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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. 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 perform simple math in interpolations, allowing you to write expressions such as ${count.index + 1}. And you can also use conditionals to determine a value based on some logic.

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

Available Variables

There are a variety of available variable references you can use.

User string variables

Use the var. prefix followed by the variable name. For example, ${var.foo} will interpolate the foo variable value.

User map variables

The syntax is var.MAP["KEY"]. For example, ${var.amis["us-east-1"]} would get the value of the us-east-1 key within the amis map variable.

User list variables

The syntax is "${var.LIST}". For example, "${var.subnets}" would get the value of the subnets list, as a list. You can also return list elements by index: ${var.subnets[idx]}.

Attributes of your own resource

The syntax is self.ATTRIBUTE. For example ${self.private_ip} will interpolate that resource's private IP address.

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}.

Attributes of a data source

The syntax is data.TYPE.NAME.ATTRIBUTE. For example. ${data.aws_ami.ubuntu.id} will interpolate the id attribute from the aws_ami data source named ubuntu. If the data source has a count attribute set, you can access individual attributes with a zero-based index, such as ${data.aws_subnet.example.0.cidr_block}. You can also use the splat syntax to get a list of all the attributes: ${data.aws_subnet.example.*.cidr_block}.

Outputs from a module

The syntax is MODULE.NAME.OUTPUT. For example ${module.foo.bar} will interpolate the bar output from the foo module.

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.

Path information

The syntax is path.TYPE. TYPE can be cwd, module, or root. cwd will interpolate the current working directory. 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.

Terraform meta information

The syntax is terraform.FIELD. This variable type contains metadata about the currently executing Terraform run. FIELD can currently only be env to reference the currently active state environment.

Conditionals

Interpolations may contain conditionals to branch on the final value.

resource "aws_instance" "web" {
  subnet = "${var.env == "production" ? var.prod_subnet : var.dev_subnet}"
}

The conditional syntax is the well-known ternary operation:

CONDITION ? TRUEVAL : FALSEVAL

The condition can be any valid interpolation syntax, such as variable access, a function call, or even another conditional. The true and false value can also be any valid interpolation syntax. The returned types by the true and false side must be the same.

The supported operators are:

  • Equality: == and !=
  • Numerical comparison: >, <, >=, <=
  • Boolean logic: &&, ||, unary !

A common use case for conditionals is to enable/disable a resource by conditionally setting the count:

resource "aws_instance" "vpn" {
  count = "${var.something ? 1 : 0}"
}

In the example above, the "vpn" resource will only be included if "var.something" evaluates to true. Otherwise, the VPN resource will not be created at all.

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")}.

Supported built-in functions

The supported built-in functions are:

  • abs(float) - Returns the absolute value of a given float. Example: abs(1) returns 1, and abs(-1) would also return 1, whereas abs(-3.14) would return 3.14. See also the signum function.

  • basename(path) - Returns the last element of a path.

  • base64decode(string) - Given a base64-encoded string, decodes it and returns the original string.

  • base64encode(string) - Returns a base64-encoded representation of the given string.

  • base64gzip(string) - Compresses the given string with gzip and then encodes the result to base64. This can be used with certain resource arguments that allow binary data to be passed with base64 encoding, since Terraform strings are required to be valid UTF-8.

  • base64sha256(string) - Returns a base64-encoded representation of raw SHA-256 sum of the given string. This is not equivalent of base64encode(sha256(string)) since sha256() returns hexadecimal representation.

  • base64sha512(string) - Returns a base64-encoded representation of raw SHA-512 sum of the given string. This is not equivalent of base64encode(sha512(string)) since sha512() returns hexadecimal representation.

  • bcrypt(password, cost) - Returns the Blowfish encrypted hash of the string at the given cost. A default cost of 10 will be used if not provided.

  • ceil(float) - Returns the least integer value greater than or equal to the argument.

  • chomp(string) - Removes trailing newlines from the given string.

  • chunklist(list, size) - Returns the list items chunked by size. Examples:

  • cidrhost(iprange, hostnum) - Takes an IP address range in CIDR notation and creates an IP address with the given host number. If given host number is negative, the count starts from the end of the range. For example, cidrhost("10.0.0.0/8", 2) returns 10.0.0.2 and cidrhost("10.0.0.0/8", -2) returns 10.255.255.254.

  • cidrnetmask(iprange) - Takes an IP address range in CIDR notation and returns the address-formatted subnet mask format that some systems expect for IPv4 interfaces. For example, cidrnetmask("10.0.0.0/8") returns 255.0.0.0. Not applicable to IPv6 networks since CIDR notation is the only valid notation for IPv6.

  • cidrsubnet(iprange, newbits, netnum) - Takes an IP address range in CIDR notation (like 10.0.0.0/8) and extends its prefix to include an additional subnet number. For example, cidrsubnet("10.0.0.0/8", 8, 2) returns 10.2.0.0/16; cidrsubnet("2607:f298:6051:516c::/64", 8, 2) returns 2607:f298:6051:516c:200::/72.

  • coalesce(string1, string2, ...) - Returns the first non-empty value from the given arguments. At least two arguments must be provided.

  • coalescelist(list1, list2, ...) - Returns the first non-empty list from the given arguments. At least two arguments must be provided.

  • compact(list) - Removes empty string elements from a list. This can be useful in some cases, for example when passing joined lists as module variables or when parsing module outputs. Example: compact(module.my_asg.load_balancer_names)

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

  • contains(list, element) - Returns true if a list contains the given element and returns false otherwise. Examples: contains(var.list_of_strings, "an_element")

  • dirname(path) - Returns all but the last element of path, typically the path's directory.

  • distinct(list) - Removes duplicate items from a list. Keeps the first occurrence of each element, and removes subsequent occurrences. This function is only valid for flat lists. Example: distinct(var.usernames)

  • 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. This function only works on flat lists. Examples:

  • 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. The path is interpreted relative to the working directory. Path variables can be used to reference paths relative to other base locations. For example, when using file() from inside a module, you generally want to make the path relative to the module base, like this: file("${path.module}/file").

  • floor(float) - Returns the greatest integer value less than or equal to the argument.

  • flatten(list of lists) - Flattens lists of lists down to a flat list of primitive values, eliminating any nested lists recursively. Examples:

  • 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.

  • indent(numspaces, string) - Prepends the specified number of spaces to all but the first line of the given multi-line string. May be useful when inserting a multi-line string into an already-indented context. The first line is not indented, to allow for the indented string to be placed after some sort of already-indented preamble. Example: " \"items\": ${ indent(4, "[\n \"item1\"\n]") },"

  • index(list, elem) - Finds the index of a given element in a list. This function only works on flat lists. Example: index(aws_instance.foo.*.tags.Name, "foo-test")

  • join(delim, list) - Joins the list with the delimiter for a resultant string. This function works only on flat lists. Examples:

  • jsonencode(value) - Returns a JSON-encoded representation of the given value, which can contain arbitrarily-nested lists and maps. Note that if the value is a string then its value will be placed in quotes.

  • keys(map) - Returns a lexically sorted list of the map keys.

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

  • list(items, ...) - Returns a list consisting of the arguments to the function. This function provides a way of representing list literals in interpolation.

  • log(x, base) - Returns the logarithm of x.

  • lookup(map, key, [default]) - Performs a dynamic lookup into a map variable. The map parameter should be another variable, such as var.amis. If key does not exist in map, the interpolation will fail unless you specify a third argument, default, which should be a string value to return if no key is found in map. This function only works on flat maps and will return an error for maps that include nested lists or maps.

  • lower(string) - Returns a copy of the string with all Unicode letters mapped to their lower case.

  • map(key, value, ...) - Returns a map consisting of the key/value pairs specified as arguments. Every odd argument must be a string key, and every even argument must have the same type as the other values specified. Duplicate keys are not allowed. Examples:

  • matchkeys(values, keys, searchset) - For two lists values and keys of equal length, returns all elements from values where the corresponding element from keys exists in the searchset list. E.g. matchkeys(aws_instance.example.*.id, aws_instance.example.*.availability_zone, list("us-west-2a")) will return a list of the instance IDs of the aws_instance.example instances in "us-west-2a". No match will result in empty list. Items of keys are processed sequentially, so the order of returned values is preserved.

  • max(float1, float2, ...) - Returns the largest of the floats.

  • merge(map1, map2, ...) - Returns the union of 2 or more maps. The maps are consumed in the order provided, and duplicate keys overwrite previous entries.

  • min(float1, float2, ...) - Returns the smallest of the floats.

  • md5(string) - Returns a (conventional) hexadecimal representation of the MD5 hash of the given string.

  • pathexpand(string) - Returns a filepath string with ~ expanded to the home directory. Note: This will create a plan diff between two different hosts, unless the filepaths are the same.

  • pow(x, y) - Returns the base x of exponential y as a float.

    Example:

  • 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.

  • rsadecrypt(string, key) - Decrypts string using RSA. The padding scheme PKCS #1 v1.5 is used. The string must be base64-encoded. key must be an RSA private key in PEM format. You may use file() to load it from a file.

  • sha1(string) - Returns a (conventional) hexadecimal representation of the SHA-1 hash of the given string. Example: "${sha1("${aws_vpc.default.tags.customer}-s3-bucket")}"

  • sha256(string) - Returns a (conventional) hexadecimal representation of the SHA-256 hash of the given string. Example: "${sha256("${aws_vpc.default.tags.customer}-s3-bucket")}"

  • sha512(string) - Returns a (conventional) hexadecimal representation of the SHA-512 hash of the given string. Example: "${sha512("${aws_vpc.default.tags.customer}-s3-bucket")}"

  • signum(integer) - Returns -1 for negative numbers, 0 for 0 and 1 for positive numbers. This function is useful when you need to set a value for the first resource and a different value for the rest of the resources. Example: element(split(",", var.r53_failover_policy), signum(count.index)) where the 0th index points to PRIMARY and 1st to FAILOVER

  • slice(list, from, to) - Returns the portion of list between from (inclusive) and to (exclusive). Example: slice(var.list_of_strings, 0, length(var.list_of_strings) - 1)

  • sort(list) - Returns a lexographically sorted list of the strings contained in the list passed as an argument. Sort may only be used with lists which contain only strings. Examples: sort(aws_instance.foo.*.id), sort(var.list_of_strings)

  • 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)

  • substr(string, offset, length) - Extracts a substring from the input string. A negative offset is interpreted as being equivalent to a positive offset measured backwards from the end of the string. A length of -1 is interpreted as meaning "until the end of the string".

  • timestamp() - Returns a UTC timestamp string in RFC 3339 format. This string will change with every invocation of the function, so in order to prevent diffs on every plan & apply, it must be used with the ignore_changes lifecycle attribute.

  • timeadd(time, duration) - Returns a UTC timestamp string corresponding to adding a given duration to time in RFC 3339 format.
    For example, timeadd("2017-11-22T00:00:00Z", "10m") produces a value "2017-11-22T00:10:00Z".

  • title(string) - Returns a copy of the string with the first characters of all the words capitalized.

  • transpose(map) - Swaps the keys and list values in a map of lists of strings. For example, transpose(map("a", list("1", "2"), "b", list("2", "3")) produces a value equivalent to map("1", list("a"), "2", list("a", "b"), "3", list("b")).

  • trimspace(string) - Returns a copy of the string with all leading and trailing white spaces removed.

  • upper(string) - Returns a copy of the string with all Unicode letters mapped to their upper case.

  • urlencode(string) - Returns an URL-safe copy of the string.

  • uuid() - Returns a UUID string in RFC 4122 v4 format. This string will change with every invocation of the function, so in order to prevent diffs on every plan & apply, it must be used with the ignore_changes lifecycle attribute.

  • values(map) - Returns a list of the map values, in the order of the keys returned by the keys function. This function only works on flat maps and will return an error for maps that include nested lists or maps.

  • zipmap(list, list) - Creates a map from a list of keys and a list of values. The keys must all be of type string, and the length of the lists must be the same. For example, to output a mapping of AWS IAM user names to the fingerprint of the key used to encrypt their initial password, you might use: zipmap(aws_iam_user.users.*.name, aws_iam_user_login_profile.users.*.key_fingerprint).

Templates

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

A template data source looks like:

data "template_file" "example" {
  template = "$${hello} $${world}!"
  vars {
    hello = "goodnight"
    world = "moon"
  }
}

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

Then the rendered value would be goodnight moon!.

You may use any of the built-in functions in your template. For more details on template usage, please see the template_file documentation.

Using Templates with Count

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

variable "count" {
  default = 2
}

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

data "template_file" "web_init" {
  # Render the template once for each instance
  count    = "${length(var.hostnames)}"
  template = "${file("templates/web_init.tpl")}"
  vars {
    # count.index tells us the index of the instance we are rendering
    hostname = "${var.hostnames[count.index]}"
  }
}

resource "aws_instance" "web" {
  # Create one instance for each hostname
  count     = "${length(var.hostnames)}"

  # Pass each instance its corresponding template_file
  user_data = "${data.template_file.web_init.*.rendered[count.index]}"
}

With this, we will build a list of template_file.web_init data 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

Operator precedences is the standard mathematical order of operations: Multiply (*), Divide (/), and Modulo (%) have precedence over Add (+) and Subtract (-). Parenthesis can be used to force ordering.

"${2 * 4 + 3 * 3}" # computes to 17
"${3 * 3 + 2 * 4}" # computes to 17
"${2 * (4 + 3) * 3}" # computes to 42

You can use the terraform console command to try the math operations.

© 2018 HashiCorpLicensed under the MPL 2.0 License.
https://www.terraform.io/docs/configuration/interpolation.html