CloudFormation template Parameters syntax
Use the optional Parameters
section to customize your templates. With
parameters, you can input custom values to your template each time you create or update a stack.
By using parameters in your templates, you can build reusable and flexible templates that can be
tailored to specific scenarios.
By defining parameters of the appropriate type, you can choose from a list of identifiers of existing resources when you use the console to create your stack. For more information, see Reference existing resources and Systems Manager parameters with CloudFormation-supplied parameter types.
Parameters are a popular way to specify property values of stack resources. However, there may be settings that are region dependent or are somewhat complex for users to figure out because of other conditions or dependencies. In these cases, you might want to put some logic in the template itself so that users can specify simpler values (or none at all) to get the results that they want, such as by using a mapping. For more information, see CloudFormation template Mappings syntax.
Syntax
You declare parameters in a template's Parameters
section, which uses the
following general syntax:
JSON
"Parameters" : { "
ParameterLogicalID
" : { "Description
": "Information about the parameter
", "Type" : "DataType
", "Default
" : "value
", "AllowedValues
" : ["value1
", "value2
"] } }
YAML
Parameters:
ParameterLogicalID
:Description
:Information about the parameter
Type:DataType
Default
:value
AllowedValues
: -value1
-value2
A parameter contains a list of attributes that define its value and constraints against
its value. The only required attribute is Type
, which can be String
,
Number
, or a CloudFormation-supplied parameter type. You can also add a
Description
attribute that describes what kind of value you should specify. The
parameter's name and description appear in the Specify Parameters page
when you use the template in the Create Stack wizard.
Note
By default, the CloudFormation console lists input parameters alphabetically by their
logical ID. To override this default ordering and group related parameters together, you can
use the AWS::CloudFormation::Interface
metadata key in your template. For more
information, see AWS::CloudFormation::Interface.
For parameters with default values, CloudFormation uses the default values unless users specify another value. If you omit the default attribute, users are required to specify a value for that parameter. However, requiring the user to input a value does not ensure that the value is valid. To validate the value of a parameter, you can declare constraints or specify an Amazon-specific parameter type.
For parameters without default values, users must specify a key name value at stack creation. If they don’t, CloudFormation fails to create the stack and throws an exception:
Parameters: [KeyName] must have values
Properties
AllowedPattern
-
A regular expression that represents the patterns to allow for
String
orCommaDelimitedList
types. When applied on a parameter of typeString
, the pattern must match the entire parameter value provided. When applied to a parameter of typeCommaDelimitedList
, the pattern must match each value in the list.Required: No
AllowedValues
-
An array containing the list of values allowed for the parameter. When applied to a parameter of type
String
, the parameter value must be one of the allowed values. When applied to a parameter of typeCommaDelimitedList
, each value in the list must be one of the specified allowed values.Required: No
Note
If you're using YAML and you want to use
Yes
andNo
strings forAllowedValues
, use single-quotes to prevent the YAML parser from considering these boolean values. ConstraintDescription
-
A string that explains a constraint when the constraint is violated. For example, without a constraint description, a parameter that has an allowed pattern of
[A-Za-z0-9]+
displays the following error message when the user specifies an invalid value:Malformed input-Parameter MyParameter must match pattern [A-Za-z0-9]+
By adding a constraint description, such as must only contain letters (uppercase and lowercase) and numbers, you can display the following customized error message:
Malformed input-Parameter MyParameter must only contain uppercase and lowercase letters and numbers
Required: No
Default
-
A value of the appropriate type for the template to use if no value is specified when a stack is created. If you define constraints for the parameter, you must specify a value that adheres to those constraints.
Required: No
Description
-
A string of up to 4000 characters that describes the parameter.
Required: No
MaxLength
-
An integer value that determines the largest number of characters you want to allow for
String
types.Required: No
MaxValue
-
A numeric value that determines the largest numeric value you want to allow for
Number
types.Required: No
MinLength
-
An integer value that determines the smallest number of characters you want to allow for
String
types.Required: No
MinValue
-
A numeric value that determines the smallest numeric value you want to allow for
Number
types.Required: No
NoEcho
-
Whether to mask the parameter value to prevent it from being displayed in the console, command line tools, or API. If you set the
NoEcho
attribute totrue
, CloudFormation returns the parameter value masked as asterisks (*****) for any calls that describe the stack or stack events, except for information stored in the locations specified below.Required: No
Important
Using the
NoEcho
attribute does not mask any information stored in the following:-
The
Metadata
template section. CloudFormation does not transform, modify, or redact any information you include in theMetadata
section. For more information, see CloudFormation template Metadata syntax. -
The
Outputs
template section. For more information, see CloudFormation template Outputs syntax. -
The
Metadata
attribute of a resource definition. For more information, see Metadata attribute.
We strongly recommend you do not use these mechanisms to include sensitive information, such as passwords or secrets.
Important
Rather than embedding sensitive information directly in your CloudFormation templates, we recommend you use dynamic parameters in the stack template to reference sensitive information that is stored and managed outside of CloudFormation, such as in the Amazon Systems Manager Parameter Store or Amazon Secrets Manager.
For more information, see the Do not embed credentials in your templates best practice.
Important
We strongly recommend against including
NoEcho
parameters, or any sensitive data, in resource properties that are part of a resource's primary identifier.When a
NoEcho
parameter is included in a property that forms a primary resource identifier, CloudFormation may use the actual plaintext value in the primary resource identifier. This resource ID may appear in any derived outputs or destinations.To determine which resource properties comprise a resource type's primary identifier, refer to the resource reference documentation for that resource in the Amazon resource and property types reference. In the Return values section, the
Ref
function return value represents the resource properties that comprise the resource type's primary identifier. -
Type
-
The data type for the parameter (
DataType
).Required: Yes
CloudFormation supports the following parameter types:
String
-
A literal string. You can use the following attributes to declare constraints:
MinLength
,MaxLength
,Default
,AllowedValues
, andAllowedPattern
.For example, users could specify
"MyUserName"
. Number
-
An integer or float. CloudFormation validates the parameter value as a number; however, when you use the parameter elsewhere in your template (for example, by using the
Ref
intrinsic function), the parameter value becomes a string.You can use the following attributes to declare constraints:
MinValue
,MaxValue
,Default
, andAllowedValues
.For example, users could specify
"8888"
. List<Number>
-
An array of integers or floats that are separated by commas. CloudFormation validates the parameter value as numbers; however, when you use the parameter elsewhere in your template (for example, by using the
Ref
intrinsic function), the parameter value becomes a list of strings.For example, users could specify
"80,20"
, and aRef
would result in["80","20"]
. CommaDelimitedList
-
An array of literal strings that are separated by commas. The total number of strings should be one more than the total number of commas. Also, each member string is space trimmed.
For example, users could specify
"test,dev,prod"
, and aRef
would result in["test","dev","prod"]
. - Amazon-specific parameter types
-
Amazon values such as Amazon EC2 key pair names and VPC IDs. For more information, see Use CloudFormation-supplied parameter types.
- Systems Manager parameter types
-
Parameters that correspond to existing parameters in Systems Manager Parameter Store. You specify a Systems Manager parameter key as the value of the Systems Manager parameter type, and CloudFormation retrieves the latest value from Parameter Store to use for the stack. For more information, see Use CloudFormation-supplied parameter types.
General requirements for parameters
The following requirements apply when using parameters:
-
You can have a maximum of 200 parameters in a CloudFormation template.
-
Each parameter must be given a logical name (also called logical ID) that must be alphanumeric and unique among all logical names within the template.
-
Each parameter must be assigned a parameter type that's supported by CloudFormation. For more information, see Type.
-
Each parameter must be assigned a value at runtime for CloudFormation to successfully provision the stack. You can optionally specify a default value for CloudFormation to use unless another value is provided.
-
Parameters must be declared and referenced from within the same template. You can reference parameters from the
Resources
andOutputs
sections of the template.
Examples
Topics
Simple string parameter
The following example declares a parameter named InstanceTypeParameter
of
type String
. This parameter lets you specify the Amazon EC2 instance type for the
stack. If no value is provided during stack creation or update, CloudFormation uses the default
value of t2.micro
.
JSON
"Parameters" : { "InstanceTypeParameter" : { "Description" : "Enter t2.micro, m1.small, or m1.large. Default is t2.micro.", "Type" : "String", "Default" : "t2.micro", "AllowedValues" : ["t2.micro", "m1.small", "m1.large"] } }
YAML
Parameters: InstanceTypeParameter: Description: Enter t2.micro, m1.small, or m1.large. Default is t2.micro. Type: String Default: t2.micro AllowedValues: - t2.micro - m1.small - m1.large
Password parameter
The following example declares a parameter named DBPwd
of type
String
with no default value. The NoEcho
property is set to
true
to prevent the parameter value from being displayed in stack
descriptions. The minimum length that can be specified is 1
, and the maximum
length that can be specified is 41
. The pattern allows lowercase and uppercase
alphabetical characters and numerals. This example also illustrates the use of a regular
expression for the AllowedPattern
property.
JSON
"Parameters" : { "DBPwd" : { "NoEcho" : "true", "Description" : "The database admin account password", "Type" : "String", "MinLength" : "1", "MaxLength" : "41", "AllowedPattern" : "^[a-zA-Z0-9]*$" } }
YAML
Parameters: DBPwd: NoEcho: true Description: The database admin account password Type: String MinLength: 1 MaxLength: 41 AllowedPattern: ^[a-zA-Z0-9]*$
Referencing parameters
You use the Ref
intrinsic function to reference a parameter, and CloudFormation
uses the parameter's value to provision the stack. You can reference parameters from the
Resources
and Outputs
sections of the same template.
In the following example, the InstanceType
property of the EC2 instance
resource references the InstanceTypeParameter
parameter value:
JSON
"Ec2Instance" : { "Type" : "AWS::EC2::Instance", "Properties" : { "InstanceType" : { "Ref" : "InstanceTypeParameter" }, "ImageId" : "ami-0ff8a91507f77f867" } }
YAML
Ec2Instance: Type: AWS::EC2::Instance Properties: InstanceType: Ref: InstanceTypeParameter ImageId: ami-0ff8a91507f77f867
Comma-delimited list parameter
The CommaDelimitedList
parameter type can be useful when you need to
provide multiple values for a single property. The following example declares a parameter
named DbSubnetIpBlocks
with a default value of three CIDR blocks separated by
commas.
JSON
"Parameters" : { "DbSubnetIpBlocks": { "Description": "Comma-delimited list of three CIDR blocks", "Type": "CommaDelimitedList", "Default": "10.0.48.0/24, 10.0.112.0/24, 10.0.176.0/24" } }
YAML
Parameters: DbSubnetIpBlocks: Description: "Comma-delimited list of three CIDR blocks" Type: CommaDelimitedList Default: "10.0.48.0/24, 10.0.112.0/24, 10.0.176.0/24"
Return a value from a comma-delimited list parameter
To refer to a specific value in a parameter's comma-delimited list, use the
Fn::Select
intrinsic function in the Resources
section of your
template. Pass the index value of the object that you want and a list of objects, as shown
in the following example.
JSON
{ "Parameters": { "VPC": { "Type": "String", "Default": "vpc-123456" }, "VpcAzs": { "Type": "CommaDelimitedList", "Default": "us-west-2a, us-west-2b, us-west-2c" }, "DbSubnetIpBlocks": { "Type": "CommaDelimitedList", "Default": "172.16.0.0/26, 172.16.0.64/26, 172.16.0.128/26" } }, "Resources": { "DbSubnet1": { "Type": "AWS::EC2::Subnet", "Properties": { "AvailabilityZone": { "Fn::Sub": [ "${AWS::Region}${AZ}", { "AZ": { "Fn::Select": [ 0, { "Ref": "VpcAzs" } ] } } ] }, "VpcId": { "Ref": "VPC" }, "CidrBlock": { "Fn::Select": [ 0, { "Ref": "DbSubnetIpBlocks" } ] } } }, "DbSubnet2": { "Type": "AWS::EC2::Subnet", "Properties": { "AvailabilityZone": { "Fn::Sub": [ "${AWS::Region}${AZ}", { "AZ": { "Fn::Select": [ 1, { "Ref": "VpcAzs" } ] } } ] }, "VpcId": { "Ref": "VPC" }, "CidrBlock": { "Fn::Select": [ 1, { "Ref": "DbSubnetIpBlocks" } ] } } }, "DbSubnet3": { "Type": "AWS::EC2::Subnet", "Properties": { "AvailabilityZone": { "Fn::Sub": [ "${AWS::Region}${AZ}", { "AZ": { "Fn::Select": [ 2, { "Ref": "VpcAzs" } ] } } ] }, "VpcId": { "Ref": "VPC" }, "CidrBlock": { "Fn::Select": [ 2, { "Ref": "DbSubnetIpBlocks" } ] } } } } }
YAML
Parameters: VPC: Type: String Default: vpc-123456 VpcAzs: Type: CommaDelimitedList Default: us-west-2a, us-west-2b, us-west-2c DbSubnetIpBlocks: Type: CommaDelimitedList Default: 172.16.0.0/26, 172.16.0.64/26, 172.16.0.128/26 Resources: DbSubnet1: Type: AWS::EC2::Subnet Properties: AvailabilityZone: !Sub - ${AWS::Region}${AZ} - AZ: !Select - 0 - !Ref VpcAzs VpcId: !Ref VPC CidrBlock: !Select - 0 - !Ref DbSubnetIpBlocks DbSubnet2: Type: AWS::EC2::Subnet Properties: AvailabilityZone: !Sub - ${AWS::Region}${AZ} - AZ: !Select - 1 - !Ref VpcAzs VpcId: !Ref VPC CidrBlock: !Select - 1 - !Ref DbSubnetIpBlocks DbSubnet3: Type: AWS::EC2::Subnet Properties: AvailabilityZone: !Sub - ${AWS::Region}${AZ} - AZ: !Select - 2 - !Ref VpcAzs VpcId: !Ref VPC CidrBlock: !Select - 2 - !Ref DbSubnetIpBlocks
Related resources
CloudFormation also supports the use of dynamic references to specify property values dynamically. For example, you might need to reference secure strings stored in Systems Manager Parameter Store. For more information, see Get values stored in other services using dynamic references.
You can also use pseudo parameters within a Ref
or a Sub
function to dynamically populate values. For more information, see Pseudo parameters reference.