Expressions

Expressions are used to compute values in Terraform. They can be simple literal values or complex combinations of references, operators, and function calls.

Types of Expressions

Literal Values

Direct value representations:

# Strings
name = "example"
multiline = <<EOT
Line 1
Line 2
EOT

# Numbers
count = 5
pi = 3.14159

# Booleans
enabled = true
disabled = false

# Null
optional_value = null

References

Reference values from other parts of your configuration:

# Resource attributes
ami = aws_ami.ubuntu.id
subnet_id = aws_subnet.public.id

# Variables
instance_type = var.instance_type
region = var.aws_region

# Local values
common_tags = local.default_tags
az_count = local.availability_zone_count

# Data sources
vpc_id = data.aws_vpc.main.id

# Module outputs
endpoint = module.database.connection_string

# Path references
config_file = file("${path.module}/config.json")
template = file("${path.root}/templates/init.sh")

Operators

Arithmetic Operators

locals {
  addition       = 5 + 3        # 8
  subtraction    = 10 - 4       # 6
  multiplication = 3 * 4        # 12
  division       = 20 / 4       # 5
  modulo         = 10 % 3       # 1
  negation       = -5           # -5
}

Comparison Operators

locals {
  equal              = 5 == 5          # true
  not_equal          = 5 != 3          # true
  less_than          = 3 < 5           # true
  less_or_equal      = 5 <= 5          # true
  greater_than       = 5 > 3           # true
  greater_or_equal   = 5 >= 5          # true
}

Logical Operators

locals {
  and_operation = true && false        # false
  or_operation  = true || false        # true
  not_operation = !false               # true
  
  # Combining operators
  complex = (var.environment == "prod") && (var.enabled == true)
}

Conditional Expressions

Ternary operator for conditional logic:

instance_type = var.environment == "production" ? "t2.large" : "t2.micro"

count = var.create_resource ? 1 : 0

tags = merge(
  var.common_tags,
  var.environment == "production" ? { Critical = "true" } : {}
)

Collection Operations

List and Tuple Access

# Index access
first_az = var.availability_zones[0]
second_az = var.availability_zones[1]

# Negative indices (from end)
last_az = var.availability_zones[-1]

# Length
az_count = length(var.availability_zones)

Map and Object Access

# Dot notation
region = var.config.region

# Bracket notation
region = var.config["region"]

# With variables
value = var.settings[var.key_name]

Splat Expressions

Extract attributes from lists:

# For resources with count
instance_ids = aws_instance.web[*].id
private_ips = aws_instance.web[*].private_ip

# For resources with for_each
instance_ids = values(aws_instance.web)[*].id

# Nested attribute access
vpc_ids = aws_subnet.private[*].vpc_id

For Expressions

Transform and filter collections:

List Transformation

# Transform elements
uppercase_names = [for name in var.names : upper(name)]

# With index
indexed_names = [for i, name in var.names : "${i}: ${name}"]

# Filtering
production_instances = [
  for instance in var.instances :
  instance.id if instance.environment == "production"
]

Map Transformation

# Transform map
port_map = {
  for key, value in var.ports :
  key => value + 1000
}

# Create map from list
instance_map = {
  for instance in aws_instance.web :
  instance.id => instance.private_ip
}

# Filtering maps
active_users = {
  for username, user in var.users :
  username => user if user.active
}

Grouping

# Group by attribute
instances_by_az = {
  for instance in aws_instance.web :
  instance.availability_zone => instance...
}

String Templates

Interpolation

Embed expressions in strings:

greeting = "Hello, ${var.name}!"

path = "/var/lib/${var.service_name}/data"

url = "https://${aws_instance.web.public_ip}:${var.port}"

String Directives

Control flow in strings:

# For loops
user_list = <<EOT
Users:
%{for user in var.users~}
  - ${user}
%{endfor~}
EOT

# Conditionals
config = <<EOT
%{if var.debug_mode~}
debug = true
log_level = verbose
%{else~}
debug = false
log_level = info
%{endif~}
EOT

Heredoc Strings

Multi-line strings with indentation control:

# Standard heredoc
script = <<EOT
#!/bin/bash
echo "Hello World"
EOT

# Indented heredoc (strips leading whitespace)
script = <<-EOT
  #!/bin/bash
  echo "Hello World"
EOT

Function Calls

Call built-in functions:

# String functions
uppercase = upper(var.name)
lowercase = lower(var.name)
joined = join(", ", var.list)

# Collection functions
list_length = length(var.items)
merged_map = merge(var.map1, var.map2)
flattened = flatten(var.nested_list)

# Numeric functions
absolute = abs(-5)
ceiling = ceil(3.7)
floor = floor(3.7)

# Date functions
current_time = timestamp()
future_time = timeadd(timestamp(), "24h")

# Encoding functions
base64_encoded = base64encode("hello")
json_encoded = jsonencode({name = "value"})

# File functions
file_content = file("${path.module}/config.json")
file_exists = fileexists("${path.module}/optional.txt")

Type Conversions

Explicit type conversions:

# Convert to string
string_value = tostring(123)

# Convert to number
number_value = tonumber("42")

# Convert to bool
bool_value = tobool("true")

# Convert to list
list_value = tolist(["a", "b"])

# Convert to set
set_value = toset(["a", "b", "a"])  # Removes duplicates

# Convert to map
map_value = tomap({key = "value"})

Dynamic Blocks

Generate nested blocks dynamically:

resource "aws_security_group" "example" {
  name = "example"
  
  dynamic "ingress" {
    for_each = var.ingress_rules
    content {
      from_port   = ingress.value.from_port
      to_port     = ingress.value.to_port
      protocol    = ingress.value.protocol
      cidr_blocks = ingress.value.cidr_blocks
    }
  }
}

With iterator:

dynamic "ingress" {
  for_each = var.ingress_rules
  iterator = rule
  content {
    from_port   = rule.value.from_port
    to_port     = rule.value.to_port
    protocol    = rule.value.protocol
  }
}

Special Values

Path References

path.module  # Path to the current module
path.root    # Path to the root module
path.cwd     # Current working directory

Terraform Values

terraform.workspace  # Current workspace name

Resource Meta-Arguments

count.index       # Current index in count
each.key         # Current key in for_each
each.value       # Current value in for_each
self.<ATTRIBUTE> # Current resource's attribute

Try and Can Functions

Handle errors gracefully:

# Try multiple expressions
value = try(
  var.value,
  local.default_value,
  "fallback"
)

# Check if expression succeeds
is_valid = can(regex("^[a-z]+$", var.name))

# Conditional based on validity
value = can(var.optional_value) ? var.optional_value : "default"

Type Constraints

Type specifications for validation:

variable "example" {
  type = object({
    name    = string
    age     = number
    active  = bool
    tags    = map(string)
    items   = list(string)
    config  = object({
      enabled = bool
      value   = string
    })
  })
}

Optional Attributes

variable "server" {
  type = object({
    name = string
    port = optional(number, 8080)  # Optional with default
    ssl  = optional(bool)           # Optional without default
  })
}

References and Dependency Detection

Terraform automatically detects dependencies:

resource "aws_instance" "web" {
  ami           = data.aws_ami.ubuntu.id      # Depends on data source
  subnet_id     = aws_subnet.public.id        # Depends on subnet
  instance_type = var.instance_type           # Depends on variable
  
  # Explicit reference creates dependency
  user_data = templatefile("init.sh", {
    db_endpoint = aws_db_instance.main.endpoint
  })
}

Expression Examples

Complex Conditionals

locals {
  instance_type = (
    var.environment == "production" ? "t2.large" :
    var.environment == "staging" ? "t2.medium" :
    "t2.micro"
  )
}

Nested For Expressions

locals {
  all_instance_ips = flatten([
    for az, instances in var.instances_by_az : [
      for instance in instances : instance.ip
    ]
  ])
}

Map Merging

locals {
  tags = merge(
    var.common_tags,
    var.environment_tags,
    {
      Name      = var.instance_name
      Terraform = "true"
    }
  )
}

Filtering and Transformation

locals {
  # Filter and transform
  active_user_emails = [
    for username, user in var.users :
    user.email if user.active && user.email != null
  ]
  
  # Group and count
  instances_per_az = {
    for az in distinct([for i in aws_instance.web : i.availability_zone]) :
    az => length([for i in aws_instance.web : i if i.availability_zone == az])
  }
}

Best Practices

Use Locals for Complexity

Move complex expressions to locals for readability and reusability.

Avoid Deep Nesting

Break down deeply nested expressions into multiple steps.

Add Comments

Document complex expressions to explain the logic.

Handle Errors

Use try and can to handle potential errors gracefully.

Common Patterns

Conditional Resource Creation

resource "aws_eip" "web" {
  count = var.create_public_ip ? 1 : 0
  
  instance = aws_instance.web.id
}

Dynamic Tagging

tags = merge(
  var.common_tags,
  {
    Name        = "${var.environment}-${var.service}"
    Environment = var.environment
    ManagedBy   = "Terraform"
  },
  var.additional_tags
)

Safe Attribute Access

# Using try for optional attributes
endpoint = try(aws_db_instance.main.endpoint, "")

# Using can to check existence
has_public_ip = can(aws_instance.web.public_ip)

Getting Started

Core Concepts

Configuration Language

Workflow

State Management

Providers

Advanced Features

Documentation Index

​Expressions

​Types of Expressions

​Literal Values

​References

​Operators

​Arithmetic Operators

​Comparison Operators

​Logical Operators

​Conditional Expressions

​Collection Operations

​List and Tuple Access

​Map and Object Access

​Splat Expressions

​For Expressions

​List Transformation

​Map Transformation

​Grouping

​String Templates

​Interpolation

​String Directives

​Heredoc Strings

​Function Calls

​Type Conversions

​Dynamic Blocks

​Special Values

​Path References

​Terraform Values

​Resource Meta-Arguments

​Try and Can Functions

​Type Constraints

​Optional Attributes

​References and Dependency Detection

​Expression Examples

​Complex Conditionals

​Nested For Expressions

​Map Merging

​Filtering and Transformation

​Best Practices