How to Pass Variables to Terraform: Simple Guide

To pass a variable to terraform, define it in a variable block in your configuration, then provide its value using a -var command-line option, a .tfvars file, or environment variables. This lets you customize your infrastructure without changing the code.

Syntax

Define variables in Terraform using a variable block with a name and optional default value. Pass values when running Terraform with -var or -var-file. Variables can also be set via environment variables.

• variable "name" {}: Declares a variable.
• -var 'name=value': Passes a variable value via CLI.
• -var-file=filename.tfvars: Loads variables from a file.
• Environment variables: Use TF_VAR_name to set variables.

hcl

variable "region" {
  description = "AWS region to deploy"
  type        = string
  default     = "us-west-2"
}

# Passing variable via CLI:
# terraform apply -var='region=us-east-1'

# Passing variable via file:
# terraform apply -var-file="custom.tfvars"

Example

This example shows how to define a variable for AWS region and pass its value using the command line.

hcl

variable "region" {
  description = "AWS region to deploy"
  type        = string
  default     = "us-west-2"
}

provider "aws" {
  region = var.region
}

resource "aws_s3_bucket" "example" {
  bucket = "my-example-bucket-12345"
  acl    = "private"
}

# Run this with:
# terraform init
# terraform apply -var='region=us-east-1' -auto-approve

Output

aws_s3_bucket.example: Creating...
aws_s3_bucket.example: Creation complete after 2s [id=my-example-bucket-12345]

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

Common Pitfalls

Common mistakes when passing variables include:

• Not defining the variable in the configuration before passing it.
• Using incorrect variable names or typos in -var or .tfvars files.
• Forgetting to run terraform init after adding variables.
• Passing sensitive data directly on the command line, which can be insecure.

Always verify variable names and prefer .tfvars files or environment variables for sensitive values.

hcl

/* Wrong: Passing undefined variable */
# terraform apply -var='region=us-east-1'

/* Right: Define variable first */
variable "region" {
  type = string
}

# Then pass value as shown in previous examples

Quick Reference

Method	How to Use	Notes
Variable Block	Define with variable "name" {}	Required to declare variables
CLI Option	Use -var 'name=value'	Overrides default or file values
Variable File	Use -var-file=filename.tfvars	Good for multiple variables
Environment Variable	Set TF_VAR_name=value	Useful for automation and secrets

Key Takeaways

Declare variables in Terraform using the variable block before passing values.

Pass variable values via CLI with -var, variable files with -var-file, or environment variables.

Use .tfvars files for managing multiple variables cleanly.

Avoid passing sensitive data directly on the command line for security.

Always run terraform init after adding or changing variables.