add syntax-json for HCL
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Adrien Delorme
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---
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layout: "docs"
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page_title: "JSON Configuration Syntax - Configuration Language"
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sidebar_current: configuration-syntax-json
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description: |-
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In addition to the native syntax that is most commonly used with Packer,
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the HCL language can also be expressed in a JSON-compatible syntax.
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---
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# JSON Configuration Syntax
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Most Packer configurations are written in [the native HCL
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syntax](./syntax.html), which is designed to be easy for humans to read and
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update.
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Packer also supports an alternative syntax that is JSON-compatible. This
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syntax is useful when generating portions of a configuration programmatically,
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since existing JSON libraries can be used to prepare the generated
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configuration files.
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The JSON syntax is defined in terms of the native syntax. Everything that can
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be expressed in native syntax can also be expressed in JSON syntax, but some
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constructs are more complex to represent in JSON due to limitations of the
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JSON grammar.
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Packer expects native syntax for files named with a `.pkr.hcl` suffix, and JSON
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syntax for files named with a `.pkr.json` suffix.
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The low-level JSON syntax, just as with the native syntax, is defined in terms
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of a specification called _HCL_. It is not necessary to know all of the details
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of HCL syntax or its JSON mapping in order to use Packer, and so this page
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summarizes the most important differences between native and JSON syntax. If
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you are interested, you can find a full definition of HCL's JSON syntax in [its
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specification](https://github.com/hashicorp/hcl2/blob/master/hcl/spec.md).
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## JSON File Structure
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At the root of any JSON-based Packer configuration is a JSON object. The
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properties of this object correspond to the top-level block types of the
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Packer language. For example:
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```json
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{
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"variables": {
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"example": "value"
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}
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}
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```
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Each top-level object property must match the name of one of the expected
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top-level block types. Block types that expect labels, such as `variable` shown
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above, are represented by one nested object value for each level of label.
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`source` blocks expect two labels, so two levels of nesting are required:
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```json
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{
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"source": {
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"amazon-ebs": {
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"example": {
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"instance_type": "t2.micro",
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"ami_name": "ami-abc123"
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}
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}
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}
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}
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```
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After any nested objects representing the labels, finally one more nested
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object represents the body of the block itself. In the above example the
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`instance_type` and `ami_name` arguments for `source "amazon-ebs" "example"`
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are specified.
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Taken together, the above two configuration files are equivalent to the
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following blocks in the native syntax:
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```hcl
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variables {
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example = "value"
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}
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source "amazon-ebs" "example" {
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instance_type = "t2.micro"
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ami_name = "ami-abc123"
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}
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```
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Within each top-level block type the rules for mapping to JSON are slightly
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different (see [Block-type-specific Exceptions][inpage-exceptions] below), but the following general rules apply in most cases:
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* The JSON object representing the block body contains properties that
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correspond either to argument names or to nested block type names.
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* Where a property corresponds to an argument that accepts
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[arbitrary expressions](./expressions.html) in the native syntax, the
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property value is mapped to an expression as described under
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[_Expression Mapping_](#expression-mapping) below. For arguments that
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do _not_ accept arbitrary expressions, the interpretation of the property
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value depends on the argument, as described in the
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[block-type-specific exceptions](#block-type-specific-exceptions)
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given later in this page.
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* Where a property name corresponds to an expected nested block type name,
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the value is interpreted as described under
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[_Nested Block Mapping_](#nested-block-mapping) below, unless otherwise
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stated in [the block-type-specific exceptions](#block-type-specific-exceptions)
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given later in this page.
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## Expression Mapping
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Since JSON grammar is not able to represent all of the Packer language
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[expression syntax](./expressions.html), JSON values interpreted as expressions
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are mapped as follows:
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| JSON | Packer Language Interpretation |
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| ------- | ------------------------------------------------------------------------------------------------------------- |
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| Boolean | A literal `bool` value. |
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| Number | A literal `number` value. |
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| String | Parsed as a [string template](./expressions.html#string-templates) and then evaluated as described below. |
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| Object | Each property value is mapped per this table, producing an `object(...)` value with suitable attribute types. |
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| Array | Each element is mapped per this table, producing a `tuple(...)` value with suitable element types. |
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| Null | A literal `null`. |
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When a JSON string is encountered in a location where arbitrary expressions are
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expected, its value is first parsed as a [string template](./expressions.html#string-templates)
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and then it is evaluated to produce the final result.
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If the given template consists _only_ of a single interpolation sequence,
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the result of its expression is taken directly, without first converting it
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to a string. This allows non-string expressions to be used within the
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JSON syntax.
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## Nested Block Mapping
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When a JSON object property is named after a nested block type, the value
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of this property represents one or more blocks of that type. The value of
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the property must be either a JSON object or a JSON array.
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The simplest situation is representing only a single block of the given type
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when that type expects no labels, as with the `tags` nested block used
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within `source` blocks:
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```json
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{
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"source": {
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"amazon-ebs": {
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"example": {
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"tags": {
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"key": "value"
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}
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}
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}
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}
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}
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```
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The above is equivalent to the following native syntax configuration:
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```hcl
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source "amazon-ebs" "example" {
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tags {
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key = "value"
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}
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}
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```
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When the nested block type requires one or more labels, or when multiple
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blocks of the same type can be given, the mapping gets a little more
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complicated. For example, the `provisioner` nested block type used
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within `source` blocks expects a label giving the provisioner to use,
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and the ordering of provisioner blocks is significant to decide the order
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of operations.
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The following native syntax example shows a `source` block with a number
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of provisioners of different types:
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```hcl
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source "amazon-ebs" "example" {
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# (source configuration omitted for brevity)
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provisioner "shell-local" {
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inline = ["echo 'Hello World' >example.txt"]
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}
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provisioner "file" {
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source = "example.txt"
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destination = "/tmp/example.txt"
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}
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provisioner "shell" {
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inline = [
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"sudo install-something -f /tmp/example.txt",
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]
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}
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}
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```
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In order to preserve the order of these blocks, you must use a JSON array
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as the direct value of the property representing this block type, as in
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this JSON equivalent of the above:
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```json
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{
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"source": {
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"amazon-ebs": {
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"example": {
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"provisioner": [
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{
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"shell-local": {
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"inline": ["echo 'Hello World' >example.txt"]
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}
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},
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{
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"file": {
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"source": "example.txt",
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"destination": "/tmp/example.txt"
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}
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},
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{
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"shell": {
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"inline": ["sudo install-something -f /tmp/example.txt"]
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}
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}
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]
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}
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}
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}
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}
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```
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Each element of the `provisioner` array is an object with a single property
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whose name represents the label for each `provisioner` block. For block types
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that expect multiple labels, this pattern of alternating array and object
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nesting can be used for each additional level.
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If a nested block type requires labels but the order does _not_ matter, you
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may omit the array and provide just a single object whose property names
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correspond to unique block labels. This is allowed as a shorthand for the above
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for simple cases, but the alternating array and object approach is the most
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general. We recommend using the most general form if systematically converting
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from native syntax to JSON, to ensure that the meaning of the configuration is
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preserved exactly.
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### Comment Properties
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Although we do not recommend hand-editing of JSON syntax configuration files
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-- this format is primarily intended for programmatic generation and consumption --
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a limited form of _comments_ are allowed inside JSON objects that represent
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block bodies using a special property name:
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```json
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{
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"source": {
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"amazon-ebs": {
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"example": {
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"//": "This instance runs the scheduled tasks for backup",
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"instance_type": "t2.micro",
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"ami_name": "ami-abc123"
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}
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}
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}
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}
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```
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In any object that represents a block body, properties named `"//"` are
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ignored by Packer entirely. This exception does _not_ apply to objects
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that are being [interpreted as expressions](#expression-mapping), where this
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would be interpreted as an object type attribute named `"//"`.
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This special property name can also be used at the root of a JSON-based
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configuration file. This can be useful to note which program created the file.
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```json
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{
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"//": "This file is generated by generate-outputs.py. DO NOT HAND-EDIT!",
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}
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```
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@ -8,6 +8,9 @@
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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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<li<%= sidebar_current("configuration-syntax-json") %>>
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<a href="/docs/configuration/from-1.5/syntax-json.html">JSON Syntax</a>
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</li>
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</ul>
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</li>
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