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

---
layout: "docs"
page_title: "Interpolation Syntax"
sidebar_current: "docs-config-interpolation"
description: |-
  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.

## 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 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](/docs/configuration/resources.html).

**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](/docs/modules/index.html).

**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(args...)` - Concatenates the values of multiple arguments into
      a single string.

  * `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.

  * `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)`

  * `lookup(map, key)` - Performs a dynamic lookup into a mapping
      variable.