How to Reference Resource in Terraform: Syntax and Examples

In Terraform, you reference a resource by using the syntax resource_type.resource_name.attribute. This lets you access properties of a resource defined elsewhere in your configuration to connect or configure other resources.

Syntax

To reference a resource in Terraform, use the format resource_type.resource_name.attribute. Here:

• resource_type is the kind of resource, like aws_instance.
• resource_name is the name you gave the resource in your Terraform file.
• attribute is a property of that resource, such as id or arn.

terraform

resource_type.resource_name.attribute

Example

This example shows how to create an AWS EC2 instance and then reference its ID to create a security group rule that allows traffic to that instance.

terraform

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
}

resource "aws_security_group" "example_sg" {
  name = "example_sg"

  ingress {
    from_port   = 22
    to_port     = 22
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  tags = {
    Name = "example_sg"
  }
}

resource "aws_security_group_rule" "allow_ssh" {
  type                     = "ingress"
  from_port                = 22
  to_port                  = 22
  protocol                 = "tcp"
  security_group_id        = aws_security_group.example_sg.id
  source_security_group_id = aws_instance.example.id
}

Output

Terraform will create an EC2 instance and a security group, then link the security group rule to the instance using the instance's ID.

Common Pitfalls

Common mistakes when referencing resources include:

• Using the wrong resource name or type, which causes Terraform to fail to find the resource.
• Trying to reference an attribute that does not exist or is not exported by the resource.
• Referencing resources before they are created, which can cause dependency errors.

Always double-check resource names and attributes and use terraform plan to verify references.

terraform

/* Wrong reference example */
output "wrong_id" {
  value = aws_instance.wrong_name.id
}

/* Correct reference example */
output "correct_id" {
  value = aws_instance.example.id
}

Quick Reference

Part	Description	Example
resource_type	Type of resource defined in Terraform	aws_instance
resource_name	Name given to the resource in config	example
attribute	Property of the resource to access	id
Full reference	Combines all parts to access attribute	aws_instance.example.id

Key Takeaways

Reference resources using the format resource_type.resource_name.attribute in Terraform.

Ensure resource names and attributes are correct to avoid errors.

Use terraform plan to verify your references before applying.

Attributes expose useful information like IDs or ARNs for connecting resources.

References create dependencies so Terraform knows the order to create resources.