126 lines
4.2 KiB
Markdown
126 lines
4.2 KiB
Markdown
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# HCL
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[](https://godoc.org/github.com/hashicorp/hcl) [](https://travis-ci.org/hashicorp/hcl)
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HCL (HashiCorp Configuration Language) is a configuration language built
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by HashiCorp. The goal of HCL is to build a structured configuration language
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that is both human and machine friendly for use with command-line tools, but
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specifically targeted towards DevOps tools, servers, etc.
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HCL is also fully JSON compatible. That is, JSON can be used as completely
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valid input to a system expecting HCL. This helps makes systems
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interoperable with other systems.
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HCL is heavily inspired by
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[libucl](https://github.com/vstakhov/libucl),
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nginx configuration, and others similar.
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## Why?
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A common question when viewing HCL is to ask the question: why not
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JSON, YAML, etc.?
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Prior to HCL, the tools we built at [HashiCorp](http://www.hashicorp.com)
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used a variety of configuration languages from full programming languages
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such as Ruby to complete data structure languages such as JSON. What we
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learned is that some people wanted human-friendly configuration languages
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and some people wanted machine-friendly languages.
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JSON fits a nice balance in this, but is fairly verbose and most
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importantly doesn't support comments. With YAML, we found that beginners
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had a really hard time determining what the actual structure was, and
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ended up guessing more often than not whether to use a hyphen, colon, etc.
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in order to represent some configuration key.
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Full programming languages such as Ruby enable complex behavior
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a configuration language shouldn't usually allow, and also forces
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people to learn some set of Ruby.
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Because of this, we decided to create our own configuration language
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that is JSON-compatible. Our configuration language (HCL) is designed
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to be written and modified by humans. The API for HCL allows JSON
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as an input so that it is also machine-friendly (machines can generate
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JSON instead of trying to generate HCL).
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Our goal with HCL is not to alienate other configuration languages.
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It is instead to provide HCL as a specialized language for our tools,
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and JSON as the interoperability layer.
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## Syntax
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For a complete grammar, please see the parser itself. A high-level overview
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of the syntax and grammar is listed here.
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* Single line comments start with `#` or `//`
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* Multi-line comments are wrapped in `/*` and `*/`. Nested block comments
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are not allowed. A multi-line comment (also known as a block comment)
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terminates at the first `*/` found.
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* Values are assigned with the syntax `key = value` (whitespace doesn't
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matter). The value can be any primitive: a string, number, boolean,
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object, or list.
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* Strings are double-quoted and can contain any UTF-8 characters.
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Example: `"Hello, World"`
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* Multi-line strings start with `<<EOF` at the end of a line, and end
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with `EOF` on its own line ([here documents](https://en.wikipedia.org/wiki/Here_document)).
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Any text may be used in place of `EOF`. Example:
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```
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<<FOO
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hello
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world
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FOO
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```
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* Numbers are assumed to be base 10. If you prefix a number with 0x,
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it is treated as a hexadecimal. If it is prefixed with 0, it is
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treated as an octal. Numbers can be in scientific notation: "1e10".
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* Boolean values: `true`, `false`
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* Arrays can be made by wrapping it in `[]`. Example:
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`["foo", "bar", 42]`. Arrays can contain primitives,
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other arrays, and objects. As an alternative, lists
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of objects can be created with repeated blocks, using
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this structure:
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```hcl
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service {
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key = "value"
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}
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service {
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key = "value"
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}
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```
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Objects and nested objects are created using the structure shown below:
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```
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variable "ami" {
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description = "the AMI to use"
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}
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```
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This would be equivalent to the following json:
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``` json
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{
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"variable": {
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"ami": {
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"description": "the AMI to use"
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}
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}
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}
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```
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## Thanks
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Thanks to:
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* [@vstakhov](https://github.com/vstakhov) - The original libucl parser
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and syntax that HCL was based off of.
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* [@fatih](https://github.com/fatih) - The rewritten HCL parser
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in pure Go (no goyacc) and support for a printer.
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