---
layout: docs
page_title: Hashicorp Configuration Language (HCL) reference
description: |-
  This section contains reference material for the Hashicorp Configuration Language (HCL) as it relates to defining a Nomad job specifications. Learn about HCL syntax elements such as arguments, blocks, and expressions. Review heredoc string support and how to format decimals.
---

# Hashicorp Configuration Language (HCL) reference

Define your job specification with the Hashicorp Configuration Language (HCL),
which is a syntax specifically designed for building structured configuration
formats. Refer to the [HCL GitHub repository](https://github.com/hashicorp/hcl)
to learn more about HCL.

## Parsing context

The [Nomad API uses JSON][jobs-api], not HCL, to represent Nomad jobs.
When running commands like `nomad job run` and `nomad job plan`, the Nomad CLI
parses HCL and ultimately converts it to JSON. Because this parsing happens locally
(i.e., where the operator is running the CLI) before job submission, there are some
limits to the capabilities that can be accessed by HCL job specifications. For
example, scheduling information is not yet available, including information about
the client. Similarly, HCL features that depend on external context will take that
context from the local environment of the CLI (e.g., files, environment variables).

[jobs-api]: /nomad/api-docs/jobs

## JSON jobs

Since HCL is a superset of JSON, `nomad job run example.json` will attempt to
parse a JSON job using the HCL parser. However, the JSON format accepted by
the HCL parser is not the same as the [API's JSON format][json-jobs-api]. The
HCL parser's JSON format is unspecified, so the API format is preferred. You can
use the API format with the [`-json` command line flag][run-json]:

```shell-session
$ # Generate an HCL formatted job
$ nomad init

$ # Convert HCL to API JSON format
$ nomad job run -output example.nomad.hcl > example.json

$ # Submit with the -json flag
$ nomad job run -json example.json
```


[json-jobs-api]: /nomad/api-docs/json-jobs
[run-json]: /nomad/commands/job/run#json

## Arguments, bocks, and expressions

The syntax of the HCL language consists of only a few basic elements:

```hcl
task "example" {
  driver = "docker"
}

<BLOCK TYPE> "<BLOCK LABEL>" {
  # Block body
  <IDENTIFIER> = <EXPRESSION> # Argument
}
```

- _Blocks_ are containers for other content and usually represent the
  configuration of some kind of object, like a task. Blocks have a
  _block type,_ can have zero or more _labels,_ and have a _body_ that contains
  any number of arguments and nested blocks. Block labels must be string literals.
- _Arguments_ assign a value to a name. They appear within blocks.
- _Expressions_ represent a value, either literally or by referencing and
  combining other values. They appear as values for arguments, or within other
  expressions.

For full details about Nomad's syntax, see:

- [Configuration Syntax](/nomad/docs/reference/hcl2/syntax)
- [Expressions](/nomad/docs/reference/hcl2/expressions)

### Blocks

Block syntax is as follows, using unquoted attributes and quoted values:

```hcl
meta {
  team         = "..."
  organization = "..."
}
```

Additionally, block attributes must be [HCL valid identifiers](https://github.com/hashicorp/hcl/blob/v2.8.0/hclsyntax/spec.md#identifiers).
Generally, identifiers may only contain letters, numbers, underscore `_`,
or a dash `-`, and start with a letter. Notable,
[`meta`](/nomad/docs/job-specification/meta), and
[`env`](/nomad/docs/job-specification/env) keys may not
contain other symbols (e.g. `.`, `#`).

Task driver config fields may require extra attention if they contain invalid
identifiers. For example, docker [`sysctl`](/nomad/docs/job-declare/task-driver/docker#sysctl) must
use the map assignment syntax if the keys aren't valid:

```hcl
sysctl = {
  "net.core.somaxconn/docs/drivers/docker#sysctl" = "16384"
}
```

Additionally, task driver config fields may not nest block syntax within an
assignment syntax. The following [`mounts`](/nomad/docs/job-declare/task-driver/docker#mounts) syntax is no longer valid:

```hcl
# INVALID in Nomad 1.0
mounts = [
  {
    type = "tmpfs"
    tmpfs_options {  # <- block syntax is not valid here
      size = 10000
    }
  }
]
```

Here, the `tmpfs_options` block declaration is invalid HCL syntax, and must be an assignment instead:

```hcl
# VALID in Nomad 1.0
mounts = [
  {
    type = "tmpfs"
    tmpfs_options = {
      size = 10000
    }
  }
]
```

Or better yet, the new [`mount`](/nomad/docs/job-declare/task-driver/docker#mount) syntax, introduced in Nomad 1.0.1, is more appropriate here:

```hcl
mount {
  type = "tmpfs"
  tmpfs_options {
    size = 10000
  }
}
```

### Multiline "heredoc" string

Nomad supports multi-line string literals in the so-called "heredoc" style, inspired by Unix shell languages:

```hcl
template {
  data = <<EOF
hello
  world
  EOF
}
```

HCL trims the whitespace preceding the delimiter in the last line. So in the
above example, `data` is read as `"hello\n world\n "` in HCL1, but `"hello\n world\n"` (note lack of trailing whitespace) in HCL.

### Decimals

HCL requires a leading zero for decimal values lower than 1 (e.g. `0.3`, `0.59`, `0.9`).
