Chapter 33: Terraform Providers Resources Variables Outputs Locals and Data Sources

33.5 Locals — name a value once, use it everywhere

By Ravindra Bagale · Last updated: 25 October 2026

Story. A Netflix-style team tags every resource with the service, environment and cost centre, and someone always misspells one of them. Locals let you compute a value once and reuse it. Unlike variables, locals are not inputs; they are named expressions inside the module. Use them for naming patterns and shared tag maps.

variable "env" {
  type = string
}

locals {
  name_prefix = "badge-${var.env}"
  common_tags = {
    Service     = "badge-api"
    Environment = var.env
  }
}

resource "aws_s3_bucket" "assets" {
  bucket = "${local.name_prefix}-assets-<your-account-id>"
  tags   = merge(local.common_tags, { Purpose = "static-assets" })
}

What each line does

• variable "env" { — An input such as dev or prod.
• locals { — Opens a block of named values for this module.
• name_prefix = "badge-${var.env}" — Builds a prefix like badge-dev from the input.
• common_tags = {...} — A map of tags shared by every resource.
• Environment = var.env — Puts the environment name into the tags.
• bucket = "${local.name_prefix}-assets-..." — Uses the prefix so the bucket name follows the pattern.
• tags = merge(local.common_tags, {...}) — Combines the shared tags with one tag specific to this bucket.

When it is green.

  + bucket = "badge-dev-assets-..."
  + tags   = { "Environment" = "dev", "Purpose" = "static-assets", "Service" = "badge-api" }
Plan: 1 to add, 0 to change, 0 to destroy.

When it is red.

Error: Reference to undeclared local value
  on main.tf line 14: local.nameprefix
A local value with the name "nameprefix" has not been declared. Did you mean "name_prefix"?

Fix. A typo in the local name. Note the block is locals (plural) but the reference is local.name (singular).

What: every term in plain English

• locals block — locals { name = expression }. Names a value inside the code.
• local.NAME — How you read a local.
• merge() — Joins maps. Later maps win on the same key.
• format() — Builds a string from a pattern.
• lower() — Makes text lower case, useful for names.
• terraform console — A prompt where you can try locals and functions.

Example 1: One name prefix for everything

A Netflix-style team names everything SERVICE-ENV-THING. One local builds the prefix once.

variable "service" {
  type    = string
  default = "Checkout"
}
variable "env" {
  type    = string
  default = "dev"
}

locals {
  prefix = lower("${var.service}-${var.env}")
}

resource "aws_s3_bucket" "assets" {
  bucket = "${local.prefix}-assets-1234"
}

resource "aws_security_group" "web" {
  name = "${local.prefix}-web-sg"
}

terraform plan | grep -E 'bucket |name '

Output

+ bucket = "checkout-dev-assets-1234"
+ name   = "checkout-dev-web-sg"

What it shows: lower() fixes a capital letter that S3 would reject.

Example 2: Common tags with merge

Finance at a Myntra-style company needs four tags everywhere. One misspelling broke a cost report. Locals fix it.

locals {
  common_tags = {
    Service    = var.service
    Env        = var.env
    CostCentre = "CC-104"
    ManagedBy  = "terraform"
  }
}

resource "aws_instance" "web" {
  ami           = data.aws_ami.al2023.id
  instance_type = "t3.micro"
  tags = merge(local.common_tags, {
    Name = "${local.prefix}-web"
  })
}

echo 'merge({a="1",b="2"},{b="9"})' | terraform console
terraform plan | grep -A6 "tags "

Output

{
  "a" = "1"
  "b" = "9"
}
      + tags = {
          + "CostCentre" = "CC-104"
          + "Env"        = "dev"
          + "ManagedBy"  = "terraform"
          + "Name"       = "checkout-dev-web"
          + "Service"    = "Checkout"
        }

What it shows: The later map wins for the same key, so a resource can override one tag.

Example 3: Size by environment with a map lookup

An Ola-style team used if-else everywhere for sizes. One map in locals is easier to read.

locals {
  sizes = {
    dev  = "t3.micro"
    test = "t3.small"
    prod = "t3.medium"
  }
  instance_type = lookup(local.sizes, var.env, "t3.micro")
  is_prod       = var.env == "prod"
}

resource "aws_instance" "web" {
  ami           = data.aws_ami.al2023.id
  instance_type = local.instance_type
  monitoring    = local.is_prod
}

terraform plan -var env=prod | grep -E "instance_type|monitoring"
terraform plan -var env=dev  | grep -E "instance_type|monitoring"

Output

+ instance_type = "t3.medium"
+ monitoring    = true
+ instance_type = "t3.micro"
+ monitoring    = false

What it shows: Detailed monitoring costs money, so it is on only in prod.

Example 4: Build a list of names with a for expression

A Flipkart-style team makes one queue per service. A local with a for expression builds all the names in one place.

variable "services" {
  type    = list(string)
  default = ["Orders", "Payments", "Search"]
}

locals {
  queue_names = [for s in var.services : "${local.prefix}-${lower(s)}-queue"]
  queue_map   = { for s in var.services : lower(s) => "${local.prefix}-${lower(s)}-queue" }
}

resource "aws_sqs_queue" "svc" {
  for_each = local.queue_map
  name     = each.value
  tags     = local.common_tags
}

output "queues" {
  value = local.queue_names
}

terraform apply -auto-approve
terraform output queues
terraform state list | grep sqs

Output

Apply complete! Resources: 3 added, 0 changed, 0 destroyed.
[
  "checkout-dev-orders-queue",
  "checkout-dev-payments-queue",
  "checkout-dev-search-queue",
]
aws_sqs_queue.svc["orders"]
aws_sqs_queue.svc["payments"]
aws_sqs_queue.svc["search"]

What it shows: Adding a service means one new word in the list. The map keys keep each queue stable when the list order changes.

Example 5: Try locals in terraform console

Before you apply a long expression, test it. A TCS reviewer asks for this on every pull request with functions.

terraform console -var env=prod
> local.prefix
> local.instance_type
> format("%s-%02d", local.prefix, 7)
> exit

Output

"checkout-prod"
"t3.medium"
"checkout-prod-07"

What it shows: console reads your code and state but changes nothing.

Command cheat sheet

Verify: did it work?

1. Local value

   echo local.prefix | terraform console
   

   Output

   "checkout-dev"
   

2. Tags applied

   terraform plan | grep CostCentre
   

   Output

   + "CostCentre" = "CC-104"
   

3. No duplicate strings

   grep -c "CC-104" *.tf
   

   Output

   1
   

When it is red 1

Error: Reference to undeclared local value
  on main.tf line 9: name = "${local.prefx}-web-sg"

Cause. Typo in a local name.

Fix, step by step

1. Fix the spelling.

When it is red 2

Error: Variables not allowed
  on variables.tf line 3: default = "${var.service}-dev"

Cause. A variable default cannot use another variable.

Fix, step by step

1. Move the expression into locals.

When it is red 3

Error: Cycle: local.a, local.b

Cause. Two locals use each other.

Fix, step by step

1. Make one of them not depend on the other.

Cleanup

terraform destroy -auto-approve

Practice tasks

Homework: try at home

Recap

• Locals name a value once.
• Read them as local.NAME.
• merge joins maps for tags.
• lookup picks by environment.
• Test expressions in terraform console.

FAQ

Local vs variable?

Variables are inputs from outside. Locals are computed inside.

Can a local use a variable?

Yes. That is the main use.

Can I have many locals blocks?

Yes. All are merged.

Can a caller set a local?

No.

Do locals cost anything?

No. They are only values.

Can locals use data sources and resources?

Yes. For example local.account = data.aws_caller_identity.me.account_id. The value is known when the data is read.

Should every value be a local?

No. Too many locals make code hard to follow. Use one only when a value repeats or the expression is long.

Interview questions

1. What is a local value?

   A named expression inside a module, read as local.NAME.

2. When use a local instead of a variable?

   When the value is computed and should not be set from outside.

3. How do you apply common tags?

   A common_tags local merged into each resource, or provider default_tags.

4. How do you test an expression?

   terraform console.

5. What does merge do on duplicate keys?

   The last map wins.

6. How do you build many names from one list?

   A for expression in a local, such as [for s in var.services : lower(s)], then a map for for_each so each item keeps a stable key.

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