147 lines
5.9 KiB
Markdown
147 lines
5.9 KiB
Markdown
---
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layout: "docs"
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page_title: "Interpolation Syntax"
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sidebar_current: "docs-config-interpolation"
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description: |-
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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}`.
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---
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# Interpolation Syntax
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Embedded within strings in Terraform, whether you're using the
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Terraform syntax or JSON syntax, you can interpolate other values
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into strings. These interpolations are wrapped in `${}`, such as
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`${var.foo}`.
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The interpolation syntax is powerful and allows you to reference
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variables, attributes of resources, call functions, etc.
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You can also perform simple math in interpolations, allowing
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you to write expressions such as `${count.index+1}`.
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## Available Variables
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**To reference user variables**, use the `var.` prefix followed by the
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variable name. For example, `${var.foo}` will interpolate the
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`foo` variable value. If the variable is a mapping, then you
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can reference static keys in the map with the syntax
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`var.MAP.KEY`. For example, `${var.amis.us-east-1}` would
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get the value of the `us-east-1` key within the `amis` variable
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that is a mapping.
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**To reference attributes of your own resource**, the syntax is
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`self.ATTRIBUTE`. For example `${self.private_ip_address}` will
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interpolate that resource's private IP address. Note that this is
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only allowed/valid within provisioners.
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**To reference attributes of other resources**, the syntax is
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`TYPE.NAME.ATTRIBUTE`. For example, `${aws_instance.web.id}`
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will interpolate the ID attribute from the "aws\_instance"
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resource named "web". If the resource has a `count` attribute set,
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you can access individual attributes with a zero-based index, such
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as `${aws_instance.web.0.id}`. You can also use the splat syntax
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to get a list of all the attributes: `${aws_instance.web.*.id}`.
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This is documented in more detail in the
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[resource configuration page](/docs/configuration/resources.html).
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**To reference outputs from a module**, the syntax is
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`MODULE.NAME.OUTPUT`. For example `${module.foo.bar}` will
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interpolate the "bar" output from the "foo"
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[module](/docs/modules/index.html).
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**To reference count information**, the syntax is `count.FIELD`.
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For example, `${count.index}` will interpolate the current index
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in a multi-count resource. For more information on count, see the
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resource configuration page.
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**To reference path information**, the syntax is `path.TYPE`.
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TYPE can be `cwd`, `module`, or `root`. `cwd` will interpolate the
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cwd. `module` will interpolate the path to the current module. `root`
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will interpolate the path of the root module. In general, you probably
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want the `path.module` variable.
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## Built-in Functions
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Terraform ships with built-in functions. Functions are called with
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the syntax `name(arg, arg2, ...)`. For example,
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to read a file: `${file("path.txt")}`. The built-in functions
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are documented below.
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The supported built-in functions are:
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* `concat(args...)` - Concatenates the values of multiple arguments into
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a single string.
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* `element(list, index)` - Returns a single element from a list
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at the given index. If the index is greater than the number of
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elements, this function will wrap using a standard mod algorithm.
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A list is only possible with splat variables from resources with
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a count greater than one.
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Example: `element(aws_subnet.foo.*.id, count.index)`
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* `file(path)` - Reads the contents of a file into the string. Variables
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in this file are _not_ interpolated. The contents of the file are
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read as-is.
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* `format(format, args...)` - Formats a string according to the given
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format. The syntax for the format is standard `sprintf` syntax.
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Good documentation for the syntax can be [found here](http://golang.org/pkg/fmt/).
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Example to zero-prefix a count, used commonly for naming servers:
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`format("web-%03d", count.index+1)`.
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* `join(delim, list)` - Joins the list with the delimiter. A list is
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only possible with splat variables from resources with a count
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greater than one. Example: `join(",", aws_instance.foo.*.id)`
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* `length(list)` - Returns a number of members in a given list
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or a number of characters in a given string.
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* `${length(split(",", "a,b,c"))}` = 3
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* `${length("a,b,c")}` = 5
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* `lookup(map, key)` - Performs a dynamic lookup into a mapping
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variable. The `map` parameter should be another variable, such
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as `var.amis`.
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* `replace(string, search, replace)` - Does a search and replace on the
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given string. All instances of `search` are replaced with the value
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of `replace`. If `search` is wrapped in forward slashes, it is treated
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as a regular expression. If using a regular expression, `replace`
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can reference subcaptures in the regular expression by using `$n` where
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`n` is the index or name of the subcapture. If using a regular expression,
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the syntax conforms to the [re2 regular expression syntax](https://code.google.com/p/re2/wiki/Syntax).
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* `split(delim, string)` - Splits the string previously created by `join`
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back into a list. This is useful for pushing lists through module
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outputs since they currently only support string values.
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Example: `split(",", module.amod.server_ids)`
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## Templates
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Long strings can be managed using templates. Templates are [resources](/docs/configuration/resources.html) defined by a filename and some variables to use during interpolation. They have a computed `rendered` attribute containing the result.
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A template resource looks like:
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```
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resource "template_file" "example" {
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filename = "template.txt"
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vars {
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hello = "goodnight"
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world = "moon"
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}
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}
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output "rendered" {
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value = "${template_file.example.rendered}"
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}
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```
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Assuming `template.txt` looks like this:
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```
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${hello} ${world}!
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```
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Then the rendered value would be `goodnight moon!`.
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You may use any of the built-in functions in your template.
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