terraform/website/source/docs/configuration/interpolation.html.md

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docs	Interpolation Syntax	docs-config-interpolation	Embedded within strings in Terraform, whether you're using the Terraform syntax or JSON syntax, you can interpolate other values into strings. These interpolations are wrapped in `${}`, such as `${var.foo}`.

Interpolation Syntax

Embedded within strings in Terraform, whether you're using the Terraform syntax or JSON syntax, you can interpolate other values into strings. These interpolations are wrapped in ${}, such as ${var.foo}.

The interpolation syntax is powerful and allows you to reference variables, attributes of resources, call functions, etc.

You can also perform simple math in interpolations, allowing you to write expressions such as ${count.index + 1}.

You can escape interpolation with double dollar signs: $${foo} will be rendered as a literal ${foo}.

Available Variables

To reference user variables, use the var. prefix followed by the variable name. For example, ${var.foo} will interpolate the foo variable value. If the variable is a mapping, then you can reference static keys in the map with the syntax var.MAP.KEY. For example, ${var.amis.us-east-1} would get the value of the us-east-1 key within the amis variable that is a mapping.

To reference attributes of your own resource, the syntax is self.ATTRIBUTE. For example ${self.private_ip_address} will interpolate that resource's private IP address. Note that this is only allowed/valid within provisioners.

To reference attributes of other resources, the syntax is TYPE.NAME.ATTRIBUTE. For example, ${aws_instance.web.id} will interpolate the ID attribute from the "aws_instance" resource named "web". If the resource has a count attribute set, you can access individual attributes with a zero-based index, such as ${aws_instance.web.0.id}. You can also use the splat syntax to get a list of all the attributes: ${aws_instance.web.*.id}. This is documented in more detail in the resource configuration page.

To reference outputs from a module, the syntax is MODULE.NAME.OUTPUT. For example ${module.foo.bar} will interpolate the "bar" output from the "foo" module.

To reference count information, the syntax is count.FIELD. For example, ${count.index} will interpolate the current index in a multi-count resource. For more information on count, see the resource configuration page.

To reference path information, the syntax is path.TYPE. TYPE can be cwd, module, or root. cwd will interpolate the cwd. module will interpolate the path to the current module. root will interpolate the path of the root module. In general, you probably want the path.module variable.

Built-in Functions

Terraform ships with built-in functions. Functions are called with the syntax name(arg, arg2, ...). For example, to read a file: ${file("path.txt")}. The built-in functions are documented below.

The supported built-in functions are:

• concat(list1, list2) - Combines two or more lists into a single list. Example: concat(aws_instance.db.*.tags.Name, aws_instance.web.*.tags.Name)

• element(list, index) - Returns a single element from a list at the given index. If the index is greater than the number of elements, this function will wrap using a standard mod algorithm. A list is only possible with splat variables from resources with a count greater than one. Example: element(aws_subnet.foo.*.id, count.index)

• file(path) - Reads the contents of a file into the string. Variables in this file are not interpolated. The contents of the file are read as-is.

• format(format, args...) - Formats a string according to the given format. The syntax for the format is standard sprintf syntax. Good documentation for the syntax can be found here. Example to zero-prefix a count, used commonly for naming servers: format("web-%03d", count.index + 1).

• formatlist(format, args...) - Formats each element of a list according to the given format, similarly to format, and returns a list. Non-list arguments are repeated for each list element. For example, to convert a list of DNS addresses to a list of URLs, you might use: formatlist("https://%s:%s/", aws_instance.foo.*.public_dns, var.port). If multiple args are lists, and they have the same number of elements, then the formatting is applied to the elements of the lists in parallel. Example: formatlist("instance %v has private ip %v", aws_instance.foo.*.id, aws_instance.foo.*.private_ip). Passing lists with different lengths to formatlist results in an error.

• index(list, elem) - Finds the index of a given element in a list. Example: index(aws_instance.foo.*.tags.Name, "foo-test")

• join(delim, list) - Joins the list with the delimiter. A list is only possible with splat variables from resources with a count greater than one. Example: join(",", aws_instance.foo.*.id)

• length(list) - Returns a number of members in a given list or a number of characters in a given string.

  • ${length(split(",", "a,b,c"))} = 3
  • ${length("a,b,c")} = 5

• lookup(map, key) - Performs a dynamic lookup into a mapping variable. The map parameter should be another variable, such as var.amis.

• replace(string, search, replace) - Does a search and replace on the given string. All instances of search are replaced with the value of replace. If search is wrapped in forward slashes, it is treated as a regular expression. If using a regular expression, replace can reference subcaptures in the regular expression by using $n where n is the index or name of the subcapture. If using a regular expression, the syntax conforms to the re2 regular expression syntax.

• split(delim, string) - Splits the string previously created by join back into a list. This is useful for pushing lists through module outputs since they currently only support string values. Depending on the use, the string this is being performed within may need to be wrapped in brackets to indicate that the output is actually a list, e.g. a_resource_param = ["${split(",", var.CSV_STRING)}"]. Example: split(",", module.amod.server_ids)

Templates

Long strings can be managed using templates. Templates are resources defined by a filename and some variables to use during interpolation. They have a computed rendered attribute containing the result.

A template resource looks like:

resource "template_file" "example" {
    filename = "template.txt"
    vars {
        hello = "goodnight"
        world = "moon"
    }
}

output "rendered" {
    value = "${template_file.example.rendered}"
}

Assuming template.txt looks like this:

${hello} ${world}!

Then the rendered value would be goodnight moon!.

You may use any of the built-in functions in your template.

Using Templates with Count

Here is an example that combines the capabilities of templates with the interpolation from count to give us a parametized template, unique to each resource instance:

variable "count" {
  default = 2
}

variable "hostnames" {
  default = {
    "0" = "example1.org"
    "1" = "example2.net"
  }
}

resource "template_file" "web_init" {
  // here we expand multiple template_files - the same number as we have instances
  count = "${var.count}"
  filename = "templates/web_init.tpl"
  vars {
    // that gives us access to use count.index to do the lookup
    hostname = "${lookup(var.hostnames, count.index)}"
  }
}

resource "aws_instance" "web" {
  // ...
  count = "${var.count}"
  // here we link each web instance to the proper template_file
  user_data = "${element(template_file.web_init.*.rendered, count.index)}"
}

With this, we will build a list of template_file.web_init resources which we can use in combination with our list of aws_instance.web resources.

Math

Simple math can be performed in interpolations:

variable "count" {
  default = 2
}

resource "aws_instance" "web" {
  // ...
  count = "${var.count}"

  // tag the instance with a counter starting at 1, ie. web-001
  tags {
    Name = "${format("web-%03d", count.index + 1)}"
  }
}

The supported operations are:

• Add, Subtract, Multiply, and Divide for float types
• Add, Subtract, Multiply, Divide, and Modulo for integer types

-> Note: Since Terraform allows hyphens in resource and variable names, it's best to use spaces between math operators to prevent confusion or unexpected behavior. For example, ${var.instance-count - 1} will subtract 1 from the instance-count variable value, while ${var.instance-count-1} will interpolate the instance-count-1 variable value.