Input Variables

Input variables serve as parameters for a Nomad job, allowing aspects of the job to be customized without altering the job's own source code.

When you declare variables in the same file as the job specification, you can set their values using CLI options and environment variables.

Declaring an Input Variable

Each input variable accepted by a job must be declared using a variable block :

variable "image_id" {
  type = string
}

variable "availability_zone_names" {
  type    = list(string)
  default = ["us-west-1a"]
}

variable "docker_ports" {
  type = list(object({
    internal = number
    external = number
    protocol = string
  }))
  default = [
    {
      internal = 8300
      external = 8300
      protocol = "tcp"
    }
  ]
}

Or a less precise variables block:

variables {
  foo       = "value"
  my_secret = "foo"
}

The label after the variable keyword or a label of a variables block is a name for the variable, which must be unique among all variables in the same job. This name is used to assign a value to the variable from outside and to reference the variable's value from within the job.

The variable block can optionally include a type argument to specify what value types are accepted for the variable, as described in the following section.

The variable declaration can also include a default argument. If present, the variable is considered to be optional and the default value will be used if no value is set when calling the job or running Nomad. The default argument requires a literal value and cannot reference other objects in the configuration.

Using Input Variable Values

Within the job that declared a variable, its value can be accessed from within expressions as var.<NAME>, where <NAME> matches the label given in the declaration block:

config {
  image = var.task_image
  label = var.task_labels
}

The value assigned to a variable can be accessed only from expressions within the folder where it was declared. Note that a block label (such as the job ID or task group name) is not an expression and so can't be interpolated with a variable or local.

Type Constraints

The type argument in a variable block allows you to restrict the type of value that will be accepted as the value for a variable. If no type constraint is set then a value of any type is accepted.

While type constraints are optional, we recommend specifying them; they serve as easy reminders for users of the job, and allow Nomad to return a helpful error message if the wrong type is used.

Type constraints are created from a mixture of type keywords and type constructors. The supported type keywords are:

• string
• number
• bool

The type constructors allow you to specify complex types such as collections:

• list(<TYPE>)
• set(<TYPE>)
• map(<TYPE>)
• object({<ATTR NAME> = <TYPE>, ... })
• tuple([<TYPE>, ...])

The keyword any may be used to indicate that any type is acceptable. For more information on the meaning and behavior of these different types, as well as detailed information about automatic conversion of complex types, see Type Constraints.

If both the type and default arguments are specified, the given default value must be convertible to the specified type.

If only default is specified, the type of the default value will be used.

When the type and default are both not specified and you try to set a variable from env vars or from the command line, the variable will always be interpreted as a string.

Input Variable Documentation

Because the input variables of a job are part of its user interface, you can briefly describe the purpose of each variable using the optional description argument:

variable "image_id" {
  type        = string
  description = "The docker image used for task."
}

The description should concisely explain the purpose of the variable and what kind of value is expected. This description string might be included in documentation about the job, and so it should be written from the perspective of the user of the job rather than its maintainer. For commentary for job maintainers, use comments.

Input Variable Custom Validation Rules

Input variables support specifying arbitrary custom validation rules for a particular variable using a validation block nested within the corresponding variable block:

variable "image_id" {
  type        = string
  description = "The id of the machine image (AMI) to use for the server."

  validation {
    condition     = substr(var.image_id, 0, 4) == "ami-"
    error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."
  }
}

The condition argument is an expression that must use the value of the variable to return true if the value is valid, or false if it is invalid. The expression can refer only to the variable that the condition applies to, and must not produce errors.

If condition evaluates to false, Nomad will produce an error message that includes the sentences given in error_message. The error message string should be at least one full sentence explaining the constraint that failed, starting with an uppercase letter ( if the alphabet permits it ) and ending with a period or question mark.

Multiple validation blocks can be declared in which case error messages will be returned for all failed conditions.

Assigning Values to job Variables

Once a variable is declared in your configuration, you can set it:

• Individually, with the -var foo=bar command line option.
• In variable definitions files specified on the command line (with -var-file=input.vars).
• As environment variables, for example: NOMAD_VAR_foo=bar

The following sections describe these options in more detail.

Variables on the Command Line

To specify individual variables on the command line, use the -var option when running the nomad job run command:

$ nomad job run -var="image_id=nginx:1.19" example.nomad.hcl

The -var option can be used any number of times in a single command.

If you plan to assign variables via the command line, we strongly recommend that you at least set a default type instead of using empty blocks; this helps the HCL parser understand what is being set. Otherwise, the interpreter will assume that any variable set on the command line is a string.

Variable Definitions Files

To set lots of variables, it is more convenient to specify their values in a variable definitions file and then specify that file on the command line with -var-file:

$ nomad job run -var-file="testing.vars" example.nomad.hcl

A variable definitions file uses the same HCL basic syntax, but consists only of variable name assignments:

image_id = "nginx:1.19"
labels = [
  "testing",
  "internal",
]

Alternatively, the files can be JSON objects, with the root object properties corresponding to variable names:

{
  "image_id": "nginx:1.19",
  "labels": ["testing", "internal"]
}

Environment Variables

As a fallback for the other ways of defining variables, Nomad searches the environment of its own process for environment variables named NOMAD_VAR_ followed by the name of a declared variable.

This can be useful when running Nomad in automation, or when running a sequence of Nomad commands in succession with the same variables. For example, at a bash prompt on a Unix system:

$ export NOMAD_VAR_image_id=nginx:1.19
$ nomad job run example.nomad.hcl
...

On operating systems where environment variable names are case-sensitive, Nomad matches the variable name exactly as given in configuration, and so the required environment variable name will usually have a mix of upper and lower case letters as in the above example.

Complex-typed Values

When variable values are provided in a variable definitions file, Nomad's usual syntax can be used to assign complex-typed values, like lists and maps.

Some special rules apply to the -var command line option and to environment variables. For convenience, Nomad defaults to interpreting -var and environment variable values as literal strings, which do not need to be quoted:

$ export NOMAD_VAR_image_id=nginx:1.19

However, if an input variable uses a type constraint to require a complex value (list, set, map, object, or tuple), Nomad will instead attempt to parse its value using the same syntax used within variable definitions files, which requires careful attention to the string escaping rules in your shell:

$ export NOMAD_VAR_availability_zone_names='["us-west-1b","us-west-1d"]'

For readability, and to avoid the need to worry about shell escaping, we recommend always setting complex variable values via variable definitions files.

Variable Definition Precedence

The above mechanisms for setting variables can be used together in any combination.

Nomad loads variables in the following order, with later sources taking precedence over earlier ones:

• Environment variables (lowest priority)
• Any -var and -var-file options on the command line, in the order they are provided. (highest priority)

If the same variable is assigned multiple values using different mechanisms, Nomad uses the last value it finds, overriding any previous values. Note that the same variable cannot be assigned multiple values within a single source.

A variable value must be known:

Take the following variable for example:

variable "foo" {
  type = string
}

Here foo must have a known value but you can default it to null to make this behavior optional :