How to fix Warning: Value for undeclared variable in Terraform

Add a variable block to declare the variable in your Terraform configuration. Create a variables.tf file or add to an existing one:

variable "your_variable_name" {
  type        = string
  description = "Description of what this variable is for"
}

Replace "your_variable_name" with the actual variable name from your warning message. The variable block tells Terraform about the variable's type and purpose.

Ensure the variable name in your terraform.tfvars or -var flag exactly matches the declared variable name, including case sensitivity:

# In variables.tf
variable "instance_type" {
  type = string
}

# In terraform.tfvars - must match exactly
instance_type = "t3.micro"

Variable names are case-sensitive. "instance_type" and "InstanceType" are treated as different variables.

Ensure your variable definitions and value assignments are in the correct locations:

- Root module variables: Declare in variables.tf in the root directory
- Module-specific variables: Declare in variables.tf within the module directory
- Values for root variables: Provide in terraform.tfvars or root module via -var
- Values for module variables: Pass through the module block in parent configuration

# Root variables.tf
variable "environment" {
  type = string
}

# Module call in main.tf
module "example" {
  source = "./modules/example"
  # This passes the value to module's declared variable
}

Make your variable declaration complete with type and optional default value:

variable "region" {
  type        = string
  description = "AWS region for deployment"
  default     = "us-east-1"
}

variable "tags" {
  type = map(string)
  description = "Common tags for all resources"
  default = {
    Environment = "production"
    ManagedBy   = "terraform"
  }
}

Specifying type prevents type mismatches and adds validation.

If the variable is not actually needed, remove the assignment from your terraform.tfvars or command line:

# Remove from terraform.tfvars
# unused_variable = "value"

# Or remove from command line
# terraform apply -var="unused_variable=value"  # Delete this flag
terraform apply

Only provide values for variables you actually use in your configuration.

If you want to see warnings but with less verbosity during troubleshooting:

terraform plan -compact-warnings
terraform apply -compact-warnings

This reduces warning output clutter but still shows you the undeclared variable warnings.