398 lines
16 KiB
Markdown
398 lines
16 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. These
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interpolations are wrapped in `${}`, such as `${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](#math) in interpolations, allowing
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you to write expressions such as `${count.index + 1}`.
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You can escape interpolation with double dollar signs: `$${foo}`
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will be rendered as a literal `${foo}`.
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## Available Variables
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There are a variety of available variable references you can use.
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### User string variables
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Use the `var.` prefix followed by the variable name. For example,
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`${var.foo}` will interpolate the `foo` variable value.
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### User map variables
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The syntax is `var.MAP["KEY"]`. For example, `${var.amis["us-east-1"]}`
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would get the value of the `us-east-1` key within the `amis` map
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variable.
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### User list variables
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The syntax is `["${var.LIST}"]`. For example, `["${var.subnets}"]`
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would get the value of the `subnets` list, as a list. You can also
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return list elements by index: `${var.subnets[idx]}`.
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### Attributes of your own resource
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The syntax is `self.ATTRIBUTE`. For example `${self.private_ip_address}`
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will interpolate that resource's private IP address.
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-> **Note**: The `self.ATTRIBUTE` syntax is only allowed and valid within
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provisioners.
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### Attributes of other resources
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The syntax is `TYPE.NAME.ATTRIBUTE`. For example,
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`${aws_instance.web.id}` will interpolate the ID attribute from the
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`aws\_instance` resource named `web`. If the resource has a `count`
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attribute set, you can access individual attributes with a zero-based
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index, such as `${aws_instance.web.0.id}`. You can also use the splat
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syntax to get a list of all the attributes: `${aws_instance.web.*.id}`.
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This is documented in more detail in the [resource configuration
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page](/docs/configuration/resources.html).
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### Outputs from a module
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The syntax is `MODULE.NAME.OUTPUT`. For example `${module.foo.bar}` will
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interpolate the `bar` output from the `foo`
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[module](/docs/modules/index.html).
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### Count information
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The syntax is `count.FIELD`. For example, `${count.index}` will
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interpolate the current index in a multi-count resource. For more
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information on `count`, see the [resource configuration
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page](/docs/configuration/resources.html).
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<a id="path-variables"></a>
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### Path information
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The syntax is `path.TYPE`. TYPE can be `cwd`, `module`, or `root`.
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`cwd` will interpolate the current working directory. `module` will
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interpolate the path to the current module. `root` will interpolate the
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path of the root module. In general, you probably want the
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`path.module` variable.
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<a id="functions"></a>
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## Built-in Functions
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Terraform ships with built-in functions. Functions are called with the
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syntax `name(arg, arg2, ...)`. For example, to read a file:
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`${file("path.txt")}`.
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### Supported built-in functions
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The supported built-in functions are:
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* `base64decode(string)` - Given a base64-encoded string, decodes it and
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returns the original string.
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* `base64encode(string)` - Returns a base64-encoded representation of the
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given string.
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* `base64sha256(string)` - Returns a base64-encoded representation of raw
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SHA-256 sum of the given string.
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**This is not equivalent** of `base64encode(sha256(string))`
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since `sha256()` returns hexadecimal representation.
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* `ceil(float)` - Returns the least integer value greater than or equal
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to the argument.
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* `cidrhost(iprange, hostnum)` - Takes an IP address range in CIDR notation
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and creates an IP address with the given host number. For example,
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`cidrhost("10.0.0.0/8", 2)` returns `10.0.0.2`.
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* `cidrnetmask(iprange)` - Takes an IP address range in CIDR notation
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and returns the address-formatted subnet mask format that some
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systems expect for IPv4 interfaces. For example,
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`cidrmask("10.0.0.0/8")` returns `255.0.0.0`. Not applicable
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to IPv6 networks since CIDR notation is the only valid notation for
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IPv4.
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* `cidrsubnet(iprange, newbits, netnum)` - Takes an IP address range in
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CIDR notation (like `10.0.0.0/8`) and extends its prefix to include an
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additional subnet number. For example,
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`cidrsubnet("10.0.0.0/8", 8, 2)` returns `10.2.0.0/16`;
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`cidrsubnet("2607:f298:6051:516c::/64", 8, 2)` returns
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`2607:f298:6051:516c:200::/72`.
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* `coalesce(string1, string2, ...)` - Returns the first non-empty value from
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the given arguments. At least two arguments must be provided.
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* `compact(list)` - Removes empty string elements from a list. This can be
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useful in some cases, for example when passing joined lists as module
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variables or when parsing module outputs.
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Example: `compact(module.my_asg.load_balancer_names)`
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* `concat(list1, list2, ...)` - Combines two or more lists into a single list.
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Example: `concat(aws_instance.db.*.tags.Name, aws_instance.web.*.tags.Name)`
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* `distinct(list)` - Removes duplicate items from a list. Keeps the first
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occurrence of each element, and removes subsequent occurrences. This
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function is only valid for flat lists. Example: `distinct(var.usernames)`
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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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This function only works on flat lists. Examples:
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* `element(aws_subnet.foo.*.id, count.index)`
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* `element(var.list_of_strings, 2)`
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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. The `path` is interpreted relative to the working directory.
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[Path variables](#path-variables) can be used to reference paths relative
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to other base locations. For example, when using `file()` from inside a
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module, you generally want to make the path relative to the module base,
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like this: `file("${path.module}/file")`.
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* `floor(float)` - Returns the greatest integer value less than or equal to
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the argument.
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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](https://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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* `formatlist(format, args, ...)` - Formats each element of a list
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according to the given format, similarly to `format`, and returns a list.
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Non-list arguments are repeated for each list element.
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For example, to convert a list of DNS addresses to a list of URLs, you might use:
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`formatlist("https://%s:%s/", aws_instance.foo.*.public_dns, var.port)`.
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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.
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Example:
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`formatlist("instance %v has private ip %v", aws_instance.foo.*.id, aws_instance.foo.*.private_ip)`.
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Passing lists with different lengths to formatlist results in an error.
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* `index(list, elem)` - Finds the index of a given element in a list.
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This function only works on flat lists.
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Example: `index(aws_instance.foo.*.tags.Name, "foo-test")`
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* `join(delim, list)` - Joins the list with the delimiter for a resultant string.
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This function works only on flat lists.
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Examples:
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* `join(",", aws_instance.foo.*.id)`
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* `join(",", var.ami_list)`
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* `jsonencode(item)` - Returns a JSON-encoded representation of the given
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item, which may be a string, list of strings, or map from string to string.
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Note that if the item is a string, the return value includes the double
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quotes.
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* `keys(map)` - Returns a lexically sorted list of the map keys.
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* `length(list)` - Returns the number of members in a given list or map, or the 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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* `${length(map("key", "val"))}` = 1
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* `list(items, ...)` - Returns a list consisting of the arguments to the function.
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This function provides a way of representing list literals in interpolation.
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* `${list("a", "b", "c")}` returns a list of `"a", "b", "c"`.
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* `${list()}` returns an empty list.
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* `lookup(map, key [, default])` - Performs a dynamic lookup into a map
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variable. The `map` parameter should be another variable, such
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as `var.amis`. If `key` does not exist in `map`, the interpolation will
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fail unless you specify a third argument, `default`, which should be a
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string value to return if no `key` is found in `map`. This function
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only works on flat maps and will return an error for maps that
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include nested lists or maps.
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* `lower(string)` - Returns a copy of the string with all Unicode letters mapped to their lower case.
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* `map(key, value, ...)` - Returns a map consisting of the key/value pairs
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specified as arguments. Every odd argument must be a string key, and every
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even argument must have the same type as the other values specified.
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Duplicate keys are not allowed. Examples:
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* `map("hello", "world")`
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* `map("us-east", list("a", "b", "c"), "us-west", list("b", "c", "d"))`
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* `max(float1, float2, ...)` - Returns the largest of the floats.
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* `merge(map1, map2, ...)` - Returns the union of 2 or more maps. The maps
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are consumed in the order provided, and duplicate keys overwrite previous
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entries.
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* `${merge(map("a", "b"), map("c", "d"))}` returns `{"a": "b", "c": "d"}`
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* `min(float1, float2, ...)` - Returns the smallest of the floats.
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* `md5(string)` - Returns a (conventional) hexadecimal representation of the
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MD5 hash of the given string.
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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 `$n` where
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`n` is the index or name of the subcapture. If using a regular expression,
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the syntax conforms to the [re2 regular expression syntax](https://code.google.com/p/re2/wiki/Syntax).
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* `sha1(string)` - Returns a (conventional) hexadecimal representation of the
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SHA-1 hash of the given string.
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Example: `"${sha1("${aws_vpc.default.tags.customer}-s3-bucket")}"`
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* `sha256(string)` - Returns a (conventional) hexadecimal representation of the
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SHA-256 hash of the given string.
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Example: `"${sha256("${aws_vpc.default.tags.customer}-s3-bucket")}"`
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* `signum(int)` - Returns `-1` for negative numbers, `0` for `0` and `1` for positive numbers.
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This function is useful when you need to set a value for the first resource and
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a different value for the rest of the resources.
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Example: `element(split(",", var.r53_failover_policy), signum(count.index))`
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where the 0th index points to `PRIMARY` and 1st to `FAILOVER`
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* `sort(list)` - Returns a lexicographically sorted list of the strings contained in
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the list passed as an argument. Sort may only be used with lists which contain only
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strings.
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Examples: `sort(aws_instance.foo.*.id)`, `sort(var.list_of_strings)`
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* `split(delim, string)` - Splits the string previously created by `join`
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back into a list. This is useful for pushing lists through module
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outputs since they currently only support string values. Depending on the
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use, the string this is being performed within may need to be wrapped
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in brackets to indicate that the output is actually a list, e.g.
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`a_resource_param = ["${split(",", var.CSV_STRING)}"]`.
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Example: `split(",", module.amod.server_ids)`
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* `title(string)` - Returns a copy of the string with the first characters of all the words capitalized.
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* `trimspace(string)` - Returns a copy of the string with all leading and trailing white spaces removed.
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* `upper(string)` - Returns a copy of the string with all Unicode letters mapped to their upper case.
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* `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`](/docs/configuration/resources.html#ignore-changes) lifecycle attribute.
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* `values(map)` - Returns a list of the map values, in the order of the keys
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returned by the `keys` function. This function only works on flat maps and
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will return an error for maps that include nested lists or maps.
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* `zipmap(list, list)` - Creates a map from a list of keys and a list of
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values. The keys must all be of type string, and the length of the lists
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must be the same.
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For example, to output a mapping of AWS IAM user names to the fingerprint
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of the key used to encrypt their initial password, you might use:
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`zipmap(aws_iam_user.users.*.name, aws_iam_user_login_profile.users.*.key_fingerprint)`.
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<a id="templates"></a>
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## Templates
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Long strings can be managed using templates.
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[Templates](/docs/providers/template/index.html) are
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[data-sources](/docs/configuration/data-sources.html) defined by a
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filename and some variables to use during interpolation. They have a
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computed `rendered` attribute containing the result.
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A template data source looks like:
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```
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data "template_file" "example" {
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template = "${hello} ${world}!"
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vars {
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hello = "goodnight"
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world = "moon"
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}
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}
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output "rendered" {
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value = "${data.template_file.example.rendered}"
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}
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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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### Using Templates with Count
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Here is an example that combines the capabilities of templates with the interpolation
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from `count` to give us a parameterized template, unique to each resource instance:
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```
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variable "count" {
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default = 2
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}
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variable "hostnames" {
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default = {
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"0" = "example1.org"
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"1" = "example2.net"
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}
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}
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data "template_file" "web_init" {
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// here we expand multiple template_files - the same number as we have instances
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count = "${var.count}"
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template = "${file("templates/web_init.tpl")}"
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vars {
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// that gives us access to use count.index to do the lookup
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hostname = "${lookup(var.hostnames, count.index)}"
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}
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}
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resource "aws_instance" "web" {
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// ...
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count = "${var.count}"
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// here we link each web instance to the proper template_file
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user_data = "${element(data.template_file.web_init.*.rendered, count.index)}"
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}
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```
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With this, we will build a list of `template_file.web_init` data sources which we can
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use in combination with our list of `aws_instance.web` resources.
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<a id="math"></a>
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## Math
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Simple math can be performed in interpolations:
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```
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variable "count" {
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default = 2
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}
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resource "aws_instance" "web" {
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// ...
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count = "${var.count}"
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// tag the instance with a counter starting at 1, ie. web-001
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tags {
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Name = "${format("web-%03d", count.index + 1)}"
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}
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}
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```
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The supported operations are:
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- *Add* (`+`), *Subtract* (`-`), *Multiply* (`*`), and *Divide* (`/`) for **float** types
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- *Add* (`+`), *Subtract* (`-`), *Multiply* (`*`), *Divide* (`/`), and *Modulo* (`%`) for **integer** types
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-> **Note:** Since Terraform allows hyphens in resource and variable names,
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it's best to use spaces between math operators to prevent confusion or unexpected
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behavior. For example, `${var.instance-count - 1}` will subtract **1** from the
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`instance-count` variable value, while `${var.instance-count-1}` will interpolate
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the `instance-count-1` variable value.
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-> **Note:** Operator precedence is not the usual one where *Multiply* (`*`),
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*Divide* (`/`), and *Modulo* (`%`) have precedence over *Add* (`+`) and *Subtract* (`-`).
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The operations are made in the order they appear. Parenthesis can be used to force ordering :
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```
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"${2 * 4 + 3 * 3}" # computes to 33
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"${3 * 3 + 2 * 4}" # computes to 44
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"${(2 * 4) + (3 * 3)}" # computes to 17
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"${(3 * 3) + (2 * 4)}" # computes to 17
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```
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