Terraform Tutorial — terraform fmt and validate

3 minute read

Two commands that every Terraform user should run constantly: terraform fmt for consistent formatting and terraform validate for catching errors early. Together, they keep your code clean and correct before you ever touch real infrastructure.

terraform fmt — Auto-Format Your Code

terraform fmt rewrites your .tf files to the canonical HCL style. It handles:

  • Indentation: 2 spaces per level
  • Alignment: = signs aligned in argument blocks
  • Spacing: Consistent blank lines between blocks
  • Quotes: Normalized quoting style

Before Formatting

resource "aws_s3_bucket" "example" {
bucket = "my-bucket"
    tags={
  Name="example"
        Environment = "dev"
    }
}

After terraform fmt

resource "aws_s3_bucket" "example" {
  bucket = "my-bucket"
  tags = {
    Name        = "example"
    Environment = "dev"
  }
}

Running fmt

# Format all .tf files in current directory
$ terraform fmt

# Format recursively (all subdirectories)
$ terraform fmt -recursive

# Check if files are formatted (exit code 1 if not)
$ terraform fmt -check

# Show diff of what would change
$ terraform fmt -diff

Using fmt in CI/CD

The -check flag is perfect for CI pipelines — it fails the build if anyone committed unformatted code:

# GitHub Actions example
- name: Check Terraform formatting
  run: terraform fmt -check -recursive

If the check fails, the developer runs terraform fmt locally and pushes a fix.

terraform validate — Check for Errors

terraform validate checks your configuration for syntax errors, invalid references, and type mismatches. It does not make any API calls — it’s a pure local check.

$ terraform validate
Success! The configuration is valid.

What validate Catches

Error Type Example
Syntax errors Missing closing brace }
Invalid references aws_s3_bucket.nonexistent.arn (resource doesn’t exist)
Type mismatches Passing a string where a number is expected
Missing required arguments Resource is missing a required field
Invalid argument names Typo in an argument name

What validate Does NOT Catch

Not Caught Why
Invalid AWS resource configs Requires API call to validate
Permissions issues Requires API call
Duplicate resource names in AWS Only detected during apply
State drift Requires comparing with real infrastructure

Example: validate Catches a Typo

resource "aws_s3_bucket" "example" {
  buckettt = "my-bucket"    # Typo — extra 't'
}

$ terraform validate
╷
│ Error: Unsupported argument
│
│   on main.tf line 2, in resource "aws_s3_bucket" "example":
│    2:   buckettt = "my-bucket"
│
│ An argument named "buckettt" is not expected here. Did you mean "bucket"?
╵

💡 Tip: You must run terraform init before terraform validate. Validation needs the provider schemas (downloaded during init) to know which arguments are valid.

Difference Between fmt and validate

Aspect terraform fmt terraform validate
Purpose Code style Code correctness
Changes files Yes (rewrites in place) No (read-only check)
Requires init No Yes
Makes API calls No No
Catches Formatting issues Syntax errors, bad references

Run both, every time. Format first, then validate.

Pre-Commit Hooks

Automate both checks before every Git commit using pre-commit hooks:

$ cat .pre-commit-config.yaml
repos:
  - repo: https://github.com/antonbabenko/pre-commit-terraform
    rev: v1.86.0
    hooks:
      - id: terraform_fmt
      - id: terraform_validate

Install the pre-commit framework and the hooks:

$ pip install pre-commit
$ pre-commit install

Now every git commit automatically checks formatting and validation. If either fails, the commit is blocked until you fix the issue.

Common Formatting Rules

Terraform’s canonical format follows these rules:

Rule Example
2-space indentation All nested content uses 2 spaces
Aligned = signs Arguments in the same block align their =
One blank line between blocks Separates resource blocks for readability
No trailing whitespace Clean line endings
Closing brace on its own line } is never on the same line as content

What’s Next?

You’ve covered the entire HCL syntax chapter. Time to test your knowledge with the practice exercises.

← For Expressions Practice Exercises →

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