» Output Values

Hands-on: Try the Output Data From Terraform tutorial on HashiCorp Learn.

Output values are like the return values of a Terraform module, and have several uses:

  • A child module can use outputs to expose a subset of its resource attributes to a parent module.
  • A root module can use outputs to print certain values in the CLI output after running terraform apply.
  • When using remote state, root module outputs can be accessed by other configurations via a terraform_remote_state data source.

Resource instances managed by Terraform each export attributes whose values can be used elsewhere in configuration. Output values are a way to expose some of that information to the user of your module.

» Declaring an Output Value

Each output value exported by a module must be declared using an output block:

output "instance_ip_addr" {
  value = aws_instance.server.private_ip
}

The label immediately after the output keyword is the name, which must be a valid identifier. In a root module, this name is displayed to the user; in a child module, it can be used to access the output's value.

The value argument takes an expression whose result is to be returned to the user. In this example, the expression refers to the private_ip attribute exposed by an aws_instance resource defined elsewhere in this module (not shown). Any valid expression is allowed as an output value.

» Accessing Child Module Outputs

In a parent module, outputs of child modules are available in expressions as module.<MODULE NAME>.<OUTPUT NAME>. For example, if a child module named web_server declared an output named instance_ip_addr, you could access that value as module.web_server.instance_ip_addr.

» Optional Arguments

output blocks can optionally include description, sensitive, and depends_on arguments, which are described in the following sections.

» description — Output Value Documentation

Because the output values of a module are part of its user interface, you can briefly describe the purpose of each value using the optional description argument:

output "instance_ip_addr" {
  value       = aws_instance.server.private_ip
  description = "The private IP address of the main server instance."
}

The description should concisely explain the purpose of the output and what kind of value is expected. This description string might be included in documentation about the module, and so it should be written from the perspective of the user of the module rather than its maintainer. For commentary for module maintainers, use comments.

» sensitive — Suppressing Values in CLI Output

An output can be marked as containing sensitive material using the optional sensitive argument:

output "db_password" {
  value       = aws_db_instance.db.password
  description = "The password for logging in to the database."
  sensitive   = true
}

Setting an output value as sensitive prevents Terraform from showing its value in plan and apply. In the following scenario, our root module has an output declared as sensitive and a module call with a sensitive output, which we then use in a resource attribute.

# main.tf

module "foo" {
  source = "./mod"
}

resource "test_instance" "x" {
  some_attribute = module.mod.a # resource attribute references a sensitive output
}

output "out" {
  value     = "xyz"
  sensitive = true
}

# mod/main.tf, our module containing a sensitive output

output "a" {
  value     = "secret"
  sensitive = true
}

When we run a plan or apply, the sensitive value is redacted from output:

# CLI output

Terraform will perform the following actions:

  # test_instance.x will be created
  + resource "test_instance" "x" {
      + some_attribute    = (sensitive)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Changes to Outputs:
  + out = (sensitive value)

Sensitive output values are still recorded in the state, and so will be visible to anyone who is able to access the state data. For more information, see Sensitive Data in State.

» depends_on — Explicit Output Dependencies

Since output values are just a means for passing data out of a module, it is usually not necessary to worry about their relationships with other nodes in the dependency graph.

However, when a parent module accesses an output value exported by one of its child modules, the dependencies of that output value allow Terraform to correctly determine the dependencies between resources defined in different modules.

Just as with resource dependencies, Terraform analyzes the value expression for an output value and automatically determines a set of dependencies, but in less-common cases there are dependencies that cannot be recognized implicitly. In these rare cases, the depends_on argument can be used to create additional explicit dependencies:

output "instance_ip_addr" {
  value       = aws_instance.server.private_ip
  description = "The private IP address of the main server instance."

  depends_on = [
    # Security group rule must be created before this IP address could
    # actually be used, otherwise the services will be unreachable.
    aws_security_group_rule.local_access,
  ]
}

The depends_on argument should be used only as a last resort. When using it, always include a comment explaining why it is being used, to help future maintainers understand the purpose of the additional dependency.