12 Built-in function

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📥 Terraform Built-in Functions & Data Sources (Daily Use)

🧰 Built-in Functions (Core Utilities)

1️⃣ file(path)

• Reads a local file → returns string ✔
• Used for: scripts, SSH keys, configs

resource "aws_instance" "cerberus" {
  ami = "ami-06178cf087598769c"
  instance_type = "m5.large"
 
  key_name = aws_key_pair.cerberus-key.key_name
  user_data = file("/root/terraform-projects/project-cerberus/install-nginx.sh")
}
 
resource "aws_key_pair" "cerberus-key" {
  key_name = "cerberus"
  public_key = file("/root/terraform-projects/project-cerberus/.ssh/cerberus.pub")
}

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2️⃣ templatefile(path, vars)

• Reads a file and renders variables → returns string ✔
• e.g. line in fileecho "Deployed by ${name} in ${region}"

user_data = templatefile("nginx.sh",{
    name   = "Om"
    region = "ap-south-1"
})

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3️⃣ jsondecode(string)

• Converts JSON string → map/object 💡

locals { config = jsondecode(file("config.json")) }

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4️⃣ yamldecode(string)

• Converts YAML string → map/object 💡

locals { values = yamldecode(file("values.yaml")) }

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5️⃣ toset(list)

• Converts list → set (for for_each) ✔

variable "users" {
  type = list(string)
  default = [
    "nginx",
    "docker",
    "aws"
  ]
}
 
resource "local_file" "setup" {
  for_each = toset(var.users)
  filename = "${each.value}.txt"
  content  = "Installing ${each.value}..."
}

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6️⃣ tomap(object)

• Converts object → map ✔

tags = tomap({ Name="app", Env="prod" })

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🗂 Data Sources (Read External Data, No Resource Creation)

8️⃣ data "http"

• Calls API/URL → fetches response

data "http" "example" { url = "..." }

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9️⃣ data "aws_ssm_parameter"

• Reads value from Parameter Store ✔🔐

data "aws_ssm_parameter" "db_pass" {
  name = "/prod/db/pass"
}
 
resource "local_file" "ssm_debug" {
  filename = "db-password-from-ssm.txt"
  content  = data.aws_ssm_parameter.db_pass.value
}

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1️⃣1️⃣ data "aws_secretsmanager_secret_version"

• Reads secret from Secrets Manager ✔🔐

data "aws_secretsmanager_secret_version" "app" {
  secret_id = "cerberus-app-secret"
}
 
resource "local_file" "secret_debug" {
  filename = "app-secret.json"
  content  = data.aws_secretsmanager_secret_version.app.secret_string
}

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