Functions Operators and Conditional Expressions

OpenTofu provides a rich set of built-in functions, operators, and conditional expressions that enhance your infrastructure-as-code (IaC) workflows. This guide will show you how to leverage the interactive console and explore numeric, string, list, and map functions with practical examples.

Launching the OpenTofu Console

To experiment with functions and interpolations in a live environment, start the interactive console:

tofu console

Note

The OpenTofu console evaluates HCL expressions in real time. You can inspect variables, resources, and file contents without applying changes.

Exploring Functions in the Console

Given this configuration snippet:

resource "aws_iam_policy" "adminUser" {
  name   = "AdminUsers"
  policy = file("admin-policy.json")
}

resource "local_file" "pet_count" {
  filename = var.filename
  count    = length(var.filename)
}

resource "local_file" "pet_each" {
  filename = var.filename
  for_each = toset(var.region)
}

variable "region" {
  type        = list(string)
  default     = ["us-east-1", "us-east-1", "ca-central-1"]
  description = "A list of AWS Regions"
}

Try these commands in the console:

> file("/root/opentofu-projects/main.tf")
<<EOT
resource "aws_instance" "development" {
  ami           = "ami-0edab43b6fa892279"
  instance_type = "t2.micro"
}
EOT

> length(var.region)
3

> toset(var.region)
toset([
  "ca-central-1",
  "us-east-1",
])

file(path) reads the file’s content.
length(list) returns the number of elements (here, 3).
toset(list) converts a list to a set, removing duplicates.

Numeric Functions

Numeric functions allow you to compare values and perform rounding:

variable "num" {
  type        = set(number)
  default     = [250, 10, 11, 5]
  description = "A set of numbers"
}

> max(-1, 2, -10, 200, -250)
200

> min(-1, 2, -10, 200, -250)
-250

> max(var.num...)
250

> ceil(10.1)
11

> floor(10.9)
10

Function	Description	Example
max(...)	Returns the largest number	`max(1,5,3)` → `5`
min(...)	Returns the smallest number	`min(1,5,3)` → `1`
ceil(n)	Rounds up to the nearest integer	`ceil(10.1)` → `11`
floor(n)	Rounds down to the nearest integer	`floor(10.9)` → `10`

Note

Use the expansion operator ... to unpack a collection into individual arguments.

String Functions

Manipulate and transform text with string functions:

variable "ami" {
  type        = string
  default     = "ami-xyz,AMI-ABC,ami-efg"
  description = "A comma-separated list of AMI IDs"
}

> split(",", var.ami)
["ami-xyz", "AMI-ABC", "ami-efg"]

> lower(var.ami)
ami-xyz,ami-abc,ami-efg

> upper(var.ami)
AMI-XYZ,AMI-ABC,AMI-EFG

> title(var.ami)
Ami-Xyz,Ami-Abc,Ami-Efg

> substr(var.ami, 0, 7)
ami-xyz

> join(";", split(",", var.ami))
ami-xyz;AMI-ABC;ami-efg

Function	Description
split(sep, string)	Splits a string into a list of substrings
join(sep, list)	Joins a list into a single string
lower(string)	Converts all characters to lowercase
upper(string)	Converts all characters to uppercase
title(string)	Capitalizes the first letter of each word
substr(string, o, l)	Extracts a substring starting at offset `o`, length `l`

Collection Functions

List Operations

variable "ami_list" {
  type        = list(string)
  default     = ["ami-xyz", "AMI-ABC", "ami-efg"]
  description = "A list of AMI IDs"
}

> length(var.ami_list)
3

> index(var.ami_list, "AMI-ABC")
1

> element(var.ami_list, 2)
ami-efg

> contains(var.ami_list, "AMI-ABC")
true

Function	Description
length(list)	Returns the number of items
index(list, value)	Finds the position of `value`
element(list, index)	Retrieves the element at `index`
contains(list, value)	Checks if `value` exists (case-sensitive)

Map Operations

variable "ami_map" {
  type = map(string)
  default = {
    "us-east-1"    = "ami-xyz"
    "ca-central-1" = "ami-efg"
    "ap-south-1"   = "ami-ABC"
  }
  description = "Map of AMI IDs per region"
}

> keys(var.ami_map)
["ap-south-1", "ca-central-1", "us-east-1"]

> values(var.ami_map)
["ami-ABC", "ami-efg", "ami-xyz"]

> lookup(var.ami_map, "ca-central-1")
ami-efg

> lookup(var.ami_map, "us-west-2", "ami-pqr")
ami-pqr

Function	Description
keys(map)	Returns a list of all keys
values(map)	Returns a list of all values
lookup(map, key, default)	Retrieves `map[key]` or `default` if the key is absent

Warning

Calling lookup without a default value will error if the key does not exist.

Summary

By mastering these built-in functions, operators, and expressions, you can write more concise, powerful, and maintainable OpenTofu configurations. Experiment in the console, integrate them into your modules, and streamline your IaC development.

References

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