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Terraform Configuration Syntax Terraform configurations are written in HCL (HashiCorp Configuration Language), which is designed to be both human-readable and machine-parseable. This page covers the detailed syntax rules.
File Extensions Terraform supports two file formats:
.tf.tf.json
This documentation focuses on the native HCL syntax, which is the standard format for human-authored configurations.
Basic Syntax Blocks Blocks are the primary structural element. A block has a type, optional labels, and a body:
< BLOCK_TYPE > "<LABEL>" "<LABEL>" { # Block body < ARGUMENT > = < EXPRESSION > < NESTED_BLOCK > { # Nested block body } }
Example resource "aws_instance" "web" { ami = "ami-12345678" instance_type = "t2.micro" network_interface { device_index = 0 description = "Main network interface" } }
In this example:
resource is the block type"aws_instance" and "web" are labelsami and instance_type are argumentsnetwork_interface is a nested block
Arguments Arguments assign values within blocks:
< IDENTIFIER > = < EXPRESSION >
The identifier must be a valid variable name (letters, digits, underscores, and hyphens).
Identifiers Valid identifiers:
Start with a letter or underscore
Contain letters, digits, underscores, and hyphens
Are case-sensitive
variable "instance_type" { } # Valid variable "instance-type" { } # Valid variable "_private" { } # Valid variable "2nd_instance" { } # Invalid - starts with digit
While hyphens are allowed in identifiers, underscores are preferred for consistency with most programming languages.
Expressions Expressions produce values and can be:
Literal Values # String name = "example" # Number count = 3 pi = 3.14159 # Boolean enabled = true disabled = false # Null default_value = null
Collections # List/tuple az_list = [ "us-west-2a" , "us-west-2b" , "us-west-2c" ] # Map/object tags = { Name = "example" Environment = "production" }
References Reference other values in the configuration:
# Resource attribute ami = aws_ami . ubuntu . id # Variable instance_type = var . instance_type # Local value subnet_id = local . primary_subnet # Data source vpc_id = data . aws_vpc . main . id
Operators Terraform supports standard operators:
# Arithmetic result = 2 + 3 # 5 result = 10 - 4 # 6 result = 3 * 4 # 12 result = 10 / 2 # 5 result = 10 % 3 # 1 # Comparison equal = 5 == 5 # true not_equal = 5 != 3 # true greater = 5 > 3 # true greater_or_equal = 5 >= 5 # true # Logical and_op = true && false # false or_op = true || false # true not_op = ! false # true
Conditional Expressions instance_type = var . environment == "production" ? "t2.large" : "t2.micro"
For Expressions Transform collections:
# List transformation uppercase_names = [ for name in var . names : upper (name)] # Map transformation port_map = { for k , v in var . ports : k => v + 1000 } # Filtering production_instances = [ for i in var . instances : i if i . environment == "production" ]
Splat Expressions Shorthand for extracting attributes:
# Extract all IDs instance_ids = aws_instance . web [ * ] . id # Works with resources created with count private_ips = aws_instance . web [ * ] . private_ip
String Templates Interpolation Embed expressions in strings:
greeting = "Hello, ${ var . name } !" path = "/var/lib/ ${ var . service } /data"
Directives Control whitespace and flow:
# For loop user_names = << EOT %{ for name in var . users ~ } - ${ name } %{ endfor ~ } EOT # Conditional config = << EOT %{ if var . debug_mode ~ } debug = true %{ else ~ } debug = false %{ endif ~ } EOT
Heredoc Syntax Multi-line strings:
user_data = <<- EOT #!/bin/bash echo "Hello World" apt-get update apt-get install -y nginx EOT
Three comment styles:
# Single-line comment (preferred) // Alternative single-line comment /* Multi-line comment Useful for larger blocks */
Special Characters Escaping Escape special characters in strings:
message = "Quote: \" Hello \" and newline: \n " path = "C: \\ Users \\ Admin" template = "$${not_interpolated}"
Unicode Unicode characters are supported:
variable "π" { default = 3.14159265359 } output "emoji" { value = "🚀" }
Style Conventions
Use 2 spaces for indentation (not tabs): resource "aws_instance" "example" { ami = "ami-12345678" network_interface { device_index = 0 } }
Align values for better readability: resource "aws_instance" "web" { ami = "ami-12345678" instance_type = "t2.micro" subnet_id = var . subnet_id }
Use blank lines to separate logical sections: variable "region" { type = string } variable "instance_type" { type = string }
Terraform includes a formatter:
# Format current directory terraform fmt # Format specific file terraform fmt main.tf # Check if files are formatted terraform fmt -check
JSON Syntax For machine-generated configurations:
{ "resource" : { "aws_instance" : { "web" : { "ami" : "ami-12345678" , "instance_type" : "t2.micro" } } } }
The JSON syntax is functionally equivalent to HCL but is less human-friendly. Use it only when generating configurations programmatically.
Validation The parser validates syntax according to the HCL specification. Common errors include:
Missing closing braces
Invalid identifier names
Incorrect expression syntax
Mismatched quotes
Use terraform validate to check for syntax errors:
Next Steps
Expressions Learn about expression syntax in detail
Functions Explore built-in functions