Comments in HCL in Terraform - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the valid single-line comment syntax in HCL

Which of the following lines is a valid single-line comment in HashiCorp Configuration Language (HCL)?

A # This is a comment

B // This is a comment

C /* This is a comment */

D <!-- This is a comment -->

💡 Hint

HCL supports single-line comments starting with # or //, and multi-line comments with /* */.

In HCL, comments can start with # or // for single-line comments, and /* */ for multi-line comments. The syntax <!-- --> is used in HTML, not HCL.

❓ Configuration

intermediate

Effect of comments on resource count

Given the following Terraform configuration snippet, how many resources will be created?

Terraform

resource "aws_instance" "example1" {
  ami           = "ami-123456"
  instance_type = "t2.micro"
}

# resource "aws_instance" "example2" {
#   ami           = "ami-654321"
#   instance_type = "t2.micro"
# }

// resource "aws_instance" "example3" {
//   ami           = "ami-abcdef"
//   instance_type = "t2.micro"
// }

A 3

B 2

C 0

D 1

💡 Hint

Lines starting with # or // are ignored by Terraform.

Only the first resource block is active. The other two resource blocks are commented out and ignored, so only one resource will be created.

❓ service_behavior

advanced

Multi-line comment effect in HCL

What will happen if you use the following multi-line comment syntax in a Terraform configuration file?

Terraform

/*
resource "aws_s3_bucket" "bucket" {
  bucket = "my-bucket"
  acl    = "private"
}
*/

resource "aws_s3_bucket" "bucket2" {
  bucket = "my-bucket-2"
  acl    = "private"
}

A Only aws_s3_bucket.bucket2 will be created; the first resource is ignored.

B Both aws_s3_bucket.bucket and aws_s3_bucket.bucket2 will be created.

C Terraform will raise a syntax error due to incorrect comment syntax.

D No resources will be created because all are commented out.

💡 Hint

Multi-line comments in HCL start with /* and end with */.

The multi-line comment correctly comments out the first resource block, so only the second resource block is active and created.

❓ security

advanced

Security risk of commented sensitive data

Why is it a bad practice to leave sensitive information like passwords or API keys in commented lines in Terraform files?

A Commented lines are encrypted and safe from exposure.

B Terraform automatically removes comments before applying configurations.

C Commented sensitive data can still be exposed in version control or logs.

D Comments are ignored by all tools and cannot be accessed.

💡 Hint

Think about where your code files are stored and who can see them.

Comments remain in the source files and can be seen by anyone with access to the code repository or logs, risking exposure of sensitive data.

❓ Architecture

expert

Impact of comments on Terraform plan output

Consider a Terraform file where a resource block is commented out using single-line comments. What will be the impact on the output of the `terraform plan` command?

Terraform

resource "aws_security_group" "sg1" {
  name        = "sg1"
  description = "Allow SSH"
  ingress {
    from_port   = 22
    to_port     = 22
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

// resource "aws_security_group" "sg2" {
//   name        = "sg2"
//   description = "Allow HTTP"
//   ingress {
//     from_port   = 80
//     to_port     = 80
//     protocol    = "tcp"
//     cidr_blocks = ["0.0.0.0/0"]
//   }
// }

A Terraform plan will show changes only for aws_security_group.sg1; sg2 is ignored.

B Terraform plan will show no changes because comments disable all resources.

C Terraform plan will fail with a syntax error due to commented lines.

D Terraform plan will show changes for both sg1 and sg2 resources.

💡 Hint

Comments disable code from being processed by Terraform.

Commented resource blocks are ignored by Terraform, so only the active resource (sg1) will appear in the plan output.

Practice

1. What is the purpose of comments in Terraform's HCL language?

easy

A. To explain the code without affecting its execution

B. To execute additional commands

C. To speed up the Terraform apply process

D. To delete unused resources automatically

Solution

1. Step 1: Understand the role of comments

   Comments are notes in code that help explain what the code does but do not change how it runs.

2. Step 2: Identify the correct purpose

   Since comments do not execute or affect resources, their purpose is purely explanatory.

3. Final Answer:

   To explain the code without affecting its execution -> Option A

4. Quick Check:

   Comments explain code [OK]

Hint: Comments never change code behavior, only explain it [OK]

Common Mistakes:

• Thinking comments run code
• Assuming comments speed up deployment
• Believing comments delete resources

2. Which of the following is a valid single-line comment syntax in Terraform HCL?

easy

A. /* This is a comment */

B. # This is a comment

C. -- This is a comment

D. %% This is a comment

Solution

1. Step 1: Recall single-line comment syntax in HCL

   Terraform HCL supports # and // for single-line comments.

2. Step 2: Match options with valid syntax

   # This is a comment uses #, which is valid. /* This is a comment */ is multi-line comment syntax. Options C and D are invalid in HCL.

3. Final Answer:

   # This is a comment -> Option B

4. Quick Check:

   Single-line comment = # or // [OK]

Hint: Single-line comments start with # or // only [OK]

Common Mistakes:

• Using -- or %% which are not valid in HCL
• Confusing multi-line comment syntax for single-line
• Omitting comment symbols

3. What will happen if you run this Terraform code snippet?

resource "aws_instance" "example" {
  ami           = "ami-123456"
  instance_type = "t2.micro"  # This is a small instance
  // This line is ignored
  /* multi-line
     comment here */
}

medium

A. Terraform will create an AWS instance ignoring all comments

B. Terraform will throw a syntax error due to comments

C. Terraform will create an instance but ignore the instance_type

D. Terraform will skip the resource block entirely

Solution

1. Step 1: Understand how comments affect Terraform code

   Comments are ignored by Terraform during execution and do not affect resource creation.

2. Step 2: Analyze the code snippet

   The resource block is valid with comments in single-line (#, //) and multi-line (/* */) forms. Terraform will create the instance as specified.

3. Final Answer:

   Terraform will create an AWS instance ignoring all comments -> Option A

4. Quick Check:

   Comments ignored, resource created [OK]

Hint: Comments do not affect resource creation or syntax [OK]

Common Mistakes:

• Thinking comments cause syntax errors
• Assuming comments disable resource properties
• Believing comments skip resource blocks

4. Identify the error in this Terraform snippet:

resource "aws_s3_bucket" "bucket" {
  bucket = "my-bucket"
  /* This is a multi-line comment
  missing the closing tag

medium

A. Single-line comment used incorrectly

B. Incorrect resource type name

C. Bucket name must be numeric

D. Missing closing */ for multi-line comment

Solution

1. Step 1: Check multi-line comment syntax

   Multi-line comments start with /* and must end with */. The snippet lacks the closing */.

2. Step 2: Confirm other parts are correct

   Resource type and bucket name are valid. No single-line comment issues.

3. Final Answer:

   Missing closing */ for multi-line comment -> Option D

4. Quick Check:

   Unclosed multi-line comment [OK]

Hint: Always close multi-line comments with */ [OK]

Common Mistakes:

• Forgetting to close multi-line comments
• Confusing resource type errors with comment errors
• Assuming bucket names must be numeric

5. You want to temporarily disable a block of Terraform code without deleting it. Which comment style should you use to comment out multiple lines safely?

hard

A. Use # at the start of each line

B. Use // at the start of each line

C. Use /* */ to wrap the entire block

D. Use XML style <!-- --> comments

Solution

1. Step 1: Review comment styles for multiple lines

   Single-line comments (# or //) require prefixing each line, which is tedious for many lines.

2. Step 2: Identify the best multi-line comment method

   Using /* */ wraps multiple lines easily, disabling the entire block at once.

3. Step 3: Exclude invalid options

   XML style comments are not valid in HCL.

4. Final Answer:

   Use /* */ to wrap the entire block -> Option C

5. Quick Check:

   Multi-line comments use /* */ [OK]

Hint: Wrap multiple lines with /* and */ to comment out block [OK]

Common Mistakes:

• Using # or // for many lines instead of block comment
• Trying XML style comments which are invalid
• Not closing multi-line comment properly