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| layout | page_title | sidebar_current |
|---|---|---|
| docs | Interpolation Syntax | docs-config-interpolation |
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.