Input variable precedence order in Terraform - Step-by-Step Execution

Process Flow - Input variable precedence order

Default variable value

↓

Check terraform.tfvars file

↓

Check environment variables

↓

Check CLI -var options

↓

Final variable value used

Terraform picks variable values by checking defaults first, then tfvars files, then environment variables, and finally CLI options, with later sources overriding earlier ones.

Execution Sample

Terraform

variable "region" {
  default = "us-east-1"
}

# terraform.tfvars contains region = "eu-west-1"
# Assume environment variable TF_VAR_region="us-west-2"
# CLI: terraform apply -var='region=ap-south-1'

Shows how Terraform chooses the region variable value from multiple sources with different precedence.

Process Table

Step	Source Checked	Value Found	Overrides Previous?	Current Variable Value
1	Default value in variable block	us-east-1	N/A (initial)	us-east-1
2	terraform.tfvars file	eu-west-1	Yes	eu-west-1
3	Environment variable TF_VAR_region	us-west-2	Yes	us-west-2
4	CLI -var option	ap-south-1	Yes	ap-south-1

💡 All sources checked; CLI -var option has highest precedence and final value is 'ap-south-1'

Status Tracker

Variable	Start	After tfvars	After Env Var	After CLI
region	us-east-1	eu-west-1	us-west-2	ap-south-1

Key Moments - 3 Insights

Why does the CLI -var option override the terraform.tfvars file value?

Because the execution_table shows CLI -var is checked last and overrides all previous values, including tfvars (see step 4).

What happens if no environment variable or CLI option is set?

Terraform uses the terraform.tfvars file value if present; otherwise, it falls back to the default value (see steps 1 and 2).

Can environment variables override CLI options?

No, CLI options have higher precedence than environment variables, so CLI options override environment variables (see steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the variable 'region' value after checking the environment variable?

A us-east-1

B eu-west-1

C us-west-2

D ap-south-1

💡 Hint

Check the 'After Env Var' column in variable_tracker and step 3 in execution_table.

At which step does the variable value become 'eu-west-1'?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Source Checked' and 'Value Found' columns in execution_table.

If the CLI -var option was not provided, what would be the final variable value?

A us-west-2

B eu-west-1

C us-east-1

D ap-south-1

💡 Hint

Refer to the variable_tracker and note the value before 'After CLI' step.

Concept Snapshot

Terraform input variable precedence order:
1. Default value in variable block
2. terraform.tfvars file
3. Environment variables (TF_VAR_*)
4. CLI -var options
Later sources override earlier ones, CLI options have highest priority.

Full Transcript

Terraform variables can have values from multiple places. First, Terraform uses the default value defined in the variable block. Then it checks for values in terraform.tfvars files, which override the default. Next, it looks if an environment variable with the prefix TF_VAR_ is set, which overrides tfvars files. Finally, any CLI -var options provided during terraform apply or plan override all previous values. This order ensures the most specific input wins. For example, if the default region is 'us-east-1', tfvars file sets 'eu-west-1', environment variable sets 'us-west-2', and CLI option sets 'ap-south-1', the final region used is 'ap-south-1'.

Practice

1. In Terraform, which source has the highest priority when setting the value of an input variable?

easy

A. Terraform variable definition files (.tfvars)

B. Environment variables

C. Command-line flags (CLI flags)

D. Default values in the variable block

Solution

1. Step 1: Understand Terraform variable precedence

   Terraform uses a specific order to decide variable values: CLI flags first, then environment variables, then .tfvars files, and lastly default values.

2. Step 2: Identify the highest priority source

   Since CLI flags are checked first, they override all other sources.

3. Final Answer:

   Command-line flags (CLI flags) -> Option C

4. Quick Check:

   CLI flags > env vars > tfvars > defaults [OK]

Hint: Remember: CLI flags always override others [OK]

Common Mistakes:

• Thinking environment variables override CLI flags
• Assuming .tfvars files have highest priority
• Confusing default values as highest priority

2. Which of the following is the correct way to pass a variable value via CLI flag when running Terraform?

easy

A. terraform apply -var-file=region=us-west-1

B. terraform apply --set region=us-west-1

C. terraform apply -env region=us-west-1

D. terraform apply -var 'region=us-west-1'

Solution

1. Step 1: Recall CLI flag syntax for variables

   The correct syntax to pass a variable via CLI is using -var followed by 'key=value'.

2. Step 2: Match the correct option

   terraform apply -var 'region=us-west-1' uses -var 'region=us-west-1', which is the correct syntax.

3. Final Answer:

   terraform apply -var 'region=us-west-1' -> Option D

4. Quick Check:

   Use -var 'key=value' for CLI variable input [OK]

Hint: Use -var 'key=value' to pass variables via CLI [OK]

Common Mistakes:

• Using --set instead of -var
• Confusing environment variable syntax with CLI flags
• Using -var-file incorrectly for single variables

3. Given the following Terraform variable declaration and usage:
<pre>variable "env" { default = "dev" } # Command run: # terraform apply -var 'env=prod' # Environment variable: # TF_VAR_env=staging # tfvars file content: # env = "qa" What value will Terraform use for the variable env during this apply?

medium

A. "prod" (from CLI flag)

B. "dev" (default value)

C. "staging" (from environment variable)

D. "qa" (from tfvars file)

Solution

1. Step 1: List variable sources and their precedence

   Terraform checks CLI flags first, then environment variables, then tfvars files, then defaults.

2. Step 2: Identify the highest priority value provided

   CLI flag sets env=prod, which overrides environment variable (staging), tfvars (qa), and default (dev).

3. Final Answer:

   "prod" (from CLI flag) -> Option A

4. Quick Check:

   CLI flag value "prod" is used [OK]

Hint: CLI flags beat env vars and tfvars files [OK]

Common Mistakes:

• Choosing environment variable over CLI flag
• Picking tfvars value over environment variable
• Assuming default value is used when others exist

4. You run Terraform with the environment variable TF_VAR_region=us-east-1 set, but your configuration uses a region variable with a default value of us-west-2. You also have a terraform.tfvars file setting region = "eu-central-1". However, Terraform still uses us-west-2. What is the most likely cause?

medium

A. You passed a CLI flag overriding all other values

B. The environment variable name is incorrect; it should be TF_REGION

C. The variable is marked as sensitive and ignores external values

D. The terraform.tfvars file is not loaded automatically

Solution

1. Step 1: Check environment variable naming

   TF_VAR_region is correct naming for environment variable to set variable 'region'.

2. Step 2: Recall precedence order

   CLI flags > env vars > tfvars > defaults. Env var us-east-1 should override tfvars and default.

3. Step 3: Identify cause of default value

   To override the env var and use default us-west-2, a CLI flag like -var 'region=us-west-2' must have been passed.

4. Final Answer:

   You passed a CLI flag overriding all other values -> Option A

5. Quick Check:

   CLI flags > env vars > tfvars > defaults [OK]

Hint: CLI flags override env vars and tfvars [OK]

Common Mistakes:

• Assuming environment variable name is wrong
• Thinking sensitive variables block external values
• Ignoring possibility of CLI flag override

5. You want to ensure a Terraform variable instance_type always uses the value from a .tfvars file, ignoring any CLI flags or environment variables. Which approach correctly enforces this behavior?

hard

A. Remove default value and only set instance_type in the .tfvars file; avoid passing CLI flags or env vars

B. Terraform cannot enforce this; CLI flags always override .tfvars files

C. Use a local variable to override instance_type with the .tfvars value inside the configuration

D. Use a validation rule in the variable block to reject values not matching the .tfvars file

Solution

1. Step 1: Understand variable precedence limits

   Terraform's precedence is fixed: CLI flags > env vars > .tfvars > defaults. Cannot natively prioritize .tfvars over CLI/env.

2. Step 2: Confirm no enforcement mechanism

   There is no way to access or prioritize the .tfvars value separately from the resolved input variable.

3. Step 3: Why other options fail

   A relies on user discipline; B validation cannot reference .tfvars; C locals use the already-resolved var value from highest precedence.

4. Final Answer:

   Terraform cannot enforce this; CLI flags always override .tfvars files -> Option B

5. Quick Check:

   Precedence fixed, cannot override CLI/env with tfvars [OK]

Hint: Terraform precedence cannot be changed natively [OK]

Common Mistakes:

• Assuming Terraform can block CLI flag precedence natively
• Relying on validation rules to enforce external values
• Thinking removing defaults affects precedence order