How to Use List Variable in Terraform: Syntax and Example

In Terraform, you define a list variable using variable block with type = list(string) or similar. You can then reference this list variable in your configuration using var.variable_name to use multiple values easily.

Syntax

To declare a list variable in Terraform, use the variable block with type = list(TYPE). You can set a default list or provide values during runtime.

• variable_name: The name of the variable.
• type: Specifies the variable as a list of a certain type, e.g., list(string).
• default: Optional default list values.

terraform

variable "example_list" {
  type    = list(string)
  default = ["value1", "value2", "value3"]
}

Example

This example shows how to define a list variable and use it to create multiple AWS S3 buckets with names from the list.

terraform

variable "bucket_names" {
  type    = list(string)
  default = ["bucket-one", "bucket-two", "bucket-three"]
}

resource "aws_s3_bucket" "buckets" {
  for_each = toset(var.bucket_names)
  bucket   = each.value
  acl      = "private"
}

Output

Creates three S3 buckets named bucket-one, bucket-two, and bucket-three with private ACL.

Common Pitfalls

Common mistakes when using list variables include:

• Not specifying the type as a list, which can cause errors.
• Using a string instead of a list, e.g., default = "value1,value2" instead of default = ["value1", "value2"].
• Trying to use list variables without converting to a set when using for_each.

terraform

variable "wrong_list" {
  default = "value1,value2"
}

# Correct way:
variable "correct_list" {
  type    = list(string)
  default = ["value1", "value2"]
}

Quick Reference

Concept	Description	Example
Declare list variable	Use variable block with type list	variable "names" { type = list(string) }
Set default list	Provide default values as list	default = ["a", "b", "c"]
Reference list	Use var.variable_name to access	var.names
Use in for_each	Convert list to set for for_each	for_each = toset(var.names)

Key Takeaways

Always declare list variables with explicit type = list(TYPE) for clarity and validation.

Use square brackets [ ] to define list values, not comma-separated strings.

Access list variables with var.variable_name in your Terraform code.

Convert lists to sets with toset() when using for_each to avoid errors.

Providing default values in list variables helps avoid errors when no input is given.