# !terraform.output

The `!terraform.output` YAML function allows reading the outputs ([remote state](/stacks/remote-state))
of components directly in Atmos stack manifests by internally executing a
[`terraform output`](https://developer.hashicorp.com/terraform/cli/commands/output) or
[`tofu output`](https://opentofu.org/docs/cli/commands/output/) command.

## Usage

The `!terraform.output` function can be called with either two or three parameters:

```yaml
  # Get the `output` of the `component` in the current stack
  !terraform.output <component> <output>

  # Get the `output` of the `component` in the provided `stack`
  !terraform.output <component> <stack> <output>

  # Get the output of the `component` by evaluating the YQ expression
  !terraform.output <component> <yq-expression>

 # Get the output of the `component` in the provided `stack` by evaluating the YQ expression
  !terraform.output <component> <stack> <yq-expression>
```

## Arguments

- **`component`**
  Atmos component name
- **`stack`**
  (Optional) Atmos stack name
- **`output` or `yq-expression`**
  Terraform output or 
  [YQ](https://mikefarah.gitbook.io/yq)
   expression to evaluate the output

:::tip
You can use [Atmos Stack Manifest Templating](/templates) in the `!terraform.output` YAML function expressions.
Atmos processes the templates first, and then executes the `!terraform.output` function, allowing you to provide the parameters to
the function dynamically.
:::

:::note Type-Aware Merging
Atmos supports type-aware merging of YAML functions and concrete values, allowing them to coexist in the inheritance chain without type conflicts.
See the full explanation: [YAML Function Merging](/reference/yaml-function-merging)
:::

## `!terraform.output` Function Execution Flow

When processing the `!terraform.output` YAML function for a component in a stack, Atmos executes the following steps:

- **Stack and Component Context Resolution**
  Atmos resolves the full context for the specified component within the given stack, including all inherited and merged
  configuration layers (globals, environment and component-level config).

- **Terraform/OpenTofu Variables**
  Based on the resolved context, Atmos generates a `varfile` with all the variables for the component in the stack.

- **Terraform/OpenTofu Backend**
  Atmos generates a backend configuration file for the component in the stack based on the resolved context.

- **Terraform/OpenTofu Provider Overrides**
  Atmos generates a provider override file for the component in the stack (if provider overrides are configured in the stack manifests).

- **Terraform/OpenTofu Workspace**
  Based on the resolved context, Atmos selects (or creates new) Terraform/OpenTofu workspace by executing
  `terraform/tofu workspace` commands.

- **Terraform/OpenTofu Init**
  Atmos executes a `terraform/tofu init` command to initialize the component and download all Terraform/OpenTofu modules and
  providers (if they are not present in the cache).

- **Terraform/OpenTofu Output**
  Atmos executes a `terraform/tofu output` command to read the outputs of the component in the stack.

- **Output Parsing and Interpolation**
  The relevant output variable is extracted from the state file (using a [YQ](https://mikefarah.gitbook.io/yq/) parser).
  Atmos parses and interpolates the value into the final configuration structure, replacing the `!terraform.output`
  directive in the YAML stack manifest with the final value.

:::tip
Since Atmos executes the `terraform/tofu init` and `terraform/tofu output` commands when processing the `!terraform.output`
YAML functions, it can significantly impact performance because it requires initializing the components,
which initializes all Terraform/OpenTofu modules and downloads all Terraform/OpenTofu providers (if they are not present in the cache).

To improve performance, consider using the [**`!store`**](/functions/yaml/store)
or [**`!terraform.state`**](/functions/yaml/terraform.state)
YAML function.

To understand the performance implications of the `!terraform.output` and `!terraform.state` functions,
compare the [!terraform.output Execution Flow](/functions/yaml/terraform.output#terraformoutput-function-execution-flow) with the
[!terraform.state Execution Flow](/functions/yaml/terraform.state#terraformstate-function-execution-flow).
:::

## Using YQ Expressions to retrieve items from complex output types

To retrieve items from complex output types such as maps and lists, or do any kind of filtering or querying,
you can utilize [YQ](https://mikefarah.gitbook.io/yq) expressions.

For example:

- Retrieve the first item from a list

```yaml
subnet_id1: !terraform.output vpc .private_subnet_ids[0]
```

- Read a key from a map

```yaml
username: !terraform.output config .config_map.username
```

For more details, review the following docs:

- [YQ Guide](https://mikefarah.gitbook.io/yq)
- [YQ Recipes](https://mikefarah.gitbook.io/yq/recipes)

## Handling YQ Expressions with Bracket Notation and Quotes

When you access map keys that contain special characters (such as hyphens) by using YQ bracket notation, you must wrap the key in double quotes like `["github-dependabot"]`. This often clashes with the surrounding quotes that protect the entire expression in YAML.

To avoid conflicting quotes, wrap the entire YQ expression in single quotes while keeping the double quotes inside the brackets:

```yaml
# Use single quotes around the expression to allow double quotes inside brackets
access_key_id: !terraform.output security '.users["github-dependabot"].access_key_id'
```

Additional examples:

```yaml
# Access map keys with special characters
api_key: !terraform.output config '.api_keys["service-account-1"].key'

# Access nested maps with special characters in multiple levels
endpoint: !terraform.output services '.endpoints["my-service"]["production"].url'

# Combine with stack templating
token: !terraform.output identity {{ .stack }} '.tokens["github-actions"].value'

# Escape single quotes by doubling them when needed inside the expression
app_name: !terraform.output config '.apps["app''s-name"].display_name'
```

**Quote escaping rules**

- Wrap the entire YQ expression in single quotes when it contains double quotes.
- Use double quotes inside brackets for string keys in YQ expressions.
- If you need single quotes inside the expression, escape them by doubling: `''`.

:::tip
This quoting pattern works for every Atmos YAML function that accepts YQ expressions, including [!terraform.state](/functions/yaml/terraform.state) and [!include](/functions/yaml/include).
:::

## Using YQ Expressions to provide a default value

If the component for which you are reading the output has not been provisioned yet, or if the specific output doesn't exist,
you can specify a [default value](https://mikefarah.gitbook.io/yq/operators/alternative-default-value)
in the YQ expression using the `//` operator. Atmos will evaluate the default when the data is unavailable.

This allows you to mock outputs when executing `atmos terraform plan` where there are dependencies between components,
and the dependent components are not provisioned yet.

:::tip Looking for reusable, component-owned mock fixtures?
A `//` default is a one-off fallback baked into a single expression. If you want a producer
component to declare its mock outputs once and have every consumer resolve them consistently,
use the dedicated [`mocks`](/stacks/components/mocks) stack-config section with the
`--use-mocks` flag instead — see [Component Mocks for Terraform YAML Lookups](/changelog/terraform-component-mocks).
A `//` default in the caller's expression still applies even when the referenced component
declares no `mocks` section at all; without one, an undeclared `mocks` section is a hard error.
:::

:::tip Default Value Behavior
Atmos distinguishes between **recoverable errors** (component not provisioned, output missing) and **non-recoverable errors** (API failures, network timeouts):

- **Recoverable errors with defaults**: When a component isn't provisioned or an output doesn't exist, AND you specify a YQ default (`//`), Atmos uses the default value
- **Recoverable errors without defaults**: Returns `null` for missing outputs (backward compatible) or an error for unprovisioned components
- **Non-recoverable errors**: API failures (S3 timeouts, authentication errors, network issues) always propagate as errors, even if a default is specified

This ensures that infrastructure failures are never silently masked by default values.
:::

:::note
`!terraform.output` parses the component, optional stack, and the remaining YQ expression directly. No parser-specific quote escaping is required.

YAML is parsed first: compact JSON such as `{"key":"value"}` can be a plain scalar; use a single-quoted YAML scalar when readable JSON contains `: `; and attach the tag to a folded scalar for multi-line expressions.

```yaml
test_map: !terraform.output >-
  component-2 .output // {"key1": "fallback1"}
```

:::

For example:

- Specify a string default value.
  Read the `username` output from the `config` component in the current stack.
  If the `config` component has not been provisioned yet, return the default value `default-user`.

```yaml
username: !terraform.output config .username // "default-user"
```

- Specify a list default value.
  Read the `private_subnet_ids` output from the `vpc` component in the current stack.
  If the `vpc` component has not been provisioned yet, return the default value `["mock-subnet1", "mock-subnet2"]`.

```yaml
subnet_ids: !terraform.output vpc .private_subnet_ids // ["mock-subnet1", "mock-subnet2"]
```

- Specify a map default value.
  Read the `config_map` output from the `config` component in the current stack.
  If the `config` component has not been provisioned yet, return the default value `{"api_endpoint": "localhost:3000", "user": "test"}`.

```yaml
config_map: !terraform.output 'config .config_map // {"api_endpoint": "localhost:3000", "user": "test"}'
```

For more details, review the following docs:

- [YQ Alternative (Default value)](https://mikefarah.gitbook.io/yq/operators/alternative-default-value)

## Using YQ Expressions to modify values returned from the remote state

Since the `output` parameter of the `!terraform.output` function is a [YQ](https://mikefarah.gitbook.io/yq) expression,
you can use [YQ pipes](https://mikefarah.gitbook.io/yq/operators/pipe) and
[YQ operators](https://mikefarah.gitbook.io/yq/operators) (including [YQ string concatenation functions](https://mikefarah.gitbook.io/yq/operators/add#string-concatenation)
and [YQ arithmetic functions](https://mikefarah.gitbook.io/yq/operators/add))
to modify the values returned from the remote state.

For example, suppose you have an `aurora-postgres` Atmos component which has the output `master_hostname`.

To read the output without modification, you can use the following expressions:

```yaml
postgres_url: !terraform.output aurora-postgres master_hostname
```

```yaml
postgres_url: !terraform.output aurora-postgres .master_hostname
```

To prepend and append strings to the output, you can use YQ pipes and the `add` function (`+` operator):

```yaml
postgres_url: !terraform.output 'aurora-postgres .master_hostname | "jdbc:postgresql://" + . + ":5432/events"'
```

The outer single quotes in the example are YAML quoting, not function-parser escaping. They preserve the readable YQ string literals unchanged.

After the `!terraform.output` function is executed, the `postgres_url` variable will have the final value similar to:

```yaml
postgres_url: "jdbc:postgresql://aurora-postgres-cluster-writer.prod.plat.mydomain.net:5432/events"
```

For more details, review the following docs:

- [YQ pipe](https://mikefarah.gitbook.io/yq/operators/pipe)
- [YQ operators](https://mikefarah.gitbook.io/yq/operators)
- [YQ string concatenation](https://mikefarah.gitbook.io/yq/operators/add#string-concatenation)
- [YQ `add` function](https://mikefarah.gitbook.io/yq/operators/add)

## Examples

**File:** `stack.yaml`

```yaml
components:
  terraform:
    my_lambda_component:
      vars:
        vpc_config:
          # Output of type string
          security_group_id: !terraform.output security-group/lambda id
          security_group_id2: !terraform.output security-group/lambda2 {{ .stack }} id
          security_group_id3: !terraform.output security-group/lambda3 {{ .atmos_stack }} id
          # Output of type list
          subnet_ids: !terraform.output vpc private_subnet_ids
          # Use a YQ expression to get an item from the list
          subnet_id1: !terraform.output vpc .private_subnet_ids[0]
          # Output of type map
          config_map: !terraform.output config {{ .stack }} config_map
          # Use a YQ expression to get a value from the map
          username: !terraform.output config .config_map.username
```

## Specifying Atmos `stack`

If you call the `!terraform.output` function with three parameters, you need to specify the stack as the second argument.

There are multiple ways you can specify the Atmos stack parameter in the `!terraform.output` function.

### Hardcoded Stack Name

Use it if you want to get an output from a component from a different (well-known and static) stack.
For example, you have a `tgw` component in a stack `plat-ue2-dev` that requires the `vpc_id` output from the `vpc` component from the stack `plat-ue2-prod`:

```yaml title="plat-ue2-dev"
  components:
    terraform:
      tgw:
        vars:
          vpc_id: !terraform.output vpc plat-ue2-prod vpc_id
```

### Reference the Current Stack Name

Use the `.stack` (or `.atmos_stack`) template identifier to specify the same stack as the current component is in
(for which the `!terraform.output` function is executed):

```yaml
  !terraform.output <component> {{ .stack }} <output>
  !terraform.output <component> {{ .atmos_stack }} <output>
```

For example, you have a `tgw` component that requires the `vpc_id` output from the `vpc` component in the same stack:

```yaml
  components:
    terraform:
      tgw:
        vars:
          vpc_id: !terraform.output vpc {{ .stack }} vpc_id
```

:::note
Using the `.stack` or `.atmos_stack` template identifiers to specify the stack is the same as calling the `!terraform.output`
function with two parameters without specifying the current stack, but without using `Go` templates.
If you need to get an output of a component in the current stack, using the `!terraform.output` function with two parameters
is preferred because it has a simpler syntax and executes faster.
:::

### Use a Format Function

Use the `printf` template function to construct stack names using static strings and dynamic identifiers.
This is convenient when you want to override some identifiers in the stack name:

```yaml
  !terraform.output <component> {{ printf "%s-%s-%s" .vars.tenant .vars.environment .vars.stage }} <output>

  !terraform.output <component> {{ printf "plat-%s-prod" .vars.environment }} <output>

  !terraform.output <component> {{ printf "%s-%s-%s" .settings.context.tenant .settings.context.region .settings.context.account }} <output>
```

- **`<component>`**
  Placeholder for an actual component name (e.g. 
  `vpc`
  )
- **`<output>`**
  Placeholder for an actual Terraform output (e.g. 
  `subnet_ids`
  )

For example, you have a `tgw` component deployed in the stack `plat-ue2-dev`. The `tgw` component requires the
`vpc_id` output from the `vpc` component from the same environment (`ue2`) and same stage (`dev`), but from a different
tenant `net` (instead of `plat`):

```yaml title="plat-ue2-dev"
  components:
    terraform:
      tgw:
        vars:
          vpc_id: !terraform.output vpc {{ printf "net-%s-%s" .vars.environment .vars.stage }} vpc_id
```

:::tip Important
By using the `printf "%s-%s-%s"` function, you are constructing stack names using the stack context variables/identifiers.

For more information on Atmos stack names and how to define them, refer to `stacks.name_pattern` and `stacks.name_template`
sections in [`atmos.yaml` CLI config file](/cli/configuration/)
:::

## Caching the result of `!terraform.output` function

Atmos caches (in memory) the results of `!terraform.output` function.

The cache is per Atmos CLI command execution, e.g., each new execution of a command like `atmos terraform plan`,
`atmos terraform apply` or `atmos describe component` will create and use a new memory cache, which involves re-invoking `terraform outputs` after reinitialization.

If you define the function in stack manifests for the same component in a stack more than once, the first call will
produce the result and cache it, and all the consecutive calls will just use the cached data. This is useful when you use the
`!terraform.output` function for the same component in a stack in multiple places in Atmos stack manifests.
It will speed up the function execution and stack processing.

For example:

```
components:
  terraform:
    test2:
      vars:
        tags:
          test: !terraform.output test id
          test2: !terraform.output test id
          test3: !terraform.output test {{ .stack }} id
```

In the example, the `test2` Atmos component uses the outputs (remote state) of the `test` Atmos component from the same stack.
The YAML function `!terraform.output` is executed three times (once for each tag).

After the first execution, Atmos caches the result in memory,
and reuses it in the next two calls to the function. The caching makes the stack processing much faster.
In a production environment where many components are used, the speedup can be significant.

## Using `!terraform.output` with `static` remote state backend

Atmos supports [brownfield configuration by using the remote state of type `static`](/components/terraform/brownfield/#hacking-remote-state-with-static-backends).

For example:

**File:** `stack.yaml`

```yaml
components:
  terraform:
    # Component `static-backend` is configured with the remote state backend of type `static`
    static-backend:
      remote_state_backend_type: static
      remote_state_backend:
        static:
          region: "us-west-2"
          cluster_name: "production-cluster"
          vpc_cidr: "10.0.0.0/16"
          database:
            type: "postgresql"
            version: "12.7"
            storage_gb: 100
          allowed_ips:
            - "192.168.1.0/24"
            - "10.1.0.0/16"
          tags:
            Environment: "production"
            Owner: "infra-team"

    eks-cluster:
      vars:
        region: !terraform.output static-backend region
        cluster_name: !terraform.output static-backend cluster_name
        vpc_cidr: !terraform.output static-backend vpc_cidr
        db_type: !terraform.output static-backend database.type
        db_storage: !terraform.output static-backend database.storage_gb
        allowed_ips: !terraform.output static-backend allowed_ips
        tags: !terraform.output static-backend tags
```

When the functions are executed, Atmos detects that the `static-backend` component has the `static` remote state configured,
and instead of executing `terraform output`, it just returns the static values from the `remote_state_backend.static` section.

Executing the command `atmos describe component eks-cluster -s <stack>` produces the following result:

```shell
vars:
  region: us-west-2
  cluster_name: production-cluster
  vpc_cidr: 10.0.0.0/16
  db_type: postgresql
  db_storage: 100
  allowed_ips:
    - 192.168.1.0/24
    - 10.1.0.0/16
  tags:
    Environment: production
    Owner: infra-team
```

## Cross-account reads via the target component's `env` section

Because `!terraform.output` invokes `terraform`/`tofu` as a subprocess, the target component's `env` section (e.g. `env.AWS_PROFILE`) is exported into that subprocess's environment. This lets a single Atmos invocation read state across multiple AWS accounts when each target stack declares its own profile in `env`. No additional configuration is required — this is the default behavior of the subprocess-based reader.

If you're using this pattern and want the **faster** [`!terraform.state`](/functions/yaml/terraform.state) function to behave the same way, see [Switching AWS credentials per component via the `env` section](/functions/yaml/terraform.state#switching-aws-credentials-per-component-via-the-env-section) on the `!terraform.state` page.

## Considerations

- Using `!terraform.output` with secrets can expose sensitive data to standard output (stdout) in any commands that describe stacks or components.

- When using `!terraform.output` with [`atmos describe affected`](/cli/commands/describe/affected), Atmos requires access to all referenced remote states.
  If you operate with limited permissions (e.g., scoped to `dev`) and reference production stacks, the command will fail.

- Overusing the function within stacks to reference multiple components can significantly impact performance since Atmos internally executes a
  [`terraform output`](https://developer.hashicorp.com/terraform/cli/commands/output) or
  [`tofu output`](https://opentofu.org/docs/cli/commands/output/) command, which requires initializing the component,
  which initializes all Terraform/OpenTofu modules and downloads all Terraform/OpenTofu providers.

- Be mindful of disaster recovery (DR) implications when using it across regions.

- Consider cold-start scenarios: if the dependent component has not yet been provisioned, the function will return an error unless you provide a YQ default value using the `//` operator. See [Using YQ Expressions to provide a default value](#using-yq-expressions-to-provide-a-default-value) for details on error handling behavior.