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Variables, Locals, and Outputs

Overview

Design clean module inputs and outputs: typed variables, validation, tfvars and TF_VAR_, locals for derived values, and sensitive outputs that do not leak in logs.

Variables are the knobs at the module boundary. Locals hold computed values inside the module. Outputs export results to the CLI, parent modules, or remote state consumers. Prefer explicit types, descriptions, and validation over undocumented defaults.

This is a core tutorial in Module 7 · Variables & Outputs of the REBASH Academy Terraform for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

Learning Objectives

By the end of this tutorial, you will be able to:

  • Declare typed variables with validation blocks
  • Load values via .tfvars and TF_VAR_
  • Use locals for derived names without exposing them as inputs
  • Mark sensitive outputs and confirm CLI redaction

Architecture

This topic’s control points and relationships are shown below.

Terraform variables flow

Theory

What it is

Input variables are referenced as var.name. Declare type, description, optional default, and validation so bad values fail before any cloud API call. Values arrive from -var / -var-file, *.auto.tfvars, terraform.tfvars, TF_VAR_<name> environment variables, then defaults — pick one clear source per pipeline.

Locals hold derived expressions (local.name_prefix). Outputs export values for humans and composition. Set sensitive = true on variables or outputs to redact CLI display — state may still store the value, so protect backends separately.

Why it matters

Typed inputs are the contract between platform modules and application teams. Validation fails fast in CI. Sensitive flags reduce secret printing in pipeline logs. Locals stop copy-pasted string templates from drifting across resources.

How it works

  1. Declare variable blocks with types and validation.
  2. Supply values with -var-file=dev.tfvars or TF_VAR_environment=stage.
  3. Compute shared names and tags in locals.
  4. Wire var.* / local.* into resources; export output values.
  5. Read results with terraform output (use -json carefully — it can reveal sensitive values).

Prefer explicit -var-file in CI over ambient terraform.tfvars so the wrong environment cannot auto-load.

Key concepts and comparisons

Construct Scope Typical use
variable Module input Env name, CIDR, size
local Internal Prefixed names, tags
output Module export IDs, endpoints
Source Example
Default default = "dev"
tfvars environment = "stage"
CLI -var='environment=prod'
Environment TF_VAR_environment=prod

Common pitfalls

  • Using any everywhere — you lose validation and clarity.
  • Committing tfvars that contain credentials.
  • Expecting sensitive = true to encrypt state — it only redacts display.
  • Exposing every local as a variable — keep the interface small.

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-terraform/module-07 && cd ~/rebash-terraform/module-07

Focus: Wire input variables, locals, and outputs cleanly

Step 1 – Define variables and outputs

cat > variables.tf <<'EOF'
variable "app_name" {
  type    = string
  default = "demo"
}
variable "tags" {
  type    = map(string)
  default = { team = "platform" }
}
EOF
cat > main.tf <<'EOF'
terraform {
  required_providers {
    local = { source = "hashicorp/local", version = "~> 2.5" }
  }
}
locals {
  label = "${var.app_name}-lab"
}
resource "local_file" "config" {
  filename = "${path.module}/config.json"
  content  = jsonencode({ name = local.label, tags = var.tags })
}
output "config_path" {
  value = local_file.config.filename
}
output "label" {
  value = local.label
}
EOF
cat > terraform.tfvars <<'EOF'
app_name = "payments"
tags = { team = "platform", env = "lab" }
EOF
terraform init

Step 2 – Apply and read outputs

terraform apply -auto-approve
terraform output
cat config.json

Final step – Cleanup note

terraform destroy -auto-approve
# Workspace kept for notes; remove with: rm -rf "$(pwd)" when finished

Validation

  • Lab commands run under ~/rebash-terraform/module-07/
  • You can explain each Theory section in your own words
  • You used modern tooling where it applies to this topic
  • You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Variables, Locals, and Outputs always combines:

  1. Inspect before you change (status, plan, logs, dry-run)
  2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
  3. Capture evidence (command output, pipeline logs) for handovers
  4. Prefer current tools and APIs over legacy shortcuts
  5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

  • Treat credentials and tokens for terraform as privileged — never commit them
  • Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
  • Validate blast radius before apply/deploy/delete operations
  • Restrict who can approve production changes
  • Collect audit logs; limit who can read sensitive traces

Common Mistakes

Using any everywhere — you lose validation and clarity.

Validate assumptions against the Theory section and official docs before changing production.

Committing tfvars that contain credentials.

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

  • Encode Variables, Locals, and Outputs changes as code and review them in pull requests
  • Pin versions (images, modules, actions, provider plugins)
  • Separate environments with clear promotion gates
  • Alert on symptoms with runbooks attached
  • Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom Likely cause Fix
Auth / permission denied Wrong identity, policy, or scope Check caller identity, roles, and least-privilege policies
Timeout / no route Network, DNS, security group, or endpoint Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan Manual change or wrong state/workspace Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red Flaky step, cache, or missing secret Read failing step logs; bisect recent workflow/config changes
Cost spike Idle load balancer, NAT, oversized compute Inventory billable resources; stop/delete labs promptly

Summary

Variables, Locals, and Outputs is essential for Cloud and DevOps engineers working with terraform. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions

  1. How do input variables differ from locals?
  2. When should an output be marked sensitive?
  3. What precedence do tfvars and environment variables follow at a high level?
  4. Why can putting secrets in terraform.tfvars committed to Git be dangerous?
  5. How do typed variables improve module interfaces?

Sample answer — question 2

Outputs marked sensitive are redacted in CLI output but may still exist in state. Use them for credentials you must expose to callers carefully, and protect state storage.

Sample answer — question 4

Committed tfvars often leak passwords and keys through Git history. Prefer secret managers, CI-injected TF_VAR_ values, and never commit real secrets.

References