Terraform Tutorial — tfvars and Environment Variables

There are five ways to provide values for Terraform variables. Understanding the precedence order prevents surprises.

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Method 1: terraform.tfvars File

Create a file called terraform.tfvars in your project root:

# terraform.tfvars
region      = "us-east-1"
environment = "dev"
instance_type = "t3.micro"

Terraform loads terraform.tfvars automatically — you don’t need to pass any flags.

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Method 2: Named .tfvars Files

For environment-specific values, create separate files:

# dev.tfvars
environment   = "dev"
instance_type = "t3.micro"
instance_count = 1

# prod.tfvars
environment   = "prod"
instance_type = "t3.large"
instance_count = 5

Pass the file explicitly:

$ terraform plan -var-file="dev.tfvars"
$ terraform apply -var-file="prod.tfvars"

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Method 3: *.auto.tfvars Files

Any file named *.auto.tfvars or *.auto.tfvars.json is loaded automatically:

# common.auto.tfvars
project_name = "web-app"
team         = "backend"

This is useful for values that apply to all environments.

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Method 4: -var CLI Flag

Pass individual values on the command line:

$ terraform plan -var="environment=staging" -var="instance_type=t3.small"

Useful for overriding a single value without creating a file.

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Method 5: TF_VAR_ Environment Variables

Prefix any environment variable with TF_VAR_ and Terraform reads it:

$ export TF_VAR_region="eu-west-1"
$ export TF_VAR_environment="staging"
$ terraform plan

This is the standard method for CI/CD pipelines where secrets are injected as environment variables.

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Precedence Order (Lowest to Highest)

When the same variable is set in multiple places, the highest priority wins:

Priority	Source	Example
1 (lowest)	Default value in variable {}	default = "us-east-1"
2	terraform.tfvars	region = "eu-west-1"
3	*.auto.tfvars (alphabetical)	common.auto.tfvars
4	-var-file flag	-var-file="prod.tfvars"
5	-var flag	-var="region=ap-south-1"
6 (highest)	TF_VAR_ environment variable	TF_VAR_region=us-west-2

💡 Tip: In practice, use terraform.tfvars for common values, environment-specific .tfvars files with -var-file, and TF_VAR_ environment variables for secrets in CI/CD.

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Common Pattern: Environment-Specific Deploys

# Deploy to dev
$ terraform apply -var-file="environments/dev.tfvars"

# Deploy to staging
$ terraform apply -var-file="environments/staging.tfvars"

# Deploy to production
$ terraform apply -var-file="environments/prod.tfvars"

Each file sets the appropriate values for that environment.

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What’s Next?

You know how to provide variable values. Next, let’s add validation to ensure variables contain acceptable values.

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← Declaring Variables Variable Validation →