autotfvars

📅 October 29, 2026📖 Verified Publication

```html Auto.tfvars in Terraform: The Complete Guide to Automatic Variable Loading

Auto.tfvars in Terraform: The Complete Guide to Automatic Variable Loading

Key Takeaways

Auto.tfvars files are loaded automatically — Terraform reads .auto.tfvars and *.auto.tfvars files without any command-line flags.
Sourcing order is critical — later files override earlier ones, and auto.tfvars values are overridden by explicit -var and -var-file flags.
They are not secret-safe by default — if your auto.tfvars contains sensitive values, ensure it is listed in .gitignore.
Environment-specific naming is a best practice — files like dev.auto.tfvars, staging.auto.tfvars, and prod.auto.tfvars keep configurations clean and explicit.
AUTO_TFVARS environment variable exists — you can customize which auto.tfvars file Terraform loads via the AUTO_TFVARS variable.

What Is Auto.tfvars in Terraform?

In Terraform, an .auto.tfvars file is a special variables file that Terraform loads automatically during terraform plan and terraform apply — no -var-file flag required. This behavior applies to two patterns:

.auto.tfvars — a single auto-loaded file in the working directory.
*.auto.tfvars — every file matching the pattern (alphabetical order applies).

These files contain the same HCL variable syntax as regular .tf files, but they exist solely to provide default values that override what is declared in your configuration without manual flag passing.

Why Do Auto.tfvars Files Exist?

Terraform was designed so that core infrastructure definitions live in version-controlled .tf files, while environment-specific overrides live separately. Auto.tfvars bridge that gap — they let you maintain a single codebase across dev, staging, and production without duplicating resources or using cumbersome inline variables.

How Auto.tfvars Works: The Sourcing Order

Understanding the order in which Terraform processes variable sources is essential. Values from later sources override earlier ones.

Complete Variable Sourcing Priority (Lowest to Highest)

Variables declared directly in .tf files (with default = values)
.auto.tfvars (single file, if present)
*.auto.tfvars (all matching files, sorted alphabetically)


      -var-file= command-line flags (processed in the order given)
      -var="key=value" command-line flags
      Environment variables (TF_VAR_*)
      terraform.tfstate values (for outputs and state-dependent resolution)



    Practical Example
    # main.tf
variable "instance_type" {
  default = "t2.micro"
}

variable "env" {
  default = "dev"
}

# dev.auto.tfvars
instance_type = "t2.small"
env           = "dev"

# staging.auto.tfvars
instance_type = "t2.medium"
env           = "staging"

# terraform apply -var-file=prod.auto.tfvars
# (prod.auto.tfvars sets env = "prod" and instance_type = "t3.large")
# Result: env = "prod", instance_type = "t3.large"



  

  
    Best Practices for Using Auto.tfvars

    1. Use Environment-Specific Names
    Naming convention <environment>.auto.tfvars is the industry standard. It makes intent obvious and prevents accidental cross-environment leakage.

    2. Always Add Auto.tfvars to .gitignore
    If your auto.tfvars contain sensitive data (API keys, passwords, internal IPs), never commit them. Add these patterns to your .gitignore:
    *.auto.tfvars
.auto.tfvars


    3. Pair with Terraform Cloud or Remote State
    For team environments, store auto.tfvars values in Terraform Cloud workspaces or use a secrets manager rather than shipping raw files across the organization.

    4. Keep a Template Auto.tfvars
    Commit a file like auto.tfvars.example that documents which variables each environment needs, without exposing actual values.

    5. Avoid the Single .auto.tfvars File at Scale
    A lone .auto.tfvars file becomes unmanageable as environments grow. Use the wildcard pattern with explicit environment names instead.

    Quick Checklist
    
      
        
          Practice
          Priority
          Impact
        
      
      
        
          Name files by environment
          High
          Prevents config confusion
        
        
          Add *.auto.tfvars to .gitignore
          High
          Prevents credential leaks
        
        
          Use terraform.tfvars.example templates
          Medium
          Onboards new team members faster
        
        
          Test with -var-file before relying on auto
          Medium
          Reduces plan surprises
        
        
          Audit AUTO_TFVARS env var usage
          Low
          Avoids unexpected file selection
        
      
    
  

  

  
    Auto.tfvars vs. Terraform.tfvars vs. Explicit -var-file

    



© 2026 Balis 情報マガジン. All rights reserved.

      
        
          Feature
          .auto.tfvars
          terraform.tfvars
          -var-file= (explicit)
        
      
      
        
          Auto-loaded by Terraform
          Yes
          Yes
          No — must specify flag
        
        
          Multiple files supported
          Yes (*.auto.tfvars)

Practice	Priority	Impact
Name files by environment	High	Prevents config confusion
Add *.auto.tfvars to .gitignore	High	Prevents credential leaks
Use terraform.tfvars.example templates	Medium	Onboards new team members faster
Test with -var-file before relying on auto	Medium	Reduces plan surprises
Audit AUTO_TFVARS env var usage	Low	Avoids unexpected file selection

Feature	`.auto.tfvars`	`terraform.tfvars`	`-var-file=` (explicit)
Auto-loaded by Terraform	Yes	Yes	No — must specify flag
Multiple files supported	Yes (`*.auto.tfvars`)