AWS Documentation » AWS CloudFormation » User Guide » Working with AWS CloudFormation Templates » Template Anatomy » Parameters Declaration	« Previous Next »
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Parameters Declaration

If present, the optional Parameters section of an AWS CloudFormation template must declare at least one parameter.

The optional Parameters section enables you to pass values into your template at stack creation time. Parameters let you create templates that can be customized for each stack deployment. When you create a stack from a template containing parameters, you can specify values for those parameters. Within the template, you can use the "Ref" intrinsic function to specify those parameter values in properties values for resources. For example, you can define a string parameter with the following Parameters section:

"Parameters" : {
    "URL" : {
      "Type" : "String"
    }
    }

Note

If present, the Parameters section must declare at least one parameter. There can be a maximum of 50 parameters in an AWS CloudFormation template.

At runtime, you can use the --parameters option of cfn-create-stack to set the URL parameter to a specific value:

cfn-create-stack MyStack --template-file My.template --parameters "URL=127.0.0.1"

Multiple parameter assignments are separated with a semicolon.

Note

By default, cfn-describe-stacks returns parameter values. To prevent sensitive parameter values such as passwords from being returned, include a NoEcho property set to true.

Parameters can have rules, called constraints, that determine valid values for a parameter. These constraint properties you validate the template user's input before any resources are created. For example, you can have a constraint that a string include only alphanumeric characters or that a numeric parameter fall between 1 and 10.

Parameter Syntax and Properties

The Parameters section is composed of the key name Parameters, followed by a single colon. Braces enclose all parameter declarations. Parameters declared within the Parameters section are delimited by a comma.

Each parameter must declare a double-quoted name, followed by a colon. Parameter names must be alphanumeric and unique among all logical names within a template.

A parameter can be declared as one of following types: String, Number, or CommaDelimitedList. For a parameter that has a String or Number type, you can define constraints that AWS CloudFormation uses to validate the value of the parameter.

Parameters have the following properties:

Property	Required	Description
Type	Yes	String, Number, or CommaDelimitedList. A parameter of type `String` is simply a literal string. A parameter of type `Number` can be an integer or float. Note that AWS CloudFormation validates the parameter as a number but uses the parameter value within the template as a string. A parameter of type `CommaDelimitedList` is an array of literal strings separated by commas. The member strings are space trimmed and there is one more string than there are commas in the specified value.
Default	No	A value of the appropriate type for the template to use if no value is specified at stack creation. If the parameter has constraints defined, this value must adhere to those constraints.
NoEcho	No	If `TRUE`, the value of the parameter is masked with asterisks (*****) with `cfn-describe-stacks`.
AllowedValues	No	An array containing the list of values allowed for the parameter.
AllowedPattern	No	`String` constraint. A regular expression that represents the patterns allowed in the parameter's string value.
MaxLength	No	`String` constraint. A integer value that determines the largest number of characters in the parameter's string value.
MinLength	No	`String` constraint. A integer value that determines the smallest number of characters in the parameter's string value.
MaxValue	No	`Number` constraint. A numeric value that determines the largest numeric value allowed for the parameter.
MinValue	No	`Number` constraint. A numeric value that determines the smallest numeric value allowed for the parameter.
Description	No	A String type up to 4000 characters describing the parameter.
ConstraintDescription	No	A String type explaining the constraint requirements that appears when the constraint is violated. For example, a parameter that has an AllowedPattern of "[A-Za-z0-9]+" would display this error message when the user specified an invalid value: cfn-create-stack: Malformed input-Parameter MyParameter must match pattern [A-Za-z0-9]+ By adding a ConstraintDescription with a value "must only contain upper- and lowercase letters, and numbers", you can display a customized error message: cfn-create-stack: Malformed input-Parameter MyParameter must only contain upper and lower case letters and numbers

Parameter Examples

The following example Parameters section declares two parameters. The DBPort parameter is of type Number with a default of 3306 and a minimum value of 1150 and maximum value 65535. The DBPwd parameter is of type String with no default value, NoEcho is set to true to prevent cfn-describe-stacks from returning the parameter value, minimum length of 1, maximum length of 41, and pattern that allows lowercase and uppercase alphabetic characters and numerals. The UserRoles parameter is a CommaDelimitedList with a default list containing the string values guest and newhire.

"Parameters" : {
    "DBPort": {
      "Default": "3306",
      "Description" : "TCP/IP port for the database",
      "Type": "Number",
      "MinValue": "1150",
      "MaxValue": "65535"
    },
    "DBPwd": {
      "NoEcho": "true",
      "Description" : "The database admin account password",
      "Type": "String",
      "MinLength": "1",
      "MaxLength": "41",
      "AllowedPattern" : "[a-zA-Z0-9]*"
    },

    "UserRoles" : {
      "Type" : "CommaDelimitedList",
      "Default" : "guest,newhire",
      "NoEcho" : "TRUE"
    }
}

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