How to Use -var Flag in Terraform for Variable Input

Use the -var flag in Terraform commands to pass variable values directly from the command line, like terraform apply -var='key=value'. This lets you override default or file-based variables without editing configuration files.

Syntax

The -var flag syntax is simple: -var='variable_name=value'. You can use it multiple times to set different variables.

This flag overrides variables defined in terraform.tfvars or default values in your configuration.

bash

terraform apply -var='instance_type=t2.micro' -var='region=us-west-2'

Example

This example shows a Terraform configuration using variables and how to pass values with the -var flag during apply.

hcl

variable "instance_type" {
  description = "Type of AWS instance"
  type        = string
  default     = "t2.small"
}

provider "aws" {
  region = "us-east-1"
}

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = var.instance_type
}

# Command to apply with -var flag:
# terraform apply -var='instance_type=t2.micro'

Output

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

Common Pitfalls

• Forgetting to quote the key=value pair can cause shell errors.
• Using the wrong variable name will not override the intended variable.
• Passing complex types (like maps or lists) requires JSON syntax inside the quotes.

bash

Wrong usage:
terraform apply -var=instance_type=t2.micro

Correct usage:
terraform apply -var='instance_type=t2.micro'

Quick Reference

Flag	Description	Example
-var	Set a single variable value	terraform apply -var='name=value'
-var-file	Load variables from a file	terraform apply -var-file=custom.tfvars
Multiple -var	Set multiple variables	terraform apply -var='a=1' -var='b=2'

Key Takeaways

Use -var='key=value' to override variables directly from the command line.

Always quote the variable assignment to avoid shell parsing errors.

You can pass multiple -var flags to set several variables at once.

The -var flag overrides default and file-based variable values.

For complex variable types, use JSON syntax inside the quotes.