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Terraform offers various methods to provide input variables, making it flexible and adaptable to different use cases. These methods include command-line flags, environment variables, variable definition files (including auto-loaded files), and default values in variable declarations.
Auto-loaded variable definition files: Terraform automatically loads files with .auto.tfvars or .auto.tfvars.json extensions, without the need for the -var-file flag. They are loaded alphabetically, with later files potentially overwriting earlier ones. Example: Two .auto.tfvars files (a.auto.tfvars and b.auto.tfvars) contain different values for project_id. When you run terraform apply, Terraform loads both files, and the final project_id value is taken from b.auto.tfvars.
Command-line flags: Using the -var flag, you can pass variables directly to the terraform apply or terraform plan commands. This method is useful for passing sensitive data or for quick testing purposes. Example:
terraform apply -var="project_id=variable-tf-demo" -var="region=us-central1"
Environment variables: Set Terraform variables using environment variables by following the naming convention TF_VAR_variable_name. This method is useful for storing sensitive data or when using CI/CD pipelines.
Example:
export TF_VAR_project_id="variable-tf-demo"
export TF_VAR_region="us-central1"
terraform apply
Variable definition files (.tfvars): Use .tfvars or .tfvars.json files to store variable values. Terraform loads all such files in the working directory by default, or you can specify a file using the -var-file flag.
Example (terraform.tfvars):
credentials_file = "credentials.json"
project_id = "variable-tf-demo"
region = "us-central1"
zone = "us-central1-a"
image_name = "debian-cloud/debian-11"
instance_type = "n1-standard-1"
Default values in variable declarations: Provide default values for variables directly in the variable declaration block in your .tf files. If a value is not provided through any other method, the default value will be used.
Example (variables.tf):
variable "project_id" {
default = "variable-tf-demo"
}
variable "region" {
default = "us-central1"
}
In summary, Terraform supports various methods to input variables, each with its own use case. You can choose the method that best suits your needs and preferences.