What You Give, What You Get: Arguments & Attributes

Lesson 4 of 14 · 9 min · Intermediate

Every resource block has two sides. Arguments are the values you hand Terraform before anything exists. Attributes are the values the provider hands back once the real thing has been created. Confusing the two is the most common source of "why doesn't this exist yet" errors.

Input Side, Output Side

Arguments

ami, instance_type, subnet_id, tags — written into the config, sent to the API.

→

Attributes

id, public_ip, private_ip, arn — discovered only after the resource is real.

1. Arguments

An argument is a configuration value that tells Terraform how to create or configure a resource — think of it as the input you hand over before anything exists.

main.tf

resource "aws_instance" "web" {
  ami           = "ami-0123456789abcdef0"
  instance_type = "t3.micro"
}

main.tf

resource "aws_instance" "web" {
  ami               = "ami-0123456789abcdef0"
  instance_type     = "t3.micro"
  subnet_id         = "subnet-123456"
  key_name          = "my-key"
  availability_zone = "ap-south-1a"
}

Five Arguments, One Instance

ami, instance_type, subnet_id, key_name, and availability_zone are all arguments — every one of these values is sent to the AWS API to create the EC2 instance.

main.tf

resource "aws_s3_bucket" "logs" {
  bucket = "shan-logs"
}

main.tf

resource "aws_instance" "web" {
  ami           = "ami-123456"
  instance_type = "t2.micro"
  tags = {
    Name = "WebServer"
    Env  = "Production"
  }
}

Argument Types

STRING

instance_type = "t3.micro"

NUMBER

volume_size = 30

BOOLEAN

monitoring = true

LIST

security_groups = [
  "sg-111111",
  "sg-222222"
]

MAP

tags = {
  Name = "WebServer"
  Env  = "Prod"
}

Where Arguments Go

ami = "ami-123" · instance_type = "t3.micro"
↓
Arguments
↓
AWS API
↓
Creates EC2

2. Attributes

An attribute is a value Terraform receives back from the provider after the resource is created. You don't write these values — Terraform discovers them.

main.tf

resource "aws_instance" "web" {
  ami           = "ami-123456"
  instance_type = "t2.micro"
}

main.tf

aws_instance.web.public_ip

Reading the Reference

aws_instance
↓
web
↓
public_ip

More Attribute Examples

.id aws_instance.web.id returns i-0d12345abcd6789
.arn aws_instance.web.arn returns arn:aws:ec2:ap-south-1:123456789012:instance/i-12345
.private_ip aws_instance.web.private_ip returns 10.0.1.15

main.tf

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
}

resource "aws_subnet" "public" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"
}

aws_vpc.main.id

Terraform gets the VPC ID only after the VPC is created, then automatically passes it into the subnet's vpc_id argument — an attribute feeding straight into the next resource's argument.

main.tf

output "instance_ip" {
  value = aws_instance.web.public_ip
}

terminal

terminalRESULT

$ terraform apply
instance_ip = 54.201.120.15

Input vs Output

ami · instance_type · subnet_id · tags
↓ Arguments
Terraform creates EC2
↓ AWS returns
Instance ID · Public IP · Private IP · ARN
↓
Attributes

Arguments vs Attributes

Arguments	Attributes
Input values	Output values
Defined by you	Generated by the provider
Used to configure resources	Used to retrieve information about resources
Known before creation	Often known only after creation
Example: `ami`, `instance_type`, `tags`	Example: `id`, `public_ip`, `private_ip`, `arn`

Complete Example

main.tf

resource "aws_instance" "web" {
  ami           = "ami-0123456789abcdef0"
  instance_type = "t3.micro"
  tags = {
    Name = "WebServer"
  }
}

output "public_ip" {
  value = aws_instance.web.public_ip
}

output "instance_id" {
  value = aws_instance.web.id
}

The Flow

Arguments: ami · instance_type · tags
↓
Terraform
↓
AWS Provider
↓
AWS Creates EC2
↓
Attributes: id · public_ip · private_ip · arn
↓
Terraform Outputs