Input Variables
Note: This page is about Terraform 0.11 and earlier. For Terraform 0.12 and later, see Configuration Language: Input Variables.
Input variables serve as parameters for a Terraform module.
When used in the root module of a configuration, variables can be set from CLI arguments and environment variables. For child modules, they allow values to pass from parent to child.
This page assumes you're familiar with the configuration syntax already.
Example
Input variables can be defined as follows:
Description
The variable
block configures a single input variable for a Terraform module.
Each block declares a single variable.
The name given in the block header is used to assign a value to the variable via the CLI and to reference the variable elsewhere in the configuration.
Within the block body (between { }
) is configuration for the variable,
which accepts the following arguments:
type
(Optional) - If set this defines the type of the variable. Valid values arestring
,list
, andmap
. If this field is omitted, the variable type will be inferred based ondefault
. If nodefault
is provided, the type is assumed to bestring
.default
(Optional) - This sets a default value for the variable. If no default is provided, Terraform will raise an error if a value is not provided by the caller. The default value can be of any of the supported data types, as described below. Iftype
is also set, the given value must be of the specified type.description
(Optional) - A human-friendly description for the variable. This is primarily for documentation for users using your Terraform configuration. When a module is published in Terraform Registry, the given description is shown as part of the documentation.
The name of a variable can be any valid identifier. However, due to the
interpretation of module configuration blocks,
the names source
, version
and providers
are reserved for Terraform's own
use and are thus not recommended for any module intended to be used as a
child module.
The default value of an input variable must be a literal value, containing no interpolation expressions. To assign a name to an expression so that it may be re-used within a module, use Local Values instead.
Strings
String values are simple and represent a basic key to value mapping where the key is the variable name. An example is:
A multi-line string value can be provided using heredoc syntax.
Terraform performs automatic conversion from string values to numeric and boolean values based on context, so in practice string variables may be used to set arguments of any primitive type. For boolean values in particular there are some caveats, described under Booleans below.
Maps
A map value is a lookup table from string keys to string values. This is useful for selecting a value based on some other provided value.
A common use of maps is to create a table of machine images per region, as follows:
Lists
A list value is an ordered sequence of strings indexed by integers starting with zero. For example:
Booleans
Although Terraform can automatically convert between boolean and string values, there are some subtle implications of these conversions that should be completely understood when using boolean values with input variables.
It is recommended for now to specify boolean values for variables as the
strings "true"
and "false"
, to avoid some caveats in the conversion
process. A future version of Terraform will properly support boolean values
and so relying on the current behavior could result in
backwards-incompatibilities at that time.
For a configuration such as the following:
The false is converted to a string "0"
when running Terraform.
Then, depending on where you specify overrides, the behavior can differ:
Variables with boolean values in a
tfvars
file will likewise be converted to "0" and "1" values.Variables specified via the
-var
command line flag will be literal strings "true" and "false", so care should be taken to explicitly use "0" or "1".Variables specified with the
TF_VAR_
environment variables will be literal string values, just like-var
.
A future version of Terraform will fully support first-class boolean types which will make the behavior of booleans consistent as you would expect. This may break some of the above behavior.
When passing boolean-like variables as parameters to resource configurations that expect boolean values, they are converted consistently:
- "1" and "true" become
true
- "0" and "false" become
false
The behavior of conversion in this direction (string to boolean) will not change in future Terraform versions. Therefore, using these string values rather than literal booleans is recommended when using input variables.
Environment Variables
Environment variables can be used to set the value of an input variable in
the root module. The name of the environment variable must be
TF_VAR_
followed by the variable name, and the value is the value of the
variable.
For example, given the configuration below:
The variable can be set via an environment variable:
Maps and lists can be specified using environment variables as well using HCL syntax in the value.
For a list variable like so:
The variable could be set like so:
Similarly, for a map declared like:
The value can be set like this:
Variable Files
Values for the input variables of a root module can be gathered in
variable definition files and passed together using the -var-file=FILE
option.
For all files which match terraform.tfvars
or *.auto.tfvars
present in the
current directory, Terraform automatically loads them to populate variables. If
the file is located somewhere else, you can pass the path to the file using the
-var-file
flag. It is recommended to name such files with names ending in
.tfvars
.
Variables files use HCL or JSON syntax to define variable values. Strings, lists
or maps may be set in the same manner as the default value in a variable
block
in Terraform configuration. For example:
The -var-file
flag can be used multiple times per command invocation:
Note: Variable files are evaluated in the order in which they are specified on the command line. If a particular variable is defined in more than one variable file, the last value specified is effective.
Variable Merging
When multiple values are provided for the same input variable, map values are merged while all other values are overridden by the last definition.
For example, if you define a variable twice on the command line:
Then the value of foo
will be baz
, since it was the last definition seen.
However, for maps, the values are merged:
The resulting value of foo
will be:
There is no way currently to unset map values in Terraform. Whenever a map is modified either via variable input or being passed into a module, the values are always merged.
Variable Precedence
Both these files have the variable baz
defined:
foo.tfvars
bar.tfvars
When they are passed in the following order:
The result will be that baz
will contain the value bar
because bar.tfvars
has the last definition loaded.
Definition files passed using the -var-file
flag will always be evaluated after
those in the working directory.
Values passed within definition files or with -var
will take precedence over
TF_VAR_
environment variables, as environment variables are considered defaults.