document hcl language & syntax
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Adrien Delorme
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---
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layout: "docs"
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page_title: "Configuration Language"
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sidebar_current: "configuration"
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description: |-
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Packer uses text files to describe infrastructure and to set variables.
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These text files are called Packer _configurations_ and are
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written in the HCL language.
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---
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# HCL Configuration Language
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Packer uses the Hashicorp Configuration Language - HCL - designed to allow
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concise descriptions of the required steps to get to a build file.
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## Builds
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The main purpose of the HCL language is defining builds and sources. All other
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language features exist only to make the definition of builds more flexible and
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convenient.
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`packer build` takes one argument. When a directory is passed, all files in the
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folder with a name ending with ".pkr.hcl" or ".pkr.json" will be parsed using
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the HCL2 format. When a file ending with ".pkr.hcl" or ".pkr.json" is passed it
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will be parsed using the HCL2 schema. For every other case; the *JSON only* old
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packer schema will be used.
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## Arguments, Blocks, and Expressions
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The syntax of the HCL language consists of only a few basic elements:
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```hcl
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source "amazon-ebs" "main" {
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ami_name = "main-ami"
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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 source. 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 Packer'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 Packer'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 HCL language uses configuration files that are named with the `.pkr.hcl`
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file extension. There is also [a JSON-based variant of the
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language](./syntax-json.html) that is named with the `.pkr.json` file
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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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## Configuration Ordering
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The ordering of root blocks is not significant. The order of `provisioner` or
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`post-processor` blocks within a `build` is the only major feature where block
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order matters.
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---
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layout: "docs"
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page_title: "Syntax - Configuration Language"
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sidebar_current: configuration-syntax
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description: |-
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HCL has its own syntax, intended to combine declarative
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structure with expressions in a way that is easy for humans to read and
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understand.
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---
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# HCL Configuration Syntax
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Other pages in this section have described various configuration constructs
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that can appear in HCL. This page describes the lower-level syntax of the
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language in more detail, revealing the building blocks that those constructs
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are built from.
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This page describes the _native syntax_ of HCL, which is a rich language
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designed to be easy for humans to read and write. The constructs in HCL can
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also be expressed in [JSON syntax](/syntax-json.html), which is harder for
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humans to read and edit but easier to generate and parse programmatically.
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This low-level syntax of HCL is defined in terms of a syntax called _HCL_,
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which is also used by configuration languages in other applications, and in
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particular other HashiCorp products. It is not necessary to know all of the
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details of HCL in order to use Packer, and so this page summarizes the most
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important details. If you are interested, you can find a full definition of HCL
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syntax in [the HCL native syntax
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specification](https://github.com/hashicorp/hcl/blob/hcl2/hclsyntax/spec.md).
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## Arguments and Blocks
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HCL syntax is built around two key syntax constructs:
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arguments and blocks.
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### Arguments
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An _argument_ assigns a value to a particular name:
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```hcl
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image_id = "abc123"
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```
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The identifier before the equals sign is the _argument name_, and the expression
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after the equals sign is the argument's value.
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The context where the argument appears determines what value types are valid
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(for example, each source type has a schema that defines the types of its
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arguments), but many arguments accept arbitrary
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[expressions](/expressions.html), which allow the value to
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either be specified literally or generated from other values programmatically.
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### Blocks
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A _block_ is a container for other content:
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```hcl
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source "amazon-ebs" "example" {
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ami_name = "abc123"
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tags {
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# ...
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}
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}
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```
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A block has a _type_ (`source` in this example). Each block type defines
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how many _labels_ must follow the type keyword. The `source` block type
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expects two labels, which are `amazon-ebs` and `example` in the example above.
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A particular block type may have any number of required labels, or it may
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require none as with the nested `tags` block type.
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After the block type keyword and any labels, the block _body_ is delimited
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by the `{` and `}` characters. Within the block body, further arguments
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and blocks may be nested, creating a hierarchy of blocks and their associated
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arguments.
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HCL uses a limited number of _top-level block types,_ which
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are blocks that can appear outside of any other block in a configuration file.
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Most of Packer's features (including resources, input variables, output
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values, data sources, etc.) are implemented as top-level blocks.
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## Identifiers
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Argument names, block type names, and the names of most Packer-specific
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constructs like resources, input variables, etc. are all _identifiers_.
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Identifiers can contain letters, digits, underscores (`_`), and hyphens (`-`).
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The first character of an identifier must not be a digit, to avoid ambiguity
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with literal numbers.
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For complete identifier rules, Packer implements
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[the Unicode identifier syntax](http://unicode.org/reports/tr31/), extended to
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include the ASCII hyphen character `-`.
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## Comments
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HCL supports three different syntaxes for comments:
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* `#` begins a single-line comment, ending at the end of the line.
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* `//` also begins a single-line comment, as an alternative to `#`.
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* `/*` and `*/` are start and end delimiters for a comment that might span
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over multiple lines.
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The `#` single-line comment style is the default comment style and should be
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used in most cases. Automatic configuration formatting tools may automatically
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transform `//` comments into `#` comments, since the double-slash style is
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not idiomatic.
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## Character Encoding and Line Endings
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Packer configuration files must always be UTF-8 encoded. While the
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delimiters of the language are all ASCII characters, Packer accepts
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non-ASCII characters in identifiers, comments, and string values.
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Packer accepts configuration files with either Unix-style line endings
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(LF only) or Windows-style line endings (CR then LF), but the idiomatic style
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is to use the Unix convention, and so automatic configuration formatting tools
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may automatically transform CRLF endings to LF.
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@ -1,6 +1,17 @@
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<% wrap_layout :inner do %>
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<% content_for :sidebar do %>
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<ul class="nav docs-sidenav">
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<li<%= sidebar_current("configuration") %>>
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<a href="/docs/configuration/from-1.5/index.html">HCL</a>
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<ul class="nav">
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<li<%= sidebar_current("configuration-syntax") %>>
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<a href="/docs/configuration/from-1.5/syntax.html">Syntax</a>
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</li>
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</ul>
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</li>
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<hr>
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<li><a href="/docs/basics/terminology.html">Terminology</a></li>
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