github.com/anth0d/nomad@v0.0.0-20221214183521-ae3a0a2cad06/website/content/docs/job-specification/hcl2/syntax.mdx (about)

     1  ---
     2  layout: docs
     3  page_title: Syntax - Configuration Language
     4  description: |-
     5    HCL has its own syntax, intended to combine declarative
     6    structure with expressions in a way that is easy for humans to read and
     7    understand.
     8  ---
     9  
    10  # HCL Configuration Syntax
    11  
    12  Other pages in this section have described various configuration constructs
    13  that can appear in HCL. This page describes the lower-level syntax of the
    14  language in more detail, revealing the building blocks that those constructs
    15  are built from.
    16  
    17  This page describes the _native syntax_ of HCL, which is a rich language
    18  designed to be easy for humans to read and write.
    19  
    20  This low-level syntax of HCL is defined in terms of a syntax called _HCL_,
    21  which is also used by configuration languages in other applications, and in
    22  particular other HashiCorp products. It is not necessary to know all of the
    23  details of HCL in order to use Nomad, and so this page summarizes the most
    24  important details. If you are interested, you can find a full definition of HCL
    25  syntax in [the HCL native syntax
    26  specification](https://github.com/hashicorp/hcl/blob/hcl2/hclsyntax/spec.md).
    27  
    28  ## Arguments and Blocks
    29  
    30  HCL syntax is built around two key syntax constructs:
    31  arguments and blocks.
    32  
    33  ### Arguments
    34  
    35  An _argument_ assigns a value to a particular name:
    36  
    37  ```hcl
    38  image_id = "nginx:1.19"
    39  ```
    40  
    41  The identifier before the equals sign is the _argument name_, and the expression
    42  after the equals sign is the argument's value.
    43  
    44  The context where the argument appears determines what value types are valid
    45  (for example, each job block type has a schema that defines the types of its
    46  arguments), but many arguments accept arbitrary
    47  [expressions](/docs/job-specification/hcl2/expressions), which allow the value to
    48  either be specified literally or generated from other values programmatically.
    49  
    50  ### Blocks
    51  
    52  A _block_ is a container for other content:
    53  
    54  ```hcl
    55  task "webserver" {
    56    driver = "docker"
    57  
    58    config {
    59      # ...
    60    }
    61  }
    62  ```
    63  
    64  A block has a _type_ (`task` in this example). Each block type defines
    65  how many _labels_ must follow the type keyword. The `task` block type
    66  expects one label, which is `webserver` in the example above.
    67  A particular block type may have any number of required labels, or it may
    68  require none as with the nested `config` block type.
    69  
    70  After the block type keyword and any labels, the block _body_ is delimited
    71  by the `{` and `}` characters. Within the block body, further arguments
    72  and blocks may be nested, creating a hierarchy of blocks and their associated
    73  arguments.
    74  
    75  HCL uses a limited number of _top-level block types,_ which
    76  are blocks that can appear outside of any other block in a configuration file.
    77  
    78  ## Identifiers
    79  
    80  Argument names, block type names, and the names of most Nomad-specific
    81  constructs like tasks, input variables, etc. are all _identifiers_.
    82  
    83  Identifiers can contain letters, digits, underscores (`_`), and hyphens (`-`).
    84  The first character of an identifier must not be a digit, to avoid ambiguity
    85  with literal numbers.
    86  
    87  For complete identifier rules, Nomad implements
    88  [the Unicode identifier syntax](http://unicode.org/reports/tr31/), extended to
    89  include the ASCII hyphen character `-`.
    90  
    91  ## Comments
    92  
    93  HCL supports three different syntaxes for comments:
    94  
    95  - `#` begins a single-line comment, ending at the end of the line.
    96  - `//` also begins a single-line comment, as an alternative to `#`.
    97  - `/*` and `*/` are start and end delimiters for a comment that might span
    98    over multiple lines.
    99  
   100  The `#` single-line comment style is the default comment style and should be
   101  used in most cases. Automatic configuration formatting tools may automatically
   102  transform `//` comments into `#` comments, since the double-slash style is
   103  not idiomatic.
   104  
   105  ## Character Encoding and Line Endings
   106  
   107  Nomad configuration files must always be UTF-8 encoded. While the
   108  delimiters of the language are all ASCII characters, Nomad accepts
   109  non-ASCII characters in identifiers, comments, and string values.
   110  
   111  Nomad accepts configuration files with either Unix-style line endings
   112  (LF only) or Windows-style line endings (CR then LF), but the idiomatic style
   113  is to use the Unix convention, and so automatic configuration formatting tools
   114  may automatically transform CRLF endings to LF.