Default values in Terraform - Step-by-Step Execution

Process Flow - Default values

Start variable declaration

↓

Check if input value provided?

No→Use default value

Yes↓

Use provided input value

↓

Variable value set

↓

End

Terraform variables check if a user input is given; if not, they use the default value specified.

Execution Sample

Terraform

variable "region" {
  type    = string
  default = "us-west-1"
}

output "selected_region" {
  value = var.region
}

Defines a variable with a default region; outputs the selected region value.

Process Table

Step	Input Provided?	Variable Value Set	Output Value
1	No	"us-west-1" (default)	"us-west-1"
2	Yes (e.g., "eu-central-1")	"eu-central-1"	"eu-central-1"

💡 Execution ends after variable value is set and output is produced.

Status Tracker

Variable	Start	After No Input	After Input Provided
var.region	undefined	"us-west-1"	"eu-central-1"
output.selected_region	undefined	"us-west-1"	"eu-central-1"

Key Moments - 2 Insights

What happens if no value is provided for the variable?

Terraform uses the default value specified in the variable block, as shown in execution_table row 1.

Can the default value be overridden?

Yes, if a value is provided during terraform apply or in a tfvars file, it overrides the default, as shown in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the variable value when no input is provided?

A undefined

B "eu-central-1"

C "us-west-1"

D null

💡 Hint

Check execution_table row 1 under 'Variable Value Set'

At which step does the variable value get overridden by user input?

A Step 2

B Step 1

C Never

D Before Step 1

💡 Hint

See execution_table row 2 where input is provided

If the default value is removed, what happens when no input is provided?

A Terraform uses null as value

B Terraform throws an error

C Terraform uses an empty string

D Terraform uses a random value

💡 Hint

Default values prevent errors when no input is given; removing default causes error if no input

Concept Snapshot

Terraform variables can have default values.
If no input is given, Terraform uses the default.
User input overrides the default value.
Defaults prevent errors from missing inputs.
Use 'default = value' inside variable blocks.

Full Transcript

In Terraform, variables can have default values. When Terraform runs, it checks if the user provided a value for the variable. If not, it uses the default value specified. If the user provides a value, that value overrides the default. This behavior ensures Terraform has a value to use and avoids errors from missing inputs. The example shows a variable 'region' with a default of 'us-west-1'. If no input is given, Terraform uses 'us-west-1'. If the user provides 'eu-central-1', Terraform uses that instead. Removing the default without providing input causes an error.

Practice

1. What is the purpose of setting a default value for a variable in Terraform?

easy

A. To make the variable mandatory without a fallback

B. To provide a fallback value if no other value is given

C. To disable the variable completely

D. To automatically generate a random value

Solution

1. Step 1: Understand variable defaults in Terraform

   Default values act as fallback values when no input is provided by the user.

2. Step 2: Identify the purpose of default values

   They make variables optional and provide a safe, predictable value.

3. Final Answer:

   To provide a fallback value if no other value is given -> Option B

4. Quick Check:

   Default value = fallback [OK]

Hint: Default means fallback if no input given [OK]

Common Mistakes:

• Thinking default makes variable mandatory
• Assuming default disables the variable
• Believing default auto-generates random values

2. Which of the following is the correct syntax to set a default value for a variable named region in Terraform?

easy

A. variable "region" { default = "us-east-1" }

B. variable region { default: "us-east-1" }

C. var region = default "us-east-1"

D. variable "region" = "us-east-1"

Solution

1. Step 1: Recall Terraform variable block syntax

   Variables are declared with the keyword variable and a name in quotes.

2. Step 2: Check correct default assignment

   The default value is set inside the block with default = "value".

3. Final Answer:

   variable "region" { default = "us-east-1" } -> Option A

4. Quick Check:

   Correct syntax uses variable block and default = value [OK]

Hint: Use variable "name" { default = "value" } [OK]

Common Mistakes:

• Missing quotes around variable name
• Using colon instead of equals for default
• Assigning default outside variable block

3. Given this variable declaration:

variable "instance_type" {
  default = "t2.micro"
}

What will be the value of var.instance_type if no override is provided?

medium

A. null

B. Error: variable not set

C. empty string ""

D. "t2.micro"

Solution

1. Step 1: Identify the default value in the variable block

   The variable instance_type has a default set to "t2.micro".

2. Step 2: Understand behavior when no override is given

   If no value is provided, Terraform uses the default value automatically.

3. Final Answer:

   "t2.micro" -> Option D

4. Quick Check:

   Default used if no override [OK]

Hint: No override? Use default value [OK]

Common Mistakes:

• Assuming null if no value given
• Expecting error without override
• Thinking default is empty string

4. You wrote this variable block:

variable "env" {
  default: "production"
}

Why does Terraform give a syntax error?

medium

A. Because default uses colon instead of equals sign

B. Because variable name must not be in quotes

C. Because default value must be a number

D. Because variable blocks cannot have defaults

Solution

1. Step 1: Check the syntax for default assignment

   Terraform requires = to assign default values, not a colon.

2. Step 2: Identify the cause of syntax error

   Using default: "production" causes a syntax error because colon is invalid here.

3. Final Answer:

   Because default uses colon instead of equals sign -> Option A

4. Quick Check:

   Use = not : for default [OK]

Hint: Use = for default, not colon : [OK]

Common Mistakes:

• Using colon instead of equals for default
• Removing quotes from variable name
• Thinking default must be numeric

5. You want to create a variable availability_zones with a default list of zones ["us-east-1a", "us-east-1b"]. Which is the correct way to declare it in Terraform?

hard

A. variable "availability_zones" {
default = "us-east-1a, us-east-1b"
}

B. variable availability_zones = ["us-east-1a", "us-east-1b"]

C. variable "availability_zones" {
type = list(string)
default = ["us-east-1a", "us-east-1b"]
}

D. variable "availability_zones" {
type = string
default = ["us-east-1a", "us-east-1b"]
}

Solution

1. Step 1: Define variable with correct type for list of strings

   Use type = list(string) to specify a list of strings.

2. Step 2: Assign default as a list of strings

   Set default = ["us-east-1a", "us-east-1b"] inside the variable block.

3. Final Answer:

   variable "availability_zones" { type = list(string) default = ["us-east-1a", "us-east-1b"] } -> Option C

4. Quick Check:

   List type with list default [OK]

Hint: Use type list(string) and default list [OK]

Common Mistakes:

• Omitting type for list variables
• Assigning list to string type
• Using string with commas instead of list