Splat expressions in Terraform - Mini Project: Build & Apply

Using Splat Expressions in Terraform

📖 Scenario: You are managing cloud infrastructure using Terraform. You have multiple virtual machines (VMs) defined, and you want to collect their public IP addresses easily.

🎯 Goal: Create a Terraform configuration that defines three virtual machines with specific names and public IPs. Then, use a splat expression to extract all their public IP addresses into a list variable.

📋 What You'll Learn

Define a list variable vm_names with three VM names: "vm1", "vm2", "vm3"

Create three aws_instance resources using a for_each loop over vm_names

Each instance must have a public_ip attribute set to a unique IP string

Use a splat expression to create a list variable public_ips containing all public IPs from the instances

💡 Why This Matters

🌍 Real World

Splat expressions help you gather information from multiple cloud resources easily, such as collecting all IP addresses or IDs for further use.

💼 Career

Cloud engineers often need to manage many resources and extract their attributes efficiently; splat expressions simplify this process in Terraform.

1

Define the VM names list variable

Create a Terraform variable called vm_names as a list with these exact values: "vm1", "vm2", "vm3".

Terraform

# Define the vm_names variable as a list of three VM names
# Your code here

Hint

Use variable "vm_names" with type = list(string) and set default to the list of names.

2

Create aws_instance resources using for_each

Create aws_instance resources named example using for_each over var.vm_names. Set the public_ip attribute to "10.0.0.${index(toset(var.vm_names), each.key) + 1}" for each instance.

Terraform

variable "vm_names" {
  type    = list(string)
  default = ["vm1", "vm2", "vm3"]
}

# Create aws_instance resources with for_each over var.vm_names
# Your code here

Hint

Use for_each = toset(var.vm_names) and set public_ip using the index of each key plus 1.

3

Use a splat expression to get all public IPs

Create a local variable called public_ips that uses a splat expression to collect all public_ip values from aws_instance.example resources.

Terraform

variable "vm_names" {
  type    = list(string)
  default = ["vm1", "vm2", "vm3"]
}

resource "aws_instance" "example" {
  for_each = toset(var.vm_names)

  ami           = "ami-12345678"
  instance_type = "t2.micro"
  associate_public_ip_address = true
  public_ip     = "10.0.0.${index(toset(var.vm_names), each.key) + 1}"
}

# Create local variable public_ips using splat expression
# Your code here

Hint

Use locals block and assign public_ips = aws_instance.example[*].public_ip.

4

Output the list of public IPs

Add an output block named all_public_ips that outputs the local variable public_ips.

Terraform

variable "vm_names" {
  type    = list(string)
  default = ["vm1", "vm2", "vm3"]
}

resource "aws_instance" "example" {
  for_each = toset(var.vm_names)

  ami           = "ami-12345678"
  instance_type = "t2.micro"
  associate_public_ip_address = true
  public_ip     = "10.0.0.${index(toset(var.vm_names), each.key) + 1}"
}

locals {
  public_ips = aws_instance.example[*].public_ip
}

# Add output block all_public_ips to output local.public_ips
# Your code here

Hint

Use output "all_public_ips" block with value = local.public_ips.

Practice

1.

What does a splat expression in Terraform do?

Example: aws_instance.example[*].id

easy

A. Collects the id attribute from all aws_instance.example resources into a list

B. Creates a new resource named id for each instance

C. Deletes all aws_instance.example resources

D. Filters instances with id equal to example

Solution

1. Step 1: Understand the splat operator *

   The * means 'all items' in the list of resources.

2. Step 2: Collect attribute values

   The expression aws_instance.example[*].id collects the id attribute from every instance in aws_instance.example.

3. Final Answer:

   Collects the id attribute from all aws_instance.example resources into a list -> Option A

4. Quick Check:

   Splat expression collects attributes [OK]

Hint: Remember: * means all items, collect their attributes [OK]

Common Mistakes:

• Thinking splat creates or deletes resources
• Confusing splat with filtering
• Assuming splat returns a single value

2.

Which of the following is the correct syntax for a splat expression to get all name attributes from a resource list aws_s3_bucket.buckets?

easy

A. aws_s3_bucket.buckets[*].name

B. aws_s3_bucket.buckets.name[*]

C. aws_s3_bucket.buckets[].name

D. aws_s3_bucket.buckets.name

Solution

1. Step 1: Identify splat operator position

   The splat operator * must come immediately after the resource list name inside square brackets.

2. Step 2: Confirm attribute access

   The correct syntax is resource_list[*].attribute, so aws_s3_bucket.buckets[*].name is correct.

3. Final Answer:

   aws_s3_bucket.buckets[*].name -> Option A

4. Quick Check:

   Splat syntax = resource_list[*].attribute [OK]

Hint: Put * inside brackets right after resource list name [OK]

Common Mistakes:

• Placing * after attribute name
• Using [] without * inside
• Omitting brackets for splat

3.

Given the Terraform code snippet:

resource "aws_instance" "web" {
  count = 3
  ami           = "ami-123456"
  instance_type = "t2.micro"
}

output "instance_ids" {
  value = aws_instance.web[*].id
}

What will be the output of instance_ids after deployment?

medium

A. A list of 0 elements because no instances are created

B. A list of 3 instance IDs, one for each created instance

C. An error because splat cannot be used with count

D. A single instance ID of the first instance

Solution

1. Step 1: Understand resource count creates multiple instances

   The count = 3 creates 3 instances named aws_instance.web[0], aws_instance.web[1], and aws_instance.web[2].

2. Step 2: Splat expression collects all instance IDs

   The expression aws_instance.web[*].id collects the id attribute from all 3 instances into a list.

3. Final Answer:

   A list of 3 instance IDs, one for each created instance -> Option B

4. Quick Check:

   count 3 + splat = list of 3 IDs [OK]

Hint: count creates multiple; splat collects all attributes [OK]

Common Mistakes:

• Thinking splat fails with count
• Expecting single ID instead of list
• Assuming no instances created

4.

Identify the error in this Terraform snippet using splat expressions:

output "bucket_names" {
  value = aws_s3_bucket.buckets.name[*]
}

medium

A. Output blocks cannot use splat expressions

B. The resource name aws_s3_bucket.buckets is invalid

C. Missing quotes around the attribute name

D. The splat operator * is incorrectly placed after the attribute name

Solution

1. Step 1: Check splat operator position

   The splat operator * must be inside brackets immediately after the resource list, not after the attribute.

2. Step 2: Correct syntax

   The correct syntax is aws_s3_bucket.buckets[*].name, not aws_s3_bucket.buckets.name[*].

3. Final Answer:

   The splat operator * is incorrectly placed after the attribute name -> Option D

4. Quick Check:

   Splat must be after resource list, not attribute [OK]

Hint: Put * right after resource list in brackets, not after attribute [OK]

Common Mistakes:

• Placing * after attribute name
• Confusing resource and attribute order
• Thinking output blocks disallow splat

5.

You have a list of AWS EC2 instances created with count = 4. You want to output a list of their public IPs but only for instances that have a tag Environment set to production. Which Terraform expression correctly uses splat and filtering?

hard

A. aws_instance.web[*].public_ip

B. aws_instance.web[*].tags.Environment == "production" ? aws_instance.web[*].public_ip : []

C. [for i in aws_instance.web : i.public_ip if i.tags.Environment == "production"]

D. aws_instance.web[?tags.Environment=="production"].public_ip

Solution

1. Step 1: Understand splat collects all attributes without filtering

   The expression aws_instance.web[*].public_ip collects all public IPs, no filtering.

2. Step 2: Use for-expression with condition for filtering

   The correct way to filter is using a for-expression with an if condition: [for i in aws_instance.web : i.public_ip if i.tags.Environment == "production"].

3. Final Answer:

   [for i in aws_instance.web : i.public_ip if i.tags.Environment == "production"] -> Option C

4. Quick Check:

   Filter with for-expression + if, not splat alone [OK]

Hint: Use for-expression with if to filter, splat alone can't filter [OK]

Common Mistakes:

• Using splat without filtering
• Trying to filter inside splat syntax
• Using invalid filter syntax