Transform Inputs with Built-in Functions

Terraform · 15 min · Practical

HCL provides a rich suite of built-in functions to transform strings, query list lengths, manipulate delimeter strings, retrieve map values safely, and set evaluation fallbacks without external scripts.

Core Implementation Highlights

String Functionsupper() & lower() change string casing; join() & split() manipulate lists to/from text.

Collection Functionlength() dynamically counts elements in a list variable to determine count .

Map Lookup Functionlookup(map, key, default) retrieves map values safely using specified key.

Error Fallback Functiontry() evaluates multiple expressions sequentially, returning the first non-error value.

The Complete Terraform Code

Save the code below into main.tf. It demonstrates string casing, collection inspection, string joins/splits, map lookups, and error fallback evaluation using built-in Terraform functions.

main.tf

# Uppercase function
resource "aws_s3_bucket" "uppercasefunction" {
  bucket = "shan-${upper("uppercasefunction")}-bucket"
}


# Lowercase function
resource "aws_s3_bucket" "lowercasefunction" {
  bucket = "shan-${lower("LOWERCASEFUNCTION")}-bucket"
}


# Length function
variable "lengthfunction" {
  default = ["web", "db"]
}

resource "aws_instance" "webserver" {
  count = length(var.lengthfunction)

  ami           = "ami-xxxxxxxxxxxxxxxxx"
  instance_type = "t2.micro"

  associate_public_ip_address = true
}


# Join function
resource "aws_s3_bucket" "joinfunction" {
  bucket = join("-", ["shan", "joinfunction", "bucket"])
}

output "joinfunctionoutput" {
  value = aws_s3_bucket.joinfunction.bucket
}


# Split function
resource "aws_s3_bucket" "splitfunction" {
  bucket = join("-", split(",", "shan,split,function,bucket"))
}

output "splitfunctionoutput" {
  value = aws_s3_bucket.splitfunction.bucket
}


# Lookup function
variable "lookupfunction" {
  default = {
    dev  = "ami-0c02fb55956c7d316"
    prod = "ami-0abcdef1234567890"
  }
}

resource "aws_instance" "instanceforlookup" {
  ami           = lookup(var.lookupfunction, "dev")
  instance_type = "t2.micro"
}


# Try function
variable "tryfunction" {
  default = {
    dev = "ami-0c02fb55956c7d316"
  }
}

resource "aws_instance" "instancefortry" {
  ami = try(
    var.tryfunction["prod"],
    var.tryfunction["dev"],
    "ami-default"
  )

  instance_type = "t2.micro"
}

Resource-by-Resource Explanation

1. upper() & lower()

Converts all characters in a string to uppercase or lowercase:

upper("uppercasefunction") converts "uppercasefunction" to "UPPERCASEFUNCTION".
lower("LOWERCASEFUNCTION") converts "LOWERCASEFUNCTION" to "lowercasefunction".

main.tf

resource "aws_s3_bucket" "uppercasefunction" {
  bucket = "shan-${upper("uppercasefunction")}-bucket"
}

resource "aws_s3_bucket" "lowercasefunction" {
  bucket = "shan-${lower("LOWERCASEFUNCTION")}-bucket"
}

2. length()

Returns the number of elements in a list, map, or string length. Here, length(var.lengthfunction) counts 2 items (["web", "db"]), automatically setting EC2 count = 2.

main.tf

variable "lengthfunction" {
  default = ["web", "db"]
}

resource "aws_instance" "webserver" {
  count = length(var.lengthfunction)
  ami   = "ami-xxxxxxxxxxxxxxxxx"
  instance_type = "t2.micro"
}

3. join() & split()

Manipulates delimited lists and strings:

join("-", [...]) concatenates list items with a hyphen separator into a single bucket name string.
split(",", "shan,split,function,bucket") splits comma-separated text into a list, which is then joined with hyphen delimiters.

main.tf

resource "aws_s3_bucket" "joinfunction" {
  bucket = join("-", ["shan", "joinfunction", "bucket"])
}

resource "aws_s3_bucket" "splitfunction" {
  bucket = join("-", split(",", "shan,split,function,bucket"))
}

4. lookup()

Retrieves the value of a single key from a map variable. lookup(var.lookupfunction, "dev") searches var.lookupfunction map for key "dev", returning "ami-0c02fb55956c7d316".

main.tf

variable "lookupfunction" {
  default = {
    dev  = "ami-0c02fb55956c7d316"
    prod = "ami-0abcdef1234567890"
  }
}

resource "aws_instance" "instanceforlookup" {
  ami = lookup(var.lookupfunction, "dev")
}

5. try()

Evaluates a list of expressions in order and returns the first one that does not produce an error:

1st attempt: var.tryfunction["prod"] (Fails because "prod" key is missing).
2nd attempt: var.tryfunction["dev"] (Succeeds! Returns "ami-0c02fb55956c7d316").
3rd attempt: "ami-default" (Fallback default if all prior items fail).

main.tf

resource "aws_instance" "instancefortry" {
  ami = try(
    var.tryfunction["prod"],
    var.tryfunction["dev"],
    "ami-default"
  )
  instance_type = "t2.micro"
}

Testing & Verification Commands

You can test Terraform built-in functions interactively using terraform console or run a full plan evaluation:

Terminal

# 1. Test expressions interactively using Terraform Console
$ terraform console
> upper("uppercasefunction")
"UPPERCASEFUNCTION"
> join("-", ["shan", "joinfunction", "bucket"])
"shan-joinfunction-bucket"
> exit

# 2. Initialize provider plugins
$ terraform init

# 3. Dry-run plan evaluation to preview evaluated function values
$ terraform plan