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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.

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(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.