String Interpolation in HCL

HCL (HashiCorp Configuration Language) allows for dynamic and flexible configurations through string interpolation. String interpolation is a powerful feature that enables you to embed variables and expressions within double-quoted strings, making your Terraform configurations more adaptable and reusable. In this guide, we’ll explore the concepts and usage of string interpolation in HCL.

Basic String Interpolation

Basic string interpolation in HCL involves embedding variables within double-quoted strings. This is often used for constructing resource names, labels, and other dynamic string values.

resource "aws_instance" "example" {
  ami = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  tags = {
    Name = "web-${var.environment}-instance"
  }
}
    

In the above example, the “Name” tag is dynamically generated by interpolating the value of the “var.environment” variable into the string. This results in resource names like “web-dev-instance” or “web-prod-instance” based on the value of “var.environment.”

String Concatenation

String interpolation also allows for string concatenation. You can combine multiple strings and variables to create complex dynamic strings.

variable "service_name" {
  type = string
  default = "my-service"
}

resource "aws_s3_bucket" "example" {
  bucket = "my-${var.service_name}-bucket"
  // Other configuration settings
}
    

Here, the “bucket” attribute is constructed by concatenating the “my-” string, the value of “var.service_name,” and the “-bucket” string, resulting in a dynamic bucket name like “my-my-service-bucket.”

Numeric Interpolation

String interpolation is not limited to variables of string type. You can also interpolate numeric values within strings.

variable "port" {
  type = number
  default = 8080
}

resource "aws_security_group" "example" {
  name = "allow-port-${var.port}"
  // Other configuration settings
}
    

In this example, the “name” attribute of the security group resource is dynamically generated by interpolating the numeric value of “var.port.” This can be used to create security group names like “allow-port-8080.”

Advanced Interpolation

HCL supports more advanced interpolation functions, such as format strings, which provide precise control over how variables are formatted and included in strings.

variable "count" {
  type = number
  default = 5
}

resource "aws_instance" "example" {
  count = var.count
  ami = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  tags = {
    Name = format("web-%02d-instance", count.index)
  }
}
    

In this example, the “format” function is used to create zero-padded numeric values in the “Name” tag, ensuring that instance names like “web-01-instance” and “web-05-instance” are generated.

Summary

String interpolation in HCL is a fundamental feature for creating dynamic Terraform configurations. By embedding variables and expressions within strings, you can build resource names, labels, and other dynamic values, making your infrastructure as code adaptable to different environments and requirements.