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---
layout: "language"
page_title: "Input Variables - Configuration Language"
sidebar_current: "docs-config-variables"
description: "Input variables allow you to customize modules without altering their source code. Learn how to declare, define, and reference variables in configurations."
---
# Input Variables
> **Hands-on:** Try the [Customize Terraform Configuration with Variables](https://learn.hashicorp.com/tutorials/terraform/variables?in=terraform/configuration-language&utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) tutorial on HashiCorp Learn.
Input variables let you customize aspects of Terraform modules without altering
the module's own source code. This allows you to share modules across different
Terraform configurations, making your module composable and reusable.
When you declare variables in the root module of your configuration, you can
set their values using CLI options and environment variables.
When you declare them in [child modules](/docs/language/modules/index.html),
the calling module should pass values in the `module` block.
If you're familiar with traditional programming languages, it can be useful to
compare Terraform modules to function definitions:
- Input variables are like function arguments.
- [Output values](./outputs.html) are like function return values.
- [Local values](./locals.html) are like a function's temporary local variables.
-> **Note:** For brevity, input variables are often referred to as just
"variables" or "Terraform variables" when it is clear from context what sort of
variable is being discussed. Other kinds of variables in Terraform include
_environment variables_ (set by the shell where Terraform runs) and _expression
variables_ (used to indirectly represent a value in an
[expression](/docs/language/expressions/index.html)).
## Declaring an Input Variable
Each input variable accepted by a module must be declared using a `variable`
block:
```hcl
variable "image_id" {
type = string
}
variable "availability_zone_names" {
type = list(string)
default = ["us-west-1a"]
}
variable "docker_ports" {
type = list(object({
internal = number
external = number
protocol = string
}))
default = [
{
internal = 8300
external = 8300
protocol = "tcp"
}
]
}
```
The label after the `variable` keyword is a name for the variable, which must
be unique among all variables in the same module. This name is used to
assign a value to the variable from outside and to reference the variable's
value from within the module.
The name of a variable can be any valid [identifier](/docs/language/syntax/configuration.html#identifiers)
_except_ the following: `source`, `version`, `providers`, `count`, `for_each`, `lifecycle`, `depends_on`, `locals`.
These names are reserved for meta-arguments in
[module configuration blocks](/docs/language/modules/syntax.html), and cannot be
declared as variable names.
## Arguments
Terraform CLI defines the following optional arguments for variable declarations:
- [`default`][inpage-default] - A default value which then makes the variable optional.
- [`type`][inpage-type] - This argument specifies what value types are accepted for the variable.
- [`description`][inpage-description] - This specifies the input variable's documentation.
- [`validation`][inpage-validation] - A block to define validation rules, usually in addition to type constraints.
- [`sensitive`][inpage-sensitive] - Limits Terraform UI output when the variable is used in configuration.
- [`nullable`][inpage-nullable] - Specify if the variable can be `null` within the module.
### Default values
[inpage-default]: #default-values
The variable declaration can also include a `default` argument. If present,
the variable is considered to be _optional_ and the default value will be used
if no value is set when calling the module or running Terraform. The `default`
argument requires a literal value and cannot reference other objects in the
configuration.
### Type Constraints
[inpage-type]: #type-constraints
The `type` argument in a `variable` block allows you to restrict the
[type of value](/docs/language/expressions/types.html) that will be accepted as
the value for a variable. If no type constraint is set then a value of any type
is accepted.
While type constraints are optional, we recommend specifying them; they
can serve as helpful reminders for users of the module, and they
allow Terraform to return a helpful error message if the wrong type is used.
Type constraints are created from a mixture of type keywords and type
constructors. The supported type keywords are:
* `string`
* `number`
* `bool`
The type constructors allow you to specify complex types such as
collections:
* `list(<TYPE>)`
* `set(<TYPE>)`
* `map(<TYPE>)`
* `object({<ATTR NAME> = <TYPE>, ... })`
* `tuple([<TYPE>, ...])`
The keyword `any` may be used to indicate that any type is acceptable. For
more information on the meaning and behavior of these different types, as well
as detailed information about automatic conversion of complex types, see
[Type Constraints](/docs/language/expressions/types.html).
If both the `type` and `default` arguments are specified, the given default
value must be convertible to the specified type.
### Input Variable Documentation
[inpage-description]: #input-variable-documentation
Because the input variables of a module are part of its user interface, you can
briefly describe the purpose of each variable using the optional
`description` argument:
```hcl
variable "image_id" {
type = string
description = "The id of the machine image (AMI) to use for the server."
}
```
The description should concisely explain the purpose
of the variable 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.
### Custom Validation Rules
[inpage-validation]: #custom-validation-rules
-> This feature was introduced in Terraform CLI v0.13.0.
In addition to Type Constraints as described above, a module author can specify
arbitrary custom validation rules for a particular variable using a `validation`
block nested within the corresponding `variable` block:
```hcl
variable "image_id" {
type = string
description = "The id of the machine image (AMI) to use for the server."
validation {
condition = length(var.image_id) > 4 && substr(var.image_id, 0, 4) == "ami-"
error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."
}
}
```
The `condition` argument is an expression that must use the value of the
variable to return `true` if the value is valid, or `false` if it is invalid.
The expression can refer only to the variable that the condition applies to,
and _must not_ produce errors.
If the failure of an expression is the basis of the validation decision, use
[the `can` function](/docs/language/functions/can.html) to detect such errors. For example:
```hcl
variable "image_id" {
type = string
description = "The id of the machine image (AMI) to use for the server."
validation {
# regex(...) fails if it cannot find a match
condition = can(regex("^ami-", var.image_id))
error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."
}
}
```
If `condition` evaluates to `false`, Terraform will produce an error message
that includes the sentences given in `error_message`. The error message string
should be at least one full sentence explaining the constraint that failed,
using a sentence structure similar to the above examples.
Multiple `validation` blocks can be declared in which case error messages
will be returned for _all_ failed conditions.
### Suppressing Values in CLI Output
[inpage-sensitive]: #suppressing-values-in-cli-output
-> This feature was introduced in Terraform v0.14.0.
> **Hands-on:** Try the [Protect Sensitive Input Variables](https://learn.hashicorp.com/tutorials/terraform/sensitive-variables?in=terraform/configuration-language&utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) tutorial on HashiCorp Learn.
Setting a variable as `sensitive` prevents Terraform from showing its value in
the `plan` or `apply` output, when you use that variable elsewhere in your
configuration.
Terraform will still record sensitive values in the [state](/docs/language/state/index.html),
and so anyone who can access the state data will have access to the sensitive
values in cleartext. For more information, see
[_Sensitive Data in State_](/docs/language/state/sensitive-data.html).
Declare a variable as sensitive by setting the `sensitive` argument to `true`:
```
variable "user_information" {
type = object({
name = string
address = string
})
sensitive = true
}
resource "some_resource" "a" {
name = var.user_information.name
address = var.user_information.address
}
```
Any expressions whose result depends on the sensitive variable will be treated
as sensitive themselves, and so in the above example the two arguments of
`resource "some_resource" "a"` will also be hidden in the plan output:
```
Terraform will perform the following actions:
# some_resource.a will be created
+ resource "some_resource" "a" {
+ name = (sensitive)
+ address = (sensitive)
}
Plan: 1 to add, 0 to change, 0 to destroy.
```
In some cases where you use a sensitive variable inside a nested block, Terraform
may treat the entire block as redacted. This happens for resource types where
all of the blocks of a particular type are required to be unique, and so
disclosing the content of one block might imply the content of a sibling block.
```
# some_resource.a will be updated in-place
~ resource "some_resource" "a" {
~ nested_block {
# At least one attribute in this block is (or was) sensitive,
# so its contents will not be displayed.
}
}
```
A provider can also
[declare an attribute as sensitive](/docs/extend/best-practices/sensitive-state.html#using-the-sensitive-flag),
which will cause Terraform to hide it from regular output regardless of how
you assign it a value. For more information, see
[Sensitive Resource Attributes](/docs/language/expressions/references.html#sensitive-resource-attributes).
If you use a sensitive value from as part of an
[output value](/docs/language/values/outputs.html) then Terraform will require
you to also mark the output value itself as sensitive, to confirm that you
intended to export it.
#### Cases where Terraform may disclose a sensitive variable
A `sensitive` variable is a configuration-centered concept, and values are sent to providers without any obfuscation. A provider error could disclose a value if that value is included in the error message. For example, a provider might return the following error even if "foo" is a sensitive value: `"Invalid value 'foo' for field"`
If a resource attribute is used as, or part of, the provider-defined resource id, an `apply` will disclose the value. In the example below, the `prefix` attribute has been set to a sensitive variable, but then that value ("jae") is later disclosed as part of the resource id:
```
# random_pet.animal will be created
+ resource "random_pet" "animal" {
+ id = (known after apply)
+ length = 2
+ prefix = (sensitive)
+ separator = "-"
}
Plan: 1 to add, 0 to change, 0 to destroy.
...
random_pet.animal: Creating...
random_pet.animal: Creation complete after 0s [id=jae-known-mongoose]
```
### Disallowing Null Input Values
[inpage-nullable]: #disallowing-null-input-values
-> This feature is available in Terraform v1.1.0 and later.
The `nullable` argument in a variable block controls whether the module caller
may assign the value `null` to the variable.
```
variable "example" {
type = string
nullable = false
}
```
The default value for `nullable` is `true`. When `nullable` is `true`, `null`
is a valid value for the variable, and the module configuration must always
account for the possibility of the variable value being `null`. Passing a
`null` value as a module input argument will override any `default` value.
Setting `nullable` to `false` ensures that the variable value will never be
`null` within the module. If `nullable` is `false` and the variable has a
`default` value, then Terraform uses the default when a module input argument is `null`.
The `nullable` argument only controls where the direct value of the variable may be `null`.
For variables of collection or structural types, such as lists or objects,
the caller may still use `null` in nested elements or attributes, as long as
the collection or structure itself is not null.
## Using Input Variable Values
Within the module that declared a variable, its value can be accessed from
within [expressions](/docs/language/expressions/index.html) as `var.<NAME>`,
where `<NAME>` matches the label given in the declaration block:
-> **Note:** Input variables are _created_ by a `variable` block, but you
_reference_ them as attributes on an object named `var`.
```hcl
resource "aws_instance" "example" {
instance_type = "t2.micro"
ami = var.image_id
}
```
The value assigned to a variable can only be accessed in expressions within
the module where it was declared.
## Assigning Values to Root Module Variables
When variables are declared in the root module of your configuration, they
can be set in a number of ways:
* [In a Terraform Cloud workspace](/docs/cloud/workspaces/variables.html).
* Individually, with the `-var` command line option.
* In variable definitions (`.tfvars`) files, either specified on the command line
or automatically loaded.
* As environment variables.
The following sections describe these options in more detail. This section does
not apply to _child_ modules, where values for input variables are instead
assigned in the configuration of their parent module, as described in
[_Modules_](/docs/language/modules/index.html).
### Variables on the Command Line
To specify individual variables on the command line, use the `-var` option
when running the `terraform plan` and `terraform apply` commands:
```
terraform apply -var="image_id=ami-abc123"
terraform apply -var='image_id_list=["ami-abc123","ami-def456"]' -var="instance_type=t2.micro"
terraform apply -var='image_id_map={"us-east-1":"ami-abc123","us-east-2":"ami-def456"}'
```
The above examples show appropriate syntax for Unix-style shells, such as on
Linux or macOS. For more information on shell quoting, including additional
examples for Windows Command Prompt, see
[Input Variables on the Command Line](/docs/cli/commands/plan.html#input-variables-on-the-command-line).
You can use the `-var` option multiple times in a single command to set several
different variables.
<a id="variable-files"></a>
### Variable Definitions (`.tfvars`) Files
To set lots of variables, it is more convenient to specify their values in
a _variable definitions file_ (with a filename ending in either `.tfvars`
or `.tfvars.json`) and then specify that file on the command line with
`-var-file`:
```
terraform apply -var-file="testing.tfvars"
```
-> **Note:** This is how Terraform Cloud passes
[workspace variables](/docs/cloud/workspaces/variables.html) to Terraform.
A variable definitions file uses the same basic syntax as Terraform language
files, but consists only of variable name assignments:
```hcl
image_id = "ami-abc123"
availability_zone_names = [
"us-east-1a",
"us-west-1c",
]
```
Terraform also automatically loads a number of variable definitions files
if they are present:
* Files named exactly `terraform.tfvars` or `terraform.tfvars.json`.
* Any files with names ending in `.auto.tfvars` or `.auto.tfvars.json`.
Files whose names end with `.json` are parsed instead as JSON objects, with
the root object properties corresponding to variable names:
```json
{
"image_id": "ami-abc123",
"availability_zone_names": ["us-west-1a", "us-west-1c"]
}
```
### Environment Variables
As a fallback for the other ways of defining variables, Terraform searches
the environment of its own process for environment variables named `TF_VAR_`
followed by the name of a declared variable.
This can be useful when running Terraform in automation, or when running a
sequence of Terraform commands in succession with the same variables.
For example, at a `bash` prompt on a Unix system:
```
$ export TF_VAR_image_id=ami-abc123
$ terraform plan
...
```
On operating systems where environment variable names are case-sensitive,
Terraform matches the variable name exactly as given in configuration, and
so the required environment variable name will usually have a mix of upper
and lower case letters as in the above example.
### Complex-typed Values
When variable values are provided in a variable definitions file, you can use
Terraform's usual syntax for
[literal expressions](/docs/language/expressions/types.html#literal-expressions)
to assign complex-typed values, like lists and maps.
Some special rules apply to the `-var` command line option and to environment
variables. For convenience, Terraform defaults to interpreting `-var` and
environment variable values as literal strings, which need only shell quoting,
and no special quoting for Terraform. For example, in a Unix-style shell:
```
$ export TF_VAR_image_id='ami-abc123'
```
However, if a root module variable uses a [type constraint](#type-constraints)
to require a complex value (list, set, map, object, or tuple), Terraform will
instead attempt to parse its value using the same syntax used within variable
definitions files, which requires careful attention to the string escaping rules
in your shell:
```
$ export TF_VAR_availability_zone_names='["us-west-1b","us-west-1d"]'
```
For readability, and to avoid the need to worry about shell escaping, we
recommend always setting complex variable values via variable definitions files.
For more information on quoting and escaping for `-var` arguments,
see
[Input Variables on the Command Line](/docs/cli/commands/plan.html#input-variables-on-the-command-line).
### Values for Undeclared Variables
If you have defined a variable value, but not its corresponding `variable {}`
definition, you may get an error or warning depending on how you have provided
that value.
If you provide values for undeclared variables defined as [environment variables](#environment-variables)
you will not get an error or warning. This is because environment variables may
be declared but not used in all configurations that might be run.
If you provide values for undeclared variables defined [in a file](#variable-definitions-tfvars-files)
you will get a warning. This is to help in cases where you have provided a variable
value _meant_ for a variable declaration, but perhaps there is a mistake in the
value definition. For example, the following configuration:
```terraform
variable "moose" {
type = string
}
```
And the following `.tfvars` file:
```hcl
moose = "Moose"
```
Will cause Terraform to warn you that there is no variable declared `"mosse"`, which can help
you spot this mistake.
If you use `.tfvars` files across multiple configurations and expect to continue to see this warning,
you can use the [`-compact-warnings`](https://www.terraform.io/docs/cli/commands/plan.html#compact-warnings)
option to simplify your output.
If you provide values for undeclared variables on the [command line](#variables-on-the-command-line),
Terraform will error. To avoid this error, either declare a variable block for the value, or remove
the variable value from your Terraform call.
### Variable Definition Precedence
The above mechanisms for setting variables can be used together in any
combination. If the same variable is assigned multiple values, Terraform uses
the _last_ value it finds, overriding any previous values. Note that the same
variable cannot be assigned multiple values within a single source.
Terraform loads variables in the following order, with later sources taking
precedence over earlier ones:
* Environment variables
* The `terraform.tfvars` file, if present.
* The `terraform.tfvars.json` file, if present.
* Any `*.auto.tfvars` or `*.auto.tfvars.json` files, processed in lexical order
of their filenames.
* Any `-var` and `-var-file` options on the command line, in the order they
are provided. (This includes variables set by a Terraform Cloud
workspace.)
~> **Important:** In Terraform 0.12 and later, variables with map and object
values behave the same way as other variables: the last value found overrides
the previous values. This is a change from previous versions of Terraform, which
would _merge_ map values instead of overriding them.
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