Terraform Variables

Lets Explore

Terraform Variables with default Option
Overriding default Variable values with -var Option
Overriding default Variable values with Environment Variables Options

Terraform Variables with `default` Option

In Terraform, variables are placeholders that allow you to input data into your configurations, making your Terraform code more flexible and reusable.

Variables are containers for storing and managing values, such as text, numbers, or complex data. These values can be used within your Terraform code.
Variables are used to parameterize your configurations, allowing you to customize your infrastructure.
You can define variables for various settings like regions, instance types, etc.
Variables are declared in your Terraform code using the variable block. You specify the variable's name, type, and an optional default value.
When running Terraform, you can input values for variables, either by specifying them in a separate variable file or through command-line arguments.
Variables make your code reusable. You can use the same Terraform configuration with different input values, making it adaptable to different environment (prd, pre, uat, dev).
Variable Defaults: When declaring variables in your Terraform configuration, you can set default values. These defaults act as placeholders and are used when no other value is provided.

Example:

00_provider.tf

terraform {
required_providers {
    aws = {
    source  = "hashicorp/aws"
    version = "~> 5.0"
    }
}
}

provider "aws" {
#region = "us-east-1"
region = var.aws_region

default_tags {
    tags = {
    Terraform = "yes"
    #Owner = "Venkatesh"
    Owner = var.owner
    }
}
}

01_ec2.tf

resource "aws_instance" "myec2" {
# terraform arguments without variables
# ami = "ami-0df435f331839b2d6"
# instance_type = "t2.micro"
# count = 1

# using variables for arguments
ami           = var.ec2_ami
instance_type = var.ec2_instance_type
count         = var.instance_count

tags = {
    Name = "Linux2023"
}
}

02_variables.tf

variable "aws_region" {
description = "AWS Region In Which Resources will be Created"
type        = string
default     = "us-east-1"
}

variable "owner" {
description = "Name of the Engineer who is creating Resources"
type        = string
default     = "Venkatesh"
}

variable "ec2_ami" {
description = "AWS EC2 AMI Amazon Linux 2023"
type        = string
default     = "ami-0df435f331839b2d6" # Amazon Linux 2023
}

variable "ec2_instance_type" {
description = "EC2 Instance Type"
type        = string
default     = "t2.micro"
}

variable "instance_count" {
description = "Number of EC2"
type        = number
default     = 1
}

In the above example, We've defined five variables:
1. aws_region: default AWS Region to be used
2. owner: Name of the Engineer who is creating Resources
3. ec2_ami: AWS EC2 AMI
4. ec2_instance_type: AWS EC2 Instance type
5. instance_count: Number of EC2 to be Created
Each variable has a specific data type (string or number) and optional default values.
With these variables, you can customize the AMI, instance type, and the number of EC2 instances you want to create, making your Terraform configuration adaptable and flexible

terraform plan Output

$ terraform plan

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
+ create

Terraform will perform the following actions:

# aws_instance.myec2[0] will be created
+ resource "aws_instance" "myec2" {
    + ami                                  = "ami-0df435f331839b2d6"
    + arn                                  = (known after apply)
    + associate_public_ip_address          = (known after apply)
    + availability_zone                    = (known after apply)
    + cpu_core_count                       = (known after apply)
    + cpu_threads_per_core                 = (known after apply)
    + disable_api_stop                     = (known after apply)
    + disable_api_termination              = (known after apply)
    + ebs_optimized                        = (known after apply)
    + get_password_data                    = false
    + host_id                              = (known after apply)
    + host_resource_group_arn              = (known after apply)
    + iam_instance_profile                 = (known after apply)
    + id                                   = (known after apply)
    + instance_initiated_shutdown_behavior = (known after apply)
    + instance_lifecycle                   = (known after apply)
    + instance_state                       = (known after apply)
    + instance_type                        = "t2.micro"
    + ipv6_address_count                   = (known after apply)
    + ipv6_addresses                       = (known after apply)
    + key_name                             = (known after apply)
    + monitoring                           = (known after apply)
    + outpost_arn                          = (known after apply)
    + password_data                        = (known after apply)
    + placement_group                      = (known after apply)
    + placement_partition_number           = (known after apply)
    + primary_network_interface_id         = (known after apply)
    + private_dns                          = (known after apply)
    + private_ip                           = (known after apply)
    + public_dns                           = (known after apply)
    + public_ip                            = (known after apply)
    + secondary_private_ips                = (known after apply)
    + security_groups                      = (known after apply)
    + source_dest_check                    = true
    + spot_instance_request_id             = (known after apply)
    + subnet_id                            = (known after apply)
    + tags                                 = {
        + "Name" = "Linux2023"
        }
    + tags_all                             = {
        + "Name"      = "Linux2023"
        + "Owner"     = "Venkatesh"
        + "Terraform" = "yes"
        }
    + tenancy                              = (known after apply)
    + user_data                            = (known after apply)
    + user_data_base64                     = (known after apply)
    + user_data_replace_on_change          = false
    + vpc_security_group_ids               = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Overriding `default` Variable values with `-var` Option

In Terraform, you can override the default values defined in a variable file using the -var option while running terraform plan or terraform apply commands. This allows you to provide custom values for variables during command execution.

Variable Overrides: To customize these variables during plan or apply, you can use the -var option, followed by the variable name and the new value. This enables you to replace the default value with the value you specify.

Example:

Override owner:
```
terraform apply -var="owner=Amar"
```

Override ec2_instance_type and instance_count :

terraform apply -var="ec2_instance_type=m4.large" -var="instance_count=2"

terraform plan Output

You can notice the change in instance_type to "m4.large" and you also see 2 EC2 instances aws_instance.myec2[0] and aws_instance.myec2[1] being created.

$ terraform plan -var="ec2_instance_type=m4.large" -var="instance_count=2"

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
+ create

Terraform will perform the following actions:

# aws_instance.myec2[0] will be created
+ resource "aws_instance" "myec2" {
    + ami                                  = "ami-0df435f331839b2d6"
    + arn                                  = (known after apply)
    + associate_public_ip_address          = (known after apply)
    + availability_zone                    = (known after apply)
    + cpu_core_count                       = (known after apply)
    + cpu_threads_per_core                 = (known after apply)
    + disable_api_stop                     = (known after apply)
    + disable_api_termination              = (known after apply)
    + ebs_optimized                        = (known after apply)
    + get_password_data                    = false
    + host_id                              = (known after apply)
    + host_resource_group_arn              = (known after apply)
    + iam_instance_profile                 = (known after apply)
    + id                                   = (known after apply)
    + instance_initiated_shutdown_behavior = (known after apply)
    + instance_lifecycle                   = (known after apply)
    + instance_state                       = (known after apply)
    + instance_type                        = "m4.large"
    + ipv6_address_count                   = (known after apply)
    + ipv6_addresses                       = (known after apply)
    + key_name                             = (known after apply)
    + monitoring                           = (known after apply)
    + outpost_arn                          = (known after apply)
    + password_data                        = (known after apply)
    + placement_group                      = (known after apply)
    + placement_partition_number           = (known after apply)
    + primary_network_interface_id         = (known after apply)
    + private_dns                          = (known after apply)
    + private_ip                           = (known after apply)
    + public_dns                           = (known after apply)
    + public_ip                            = (known after apply)
    + secondary_private_ips                = (known after apply)
    + security_groups                      = (known after apply)
    + source_dest_check                    = true
    + spot_instance_request_id             = (known after apply)
    + subnet_id                            = (known after apply)
    + tags                                 = {
        + "Name" = "Linux2023"
        }
    + tags_all                             = {
        + "Name"      = "Linux2023"
        + "Owner"     = "Venkatesh"
        + "Terraform" = "yes"
        }
    + tenancy                              = (known after apply)
    + user_data                            = (known after apply)
    + user_data_base64                     = (known after apply)
    + user_data_replace_on_change          = false
    + vpc_security_group_ids               = (known after apply)
    }

# aws_instance.myec2[1] will be created
+ resource "aws_instance" "myec2" {
    + ami                                  = "ami-0df435f331839b2d6"
    + arn                                  = (known after apply)
    + associate_public_ip_address          = (known after apply)
    + availability_zone                    = (known after apply)
    + cpu_core_count                       = (known after apply)
    + cpu_threads_per_core                 = (known after apply)
    + disable_api_stop                     = (known after apply)
    + disable_api_termination              = (known after apply)
    + ebs_optimized                        = (known after apply)
    + get_password_data                    = false
    + host_id                              = (known after apply)
    + host_resource_group_arn              = (known after apply)
    + iam_instance_profile                 = (known after apply)
    + id                                   = (known after apply)
    + instance_initiated_shutdown_behavior = (known after apply)
    + instance_lifecycle                   = (known after apply)
    + instance_state                       = (known after apply)
    + instance_type                        = "m4.large"
    + ipv6_address_count                   = (known after apply)
    + ipv6_addresses                       = (known after apply)
    + key_name                             = (known after apply)
    + monitoring                           = (known after apply)
    + outpost_arn                          = (known after apply)
    + password_data                        = (known after apply)
    + placement_group                      = (known after apply)
    + placement_partition_number           = (known after apply)
    + primary_network_interface_id         = (known after apply)
    + private_dns                          = (known after apply)
    + private_ip                           = (known after apply)
    + public_dns                           = (known after apply)
    + public_ip                            = (known after apply)
    + secondary_private_ips                = (known after apply)
    + security_groups                      = (known after apply)
    + source_dest_check                    = true
    + spot_instance_request_id             = (known after apply)
    + subnet_id                            = (known after apply)
    + tags                                 = {
        + "Name" = "Linux2023"
        }
    + tags_all                             = {
        + "Name"      = "Linux2023"
        + "Owner"     = "Venkatesh"
        + "Terraform" = "yes"
        }
    + tenancy                              = (known after apply)
    + user_data                            = (known after apply)
    + user_data_base64                     = (known after apply)
    + user_data_replace_on_change          = false
    + vpc_security_group_ids               = (known after apply)
    }

Plan: 2 to add, 0 to change, 0 to destroy.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── 

Note: You didn't use the -out option to save this plan, so Terraform can't guarantee to take exactly these actions if you run "terraform apply" now.

You can also create terraform plan output file with -out option and use that file as input to apply
Note: when terraform apply is used with plan file than it wont prompt for approval instead it will apply immediately

Example:

# Generate Terraform plan file
terraform plan -var="ec2_instance_type=m4.large" -var="instance_count=2" -out tfplan_05112023.plan

# Deploy Terraform Resources using Plan file
terraform apply tfplan_05112023.plan

In above example terraform will over ride default values for ec2_instance_type and instance_count and deploy the resources using Plan file

Overriding `default` Variable values with `Environment Variables` Options

In Terraform, you can override input variables using environment variables, providing a way to customize your configurations without modifying the Terraform code.

Environment Variables:
- Environment variables are a way to store values that can be accessed by different programs or scripts.
- In Terraform, you can use environment variables to override the values of input variables during the execution of Terraform commands.
- To override input variables with environment variables, Prefix the name of the input variable with TF_VAR_
- Syntax
  - TF_VAR_variable_name=value where variable_name is name of your input variable
  - use the export command to set the Environment Variable
- Example
```
# SET Environment Variables
export TF_VAR_ec2_instance_type=t2.large
export TF_VAR_owner=Amar
echo $TF_VAR_ec2_instance_type, $TF_VAR_owner
```

terraform plan Output

You can notice the change in instance_type to "t2.large" and Owner to "Amar"

Venkatesh@LenovoPC MINGW64 /d/StudyRelated/terraform-beginners-guide/09-Terraform-Variables/09-01-Terraform-Variables-default (main)
$ export TF_VAR_ec2_instance_type=t2.large

Venkatesh@LenovoPC MINGW64 /d/StudyRelated/terraform-beginners-guide/09-Terraform-Variables/09-01-Terraform-Variables-default (main)
$ export TF_VAR_owner=Amar

Venkatesh@LenovoPC MINGW64 /d/StudyRelated/terraform-beginners-guide/09-Terraform-Variables/09-01-Terraform-Variables-default (main)
$ echo $TF_VAR_ec2_instance_type, $TF_VAR_owner
t2.large, Amar

Venkatesh@LenovoPC MINGW64 /d/StudyRelated/terraform-beginners-guide/09-Terraform-Variables/09-01-Terraform-Variables-default (main)
$ terraform plan

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
+ create

Terraform will perform the following actions:

# aws_instance.myec2[0] will be created
+ resource "aws_instance" "myec2" {
    + ami                                  = "ami-0df435f331839b2d6"
    + arn                                  = (known after apply)
    + associate_public_ip_address          = (known after apply)
    + availability_zone                    = (known after apply)
    + cpu_core_count                       = (known after apply)
    + cpu_threads_per_core                 = (known after apply)
    + disable_api_stop                     = (known after apply)
    + disable_api_termination              = (known after apply)
    + ebs_optimized                        = (known after apply)
    + get_password_data                    = false
    + host_id                              = (known after apply)
    + host_resource_group_arn              = (known after apply)
    + iam_instance_profile                 = (known after apply)
    + id                                   = (known after apply)
    + instance_initiated_shutdown_behavior = (known after apply)
    + instance_lifecycle                   = (known after apply)
    + instance_state                       = (known after apply)
    + instance_type                        = "t2.large"
    + ipv6_address_count                   = (known after apply)
    + ipv6_addresses                       = (known after apply)
    + key_name                             = (known after apply)
    + monitoring                           = (known after apply)
    + outpost_arn                          = (known after apply)
    + password_data                        = (known after apply)
    + placement_group                      = (known after apply)
    + placement_partition_number           = (known after apply)
    + primary_network_interface_id         = (known after apply)
    + private_dns                          = (known after apply)
    + private_ip                           = (known after apply)
    + public_dns                           = (known after apply)
    + public_ip                            = (known after apply)
    + secondary_private_ips                = (known after apply)
    + security_groups                      = (known after apply)
    + source_dest_check                    = true
    + spot_instance_request_id             = (known after apply)
    + subnet_id                            = (known after apply)
    + tags                                 = {
        + "Name" = "Linux2023"
        }
    + tags_all                             = {
        + "Name"      = "Linux2023"
        + "Owner"     = "Amar"
        + "Terraform" = "yes"
        }
    + tenancy                              = (known after apply)
    + user_data                            = (known after apply)
    + user_data_base64                     = (known after apply)
    + user_data_replace_on_change          = false
    + vpc_security_group_ids               = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── 

Note: You didn't use the -out option to save this plan, so Terraform can't guarantee to take exactly these actions if you run "terraform apply" 
now.

Venkatesh@LenovoPC MINGW64 /d/StudyRelated/terraform-beginners-guide/09-Terraform-Variables/09-01-Terraform-Variables-default (main)
$

Use unset command to remove the Environment Variables

# SET Environment Variables
unset TF_VAR_ec2_instance_type
unset TF_VAR_owner
echo $TF_VAR_ec2_instance_type, $TF_VAR_owner

Terraform Variable Precedence

If the same variable is assigned multiple values, Terraform uses the last value it finds, overriding any previous values.
Note that the same variable cannot be assigned multiple values within a single source.
Terraform loads variables in the following order, with later sources taking precedence over earlier ones:

5. Environment variables
4. The terraform.tfvars file, if present.
3. The terraform.tfvars.json file, if present.
2. Any *.auto.tfvars or *.auto.tfvars.json files, processed in lexical order of their filenames.
1. Any -var and -var-file options on the command line, in the order they are provided. (This includes variables set by a Terraform Cloud workspace.)

References :

Input Variables

Environment Variable TF_VAR_name

Variable Definition Precedence

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README.md

README.md

Terraform Variables