---
layout: docs
page_title: Variable Interpolation
description: Learn about the Nomad's interpolation and interpreted variables.
---

# Variable Interpolation

Nomad supports interpreting two classes of variables: node attributes and
runtime environment variables. Node attributes are interpretable in constraints,
task environment variables, and certain driver fields. Runtime environment
variables are not interpretable in constraints because they are only defined
once the scheduler has placed them on a particular node.

Nomad supports interpreting two classes of variables: [node attributes](#interpreted_node_vars)
and [runtime environment variables](#interpreted_env_vars). Node attributes are
interpretable in [constraints](/nomad/docs/job-specification/constraint),
[task environment variables](/nomad/docs/job-specification/env), and certain
task driver fields—for example the [`labels`](/nomad/docs/drivers/docker#labels)
attribute of the Docker [`config`](/nomad/docs/drivers/docker).

<Note>
Runtime environment variables are not defined until after the scheduler
has placed the job, so they are unavailable for use in job constraints.
</Note>

## Syntax

The syntax for interpreting variables in the Nomad job specification is
`${variable_name}`. The [`template` block](/nomad/docs/job-specification/template)
uses the `env` function to retrieve these variables from the environment, using
`{{env "variable_name"}}` instead. Examples can be seen below:

```hcl
task "docs" {
  driver = "docker"

  # Drivers support interpreting node attributes and runtime environment
  # variables
  config {
    image = "my-app"

    # Interpret runtime variables to inject the address to bind to and the
    # location to write logs to.
    args = [
      "--bind", "${NOMAD_ADDR_RPC}",
      "--logs", "${NOMAD_ALLOC_DIR}/logs",
    ]

    port_map {
      RPC = 6379
    }
  }

  # Constraints only support node attributes as runtime environment variables
  # are only defined after the task is placed on a node.
  constraint {
    attribute = "${attr.kernel.name}"
    value     = "linux"
  }

  template {
    destination = "template.txt"
    data = <<EOT
{{- /*
      Environment variables are available to templates via the env function,
      rather than the ${...} syntax.
*/ -}}
Running on {{env "attr.unique.hostname"}}.
EOT
  }

  # Environment variables are interpreted and can contain both runtime and
  # node attributes. These environment variables are passed into the task.
  env {
    DC      = "Running on datacenter ${node.datacenter}"
    VERSION = "Version ${NOMAD_META_VERSION}"
  }

  # Meta keys are also interpretable.
  meta {
    VERSION = "v0.3"
  }
}
```

## Node Attributes ((#interpreted_node_vars, #node-variables-))

Below is a full listing of node attributes that are interpretable. These
attributes are interpreted by **both** constraints and within the task and
driver.

| Variable              | Description                                 | Example Value                          |
|-----------------------|---------------------------------------------|----------------------------------------|
| `${node.unique.id}`   | 36 character unique client identifier       | `9afa5da1-8f39-25a2-48dc-ba31fd7c0023` |
| `${node.region}`      | Client's region                             | `global`                               |
| `${node.datacenter}`  | Client's datacenter                         | `dc1`                                  |
| `${node.unique.name}` | Client's name                               | `nomad-client-10-1-2-4`                |
| `${node.class}`       | Client's class                              | `linux-64bit`                          |
| `${node.pool}`        | Client's node pool                          | `prod`                                 |
| `${attr.<property>}`  | Property given by `property` on the client  | `${attr.cpu.arch} => amd64`            |
| `${meta.<key>}`       | Metadata value given by `key` on the client | `${meta.foo} => bar`                   |

Below is a table documenting common node properties.

| Property                                           | Description                                                                                                                                            |
| ---------------------------------------------------| ------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `${attr.cpu.arch}`                                 | CPU architecture of the client (e.g. `amd64`, `386`)                                                                                                   |
| `${attr.cpu.numcores}`                             | Number of CPU cores on the client. May differ from how many cores are available for reservation due to OS or configuration. See `cpu.reservablecores`. |
| `${attr.cpu.reservablecores}`                      | Number of CPU cores on the client available for scheduling. Number of cores used by the scheduler when placing work with `resources.cores` set.        |
| `${attr.cpu.totalcompute}`                         | `cpu.frequency × cpu.numcores` but may be overridden by `client.cpu_total_compute`                                                                     |
| `${attr.consul.datacenter}`                        | The Consul datacenter of the client (if Consul is found)                                                                                               |
| `${attr.driver.<property>}`                        | See the [task drivers](/nomad/docs/drivers) for property documentation                                                                                 |
| `${attr.unique.hostname}`                          | Hostname of the client                                                                                                                                 |
| `${attr.unique.network.ip-address}`                | The IP address fingerprinted by the client and from which task ports are allocated                                                                     |
| `${attr.kernel.arch}`                              | Kernel architecture of the client (e.g. `x86_64`, `aarch64`)                                                                                           |
| `${attr.kernel.name}`                              | Kernel of the client (e.g. `linux`, `darwin`)                                                                                                          |
| `${attr.kernel.version}`                           | Version of the client kernel (e.g. `3.19.0-25-generic`, `15.0.0`)                                                                                      |
| `${attr.platform.aws.ami-id}`                      | AMI ID of the client (if on AWS EC2)                                                                                                                   |
| `${attr.platform.aws.instance-life-cycle}`         | Instance lifecycle (e.g. spot, on-demand) of the client (if on AWS EC2)                                                                                |
| `${attr.platform.aws.instance-type}`               | Instance type of the client (if on AWS EC2)                                                                                                            |
| `${attr.platform.aws.placement.availability-zone}` | Availability Zone of the client (if on AWS EC2)                                                                                                        |
| `${attr.os.name}`                                  | Operating system of the client (e.g. `ubuntu`, `windows`, `darwin`)                                                                                    |
| `${attr.os.version}`                               | Version of the client OS                                                                                                                               |
| `${attr.os.build}`                                 | Build number (e.g `14393.5501`) of the client OS (if on Windows)                                                                                       |

The full list of node attributes can be obtained by running `nomad node status -verbose [node]`.

Here are some examples of using node attributes and properties in a job file:

```hcl
job "docs" {
  # This will constrain this job to only run on 64-bit clients.
  constraint {
    attribute = "${attr.cpu.arch}"
    value     = "amd64"
  }

  # This will restrict the job to only run on clients with 4 or more cores.
  # Note: you may also declare a resource requirement for CPU for a task.
  constraint {
    attribute = "${cpu.numcores}"
    operator  = ">="
    value     = "4"
  }

  # Only run this job on a memory-optimized AWS EC2 instance.
  constraint {
    attribute = "${attr.platform.aws.instance-type}"
    value     = "m4.xlarge"
  }
}
```

## Environment Variables ((#interpreted_env_vars))

The following are runtime environment variables that describe the environment
the task is running in. These are only defined once the task has been placed on
a particular node and as such can not be used in constraints.

Environment variables should be enclosed in brackets `${...}` for
interpolation or accessed using the `env` function inside the template
block—`{{env "..."}}`

### Dots in Variables ((#dots_in_vars))

Nomad interprets dots in names as object notation. This causes names that have
multiple consecutive dots to be considered invalid. For example, an environment
variable named `invalid...name` cannot be interpolated using the standard
`"${invalid...name}"` syntax. If you do, the parser will return an
`Extra characters after interpolation expression` error. Nomad provides a
variable—`env`—that can access any environment variable, regardless
of its name, using index syntax.

```hcl
job "sample" {
  datacenters = ["dc1"]
  group "g1" {
    task "redis" {

      # Note: to set an environment variable with an invalid name, you must
      # use the HCL2 map assignment syntax for `env`. Otherwise, the job spec
      # parser will throw an `Argument or block definition required` error
      env = {
        "invalid...name" = "value1"
        "valid.name"     = "value2"
      }

      driver = "docker"
      config {
        image  = "redis:7"
        labels {
          label1 = "${env["invalid...name"]}"
          label2 = "${valid.name}"
        }
      }
    }
  }
}
```

@include 'envvars.mdx'
