Templatefile Function: Separating Logic from Configuration

The templatefile function is powerful but can be confusing to use correctly.

The Problem

# Mixing logic and configuration
resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  
  user_data = <<-EOT
    #!/bin/bash
    echo "Hello, ${var.environment}!"
    %{for port in var.open_ports}
    firewall-cmd --permanent --add-port=${port}/tcp
    %{endfor}
    firewall-cmd --reload
  EOT
}

The Solution

# Using templatefile (templates/user_data.tftpl)
# #!/bin/bash
# echo "Hello, ${environment}!"
# %{for port in open_ports}
# firewall-cmd --permanent --add-port=${port}/tcp
# %{endfor}
# firewall-cmd --reload

# In your Terraform code:
resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  
  user_data = templatefile("${path.module}/templates/user_data.tftpl", {
    environment = var.environment
    open_ports  = var.open_ports
  })
}

Separating templates makes your configuration cleaner and more maintainable. Scalr provides visibility into these template renderings during the plan stage to help catch issues early.