168 lines
6.6 KiB
Markdown
168 lines
6.6 KiB
Markdown
---
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layout: "docs"
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page_title: "Configuration Language"
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sidebar_current: "docs-config-index"
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description: |-
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Terraform uses text files to describe infrastructure and to set variables.
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These text files are called Terraform _configurations_ and are
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written in the Terraform language.
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---
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# Configuration
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Terraform uses its own configuration language, designed to allow concise
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descriptions of infrastructure. The Terraform language is declarative,
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describing an intended goal rather than the steps to reach that goal.
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## Resources and Modules
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The main purpose of the Terraform language is declaring [resources](./resources.html).
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All other language features exist only to make the definition of resources
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more flexible and convenient.
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A group of resources can be gathered into a [module](./modules.html),
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which creates a larger unit of configuration. A resource describes a single
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infrastructure object, while a module might describe a set of objects and the
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necessary relationships between them in order to create a higher-level system.
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A _Terraform configuration_ consists of a _root module_, where evaluation
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begins, along with a tree of child modules created when one module calls
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another.
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## Arguments, Blocks, and Expressions
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The syntax of the Terraform language consists of only a few basic elements:
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```hcl
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resource "aws_vpc" "main" {
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cidr_block = var.base_cidr_block
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}
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<BLOCK TYPE> "<BLOCK LABEL>" "<BLOCK LABEL>" {
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# Block body
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<IDENTIFIER> = <EXPRESSION> # Argument
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}
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```
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- _Blocks_ are containers for other content and usually represent the
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configuration of some kind of object, like a resource. Blocks have a
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_block type,_ can have zero or more _labels,_ and have a _body_ that contains
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any number of arguments and nested blocks. Most of Terraform's features are
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controlled by top-level blocks in a configuration file.
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- _Arguments_ assign a value to a name. They appear within blocks.
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- _Expressions_ represent a value, either literally or by referencing and
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combining other values. They appear as values for arguments, or within other
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expressions.
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For full details about Terraform's syntax, see:
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- [Configuration Syntax](./syntax.html)
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- [Expressions](./expressions.html)
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## Code Organization
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The Terraform language uses configuration files that are named with the `.tf`
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file extension. There is also [a JSON-based variant of the language](./syntax-json.html)
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that is named with the `.tf.json` file extension.
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Configuration files must always use UTF-8 encoding, and by convention are
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usually maintained with Unix-style line endings (LF) rather than Windows-style
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line endings (CRLF), though both are accepted.
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A _module_ is a collection of `.tf` or `.tf.json` files kept together in a
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directory. The root module is built from the configuration files in the
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current working directory when Terraform is run, and this module may reference
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child modules in other directories, which can in turn reference other modules,
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etc.
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The simplest Terraform configuration is a single root module containing only
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a single `.tf` file. A configuration can grow gradually as more resources
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are added, either by creating new configuration files within the root module
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or by organizing sets of resources into child modules.
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## Configuration Ordering
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Because Terraform's configuration language is declarative, the ordering of
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blocks is generally not significant. (The order of `provisioner` blocks within a
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resource is the only major feature where block order matters.)
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Terraform automatically processes resources in the correct order based on
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relationships defined between them in configuration, and so you can organize
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resources into source files in whatever way makes sense for your infrastructure.
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## Terraform CLI vs. Providers
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The Terraform command line interface (CLI) is a general engine for evaluating
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and applying Terraform configuations. It defines the Terraform language syntax
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and overall structure, and coordinates sequences of changes that must be made to
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make remote infrastructure match the given configuration.
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This general engine has no knowledge about specific types of infrastructure
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objects. Instead, Terraform uses plugins called
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[providers](./providers.html) that each define and manage a
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set of resource types. Most providers are associated with a particular cloud or
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on-premises infrastructure service, allowing Terraform to manage infrastructure
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objects within that service.
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Terraform doesn't have a concept of platform-independent resource types
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— resources are always tied to a provider, since the features of similar
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resources can vary greatly from provider to provider. But Terraform CLI's shared
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configuration engine ensures that the same language constructs and syntax are
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available across all services and allows resource types from different services
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to be combined as needed.
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## Example
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The following simple example describes a simple network topology for Amazon Web
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Services, just to give a sense of the overall structure and syntax of the
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Terraform language. Similar configurations can be created for other virtual
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network services, using resource types defined by other providers, and a
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practical network configuration will often contain additional elements not
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shown here.
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```hcl
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variable "aws_region" {}
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variable "base_cidr_block" {
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description = "A /16 CIDR range definition, such as 10.1.0.0/16, that the VPC will use"
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default = "10.1.0.0/16"
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}
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variable "availability_zones" {
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description = "A list of availability zones in which to create subnets"
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type = list(string)
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}
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provider "aws" {
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region = var.aws_region
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}
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resource "aws_vpc" "main" {
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# Referencing the base_cidr_block variable allows the network address
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# to be changed without modifying the configuration.
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cidr_block = var.base_cidr_block
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}
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resource "aws_subnet" "az" {
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# Create one subnet for each given availability zone.
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count = length(var.availability_zones)
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# For each subnet, use one of the specified availability zones.
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availability_zone = var.availability_zones[count.index]
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# By referencing the aws_vpc.main object, Terraform knows that the subnet
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# must be created only after the VPC is created.
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vpc_id = aws_vpc.main.id
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# Built-in functions and operators can be used for simple transformations of
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# values, such as computing a subnet address. Here we create a /20 prefix for
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# each subnet, using consecutive addresses for each availability zone,
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# such as 10.1.16.0/20 .
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cidr_block = cidrsubnet(aws_vpc.main.cidr_block, 4, count.index+1)
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}
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```
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For more information on the configuration elements shown here, use the
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site navigation to explore the Terraform language documentation sub-sections.
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To start, see [_Resource Configuration_](./resources.html).
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