  instance_type = local.instance_type

  tags = {
    Name = "${var.project_name}-web"
  }
}

output "instance_id" {
  value = aws_instance.web.id
}

Note: Variable references are resolved at runtime, not during parsing.

11. Setting Variable Precedence

Source	Priority	Description
Environment Variables	`1`	Set via `TF_VAR_` prefix, highest priority
CLI Flags	`2`	Passed with `-var` or `-var-file`
Variable Files	`3`	Defined in `.tfvars` files, loaded with `-var-file`
Default Values	`4`	Defined in variable blocks using `default`
Backend Configuration	`5`	Values from remote state or backend configuration
Module Inputs	`6`	Passed to modules from parent configurations

Example: Setting Variables via Environment

export TF_VAR_instance_type="t3.micro"
terraform apply

Example: Setting Variables via CLI

terraform apply -var="instance_type=t3.micro"

Note: The TF_VAR_ environment variables override any values set in variable files or default values.

12. Using Nullable Variables

Property	Description	Default	Notes
nullable	Controls whether a variable can be set to null or undefined	`false`	When `true`, the variable can accept null values. When `false`, Terraform will throw an error if a null value is provided.

Example: Setting nullable variable

variable "optional_value" {
  type = string
  nullable = true
  default = null
}

Example: Using nullable variable in resource

resource "aws_instance" "example" {
  ami = "ami-12345678"
  instance_type = var.optional_value != null ? var.optional_value : "t2.micro"
}

Managing Input Variables

1. Declaring Input Variables

Example: Declaring Variables in Terraform

2. Setting Variable Types

Example: Declaring a variable with type

Example: Using a list variable

Example: Using a map variable

3. Defining Default Values

Example: Using Default Values in a Variable Definition

Example: Default Value in a Resource Block

4. Adding Variable Descriptions

Example: Variable with Description

5. Validating Variable Values

Example: Validating Instance Type

Example: Validating Numeric Range

6. Marking Variables Sensitive

Example: Declaring a Sensitive Variable

7. Setting Variables via CLI

Example: Setting Variables via CLI

8. Using Variable Files

Example: Using a .tfvars file

Example: Using a JSON variable file

9. Using Environment Variables

Example: Setting Environment Variables

10. Referencing Variables

Example: Referencing Variables in Expressions

11. Setting Variable Precedence

Example: Setting Variables via Environment

Example: Setting Variables via CLI

12. Using Nullable Variables

Example: Setting nullable variable

Example: Using nullable variable in resource

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