Terraform: Combine Variables and Strings in Configurations

• Introduction
• Step-by-Step Guide
• Code Example
• Additional Notes
• Summary
• Conclusion
• References

In Terraform, combining variables and strings is a common task for creating dynamic resource configurations. This article explains how to achieve this using direct concatenation and the join() function. We'll cover examples and important considerations for different Terraform versions.

You can combine variables and strings in Terraform using a couple of methods.

Direct Concatenation:

You can directly concatenate a variable and a string within your Terraform code. For example, if you have a variable var.id with the value "abcde", you can create an output like this:

output "output1" {
 value = "${var.id}id"
}

This will output "abcdeid".

The join() Function:

The join() function allows you to concatenate multiple strings together, including variables. For instance:

output "output2" {
 value = join("", ["This ", "is ", "a ", "string ", "with ", var.id, "."])
}

This will output "This is a string with abcde."

Important Notes:

• Interpolation in tfvars Files: You cannot use interpolation (${}) within .tfvars files. If you need to combine strings in a variable definition, do it directly in your Terraform code.

• Terraform Version 0.12.x and Above: In versions 0.12.x and later, you don't need to use ${} for variable interpolation within strings. You can directly use the variable name. For example:

  name = "My-Bucket${var.bucket_prefix}"  # Older versions
  name = "My-Bucket" + var.bucket_prefix  # Version 0.12.x and above

Remember to choose the method that best suits your needs and Terraform version.

This code snippet provides examples of string concatenation in Terraform using direct concatenation, the join() function, and concatenation within resource names. It defines variables for an ID and bucket prefix, then demonstrates their usage in outputs and an AWS S3 bucket resource. The code includes explanations and instructions for running the example to observe the output.

This code demonstrates different ways to combine variables and strings in Terraform.

variables.tf

variable "id" {
  default = "abcde"
}

variable "bucket_prefix" {
  default = "prod"
}

main.tf

# Direct Concatenation
output "output1" {
  value = "${var.id}id"
}

# Using the join() function
output "output2" {
  value = join("", ["This ", "is ", "a ", "string ", "with ", var.id, "."])
}

# Concatenation in resource names (for Terraform 0.12.x and above)
resource "aws_s3_bucket" "example" {
  bucket = "my-bucket-" + var.bucket_prefix
}

Explanation:

• variables.tf: Defines two variables, id and bucket_prefix.
• main.tf:
  • output1: Demonstrates direct concatenation by adding "id" to the value of var.id.
  • output2: Uses the join() function to combine multiple strings and the variable var.id.
  • aws_s3_bucket: Shows how to concatenate strings directly within a resource definition for Terraform versions 0.12.x and above.

Running the code:

1. Save the code as main.tf and variables.tf in the same directory.
2. Run terraform init to initialize the working directory.
3. Run terraform apply to apply the configuration.

Output:

You will see the following outputs:

output1 = abcdeid
output2 = This is a string with abcde.

This example demonstrates different ways to combine variables and strings in Terraform, allowing you to create dynamic and flexible configurations. Remember to choose the method that best suits your needs and Terraform version.

• Flexibility: Combining variables and strings is crucial for creating dynamic and reusable Terraform modules. It allows you to customize resource names, tags, and other configurations based on variables passed in at runtime.

• Readability: While direct concatenation works, using the join() function can often improve the readability of your code, especially when combining multiple strings and variables.

• Error Handling: Be mindful of potential errors when concatenating strings, such as accidentally introducing typos or mismatched data types. Terraform's type system and validation can help catch some of these errors.

• Alternative Approaches: For more complex string manipulations, consider using Terraform functions like format(), substr(), or regex() in conjunction with variable interpolation.

• Best Practices: Favor descriptive variable names and clear comments to enhance the maintainability of your Terraform code, especially when dealing with string manipulations.

• Terraform State: Remember that Terraform tracks the state of your infrastructure. If you change the way you concatenate strings in a resource name, for example, it might trigger a recreation of that resource.

Important Notes:

• Interpolation ("${}) is not allowed within .tfvars files.
• Choose the method that best suits your needs and Terraform version.

In conclusion, mastering the art of combining variables and strings in Terraform is fundamental for building dynamic and adaptable infrastructure configurations. Whether you choose direct concatenation, the join() function, or leverage the simplified syntax of Terraform 0.12.x and above, understanding these techniques empowers you to create more efficient, maintainable, and robust infrastructure as code. Remember to prioritize readability and error prevention, and always refer to the official Terraform documentation for the most up-to-date information and best practices.

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