Arguments and expressions in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an argument in Terraform?

An argument in Terraform is a key-value pair inside a resource or module block that sets a specific property or configuration.

beginner

What is an expression in Terraform?

An expression in Terraform is a combination of values, variables, functions, or operators that Terraform evaluates to produce a result.

beginner

How do you use variables inside arguments in Terraform?

You use variables inside arguments by referencing them with syntax like ${var.variable_name} or simply var.variable_name in newer versions.

intermediate

What does this expression do?

count = var.enable ? 1 : 0

This expression uses a condition to set count to 1 if var.enable is true, otherwise 0. It controls resource creation based on a variable.

intermediate

Why are expressions useful in Terraform arguments?

Expressions let you create dynamic and flexible configurations by calculating values, using conditions, or referencing variables and outputs.

In Terraform, what is an argument?

A A key-value pair inside a resource block

B A function that runs during deployment

C A type of variable

D A command line option

Arguments are key-value pairs inside resource or module blocks that set properties.

Which of the following is a valid Terraform expression?

A resource.aws_instance

B var.instance_count + 2

C terraform apply

D module.network

Expressions combine values and variables, like adding numbers.

How do you reference a variable named 'region' in Terraform 0.12+?

A region

B ${var.region}

C var.region

D variable.region

In Terraform 0.12 and later, you can reference variables directly as var.region.

What does this expression do?

enabled = var.is_active && var.has_permission

A Sets enabled to true only if both variables are true

B Sets enabled to true if either variable is true

C Sets enabled to false always

D Throws an error

The && operator means both conditions must be true for enabled to be true.

Why use expressions in Terraform arguments?

A To hardcode values

B To write comments

C To run scripts

D To create dynamic and flexible configurations

Expressions allow dynamic values based on variables and conditions.

Explain what arguments and expressions are in Terraform and how they work together.

Think about how you set properties and use variables or conditions.

Describe a simple example where you use an expression inside an argument to control resource creation.

Consider how to create or skip a resource based on a variable.

Practice

1. What is the purpose of arguments inside a Terraform resource block?

easy

A. To set specific details or properties for the resource

B. To run commands on the resource after creation

C. To define the cloud provider used

D. To store output values from the resource

Solution

1. Step 1: Understand resource blocks

   Resource blocks define cloud resources, and arguments inside them specify details like size or name.

2. Step 2: Identify argument role

   Arguments set the properties that Terraform uses to create or manage the resource.

3. Final Answer:

   To set specific details or properties for the resource -> Option A

4. Quick Check:

   Arguments = resource details [OK]

Hint: Arguments define resource details inside blocks [OK]

Common Mistakes:

• Confusing arguments with outputs
• Thinking arguments run commands
• Mixing provider definition with arguments

2. Which of the following is the correct syntax to assign a variable value to an argument in Terraform?

easy

A. name := var.instance_name

B. name => var.instance_name

C. name == var.instance_name

D. name = var.instance_name

Solution

1. Step 1: Recall Terraform assignment syntax

   Terraform uses single equals sign (=) to assign values to arguments.

2. Step 2: Match variable usage

   Variables are referenced with var.<variable_name> and assigned using =.

3. Final Answer:

   name = var.instance_name -> Option D

4. Quick Check:

   Assignment uses = and var. prefix [OK]

Hint: Use = to assign variables in Terraform [OK]

Common Mistakes:

• Using := or == instead of =
• Confusing arrow => with assignment
• Omitting var. prefix for variables

3. Given the Terraform expression count = var.enable ? 3 : 1 and var.enable = false, what will be the value of count?

medium

A. true

B. 3

C. 1

D. false

Solution

1. Step 1: Understand the ternary expression

   The expression uses a condition: if var.enable is true, count is 3; else count is 1.

2. Step 2: Substitute variable value

   Since var.enable is false, the expression returns the value after the colon, which is 1.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   False condition returns second value [OK]

Hint: Ternary returns value after colon if condition is false [OK]

Common Mistakes:

• Choosing the first value regardless of condition
• Confusing boolean with numeric output
• Ignoring variable value in expression

4. What is wrong with this Terraform argument expression?

size = var.instance_size ? "large" : "small"

assuming var.instance_size is a string variable.

medium

A. Ternary condition requires a boolean, not a string

B. Strings cannot be used in ternary expressions

C. The quotes around "large" and "small" are invalid

D. Variable names cannot be used in expressions

Solution

1. Step 1: Check ternary condition type

   The condition before ? must be boolean (true/false), but var.instance_size is a string.

2. Step 2: Identify type mismatch error

   Using a string as condition causes an error because Terraform expects a boolean expression.

3. Final Answer:

   Ternary condition requires a boolean, not a string -> Option A

4. Quick Check:

   Condition must be boolean [OK]

Hint: Ternary condition must be true/false, not string [OK]

Common Mistakes:

• Using strings as conditions
• Misplacing quotes around values
• Assuming variables can't be in expressions

5. You want to create multiple AWS EC2 instances only if var.create_instances is true. Which expression correctly sets the count argument to 5 when true, or 0 when false?

hard

A. count = var.create_instances == 5

B. count = var.create_instances ? 5 : 0

C. count = var.create_instances && 5

D. count = var.create_instances || 0

Solution

1. Step 1: Understand conditional count setting

   We want count to be 5 if var.create_instances is true, else 0.

2. Step 2: Evaluate each option

   count = var.create_instances ? 5 : 0 uses ternary: if true, 5; else 0. Options B, C, D misuse operators or return booleans.

3. Final Answer:

   count = var.create_instances ? 5 : 0 -> Option B

4. Quick Check:

   Ternary sets count correctly [OK]

Hint: Use ternary to set count based on boolean variable [OK]

Common Mistakes:

• Using boolean expressions instead of numbers for count
• Confusing && and || operators for count
• Not using ternary for conditional count