open-nomad/website/source/docs/jobspec/interpreted.html.md

layout	page_title	sidebar_current	description
docs	Interpreted Variables	docs-jobspec-interpreted	Learn about the Nomad's interpreted variables.

Interpreted Variables

Nomad support 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.

The syntax for interpreting variables is ${variable}. An example and a comprehensive list of interpretable fields can be seen below:

task "demo" {
    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"
    }

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

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

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
${node.unique.id}	The client node identifier
${node.datacenter}	The client node datacenter
${node.unique.name}	The client node name
${node.class}	The client node class
${attr.\	The attribute given by `key` on the client node.
${meta.\}	The metadata value given by `key` on the client node.

Below is a table documenting common node attributes:

Attribute	Description
arch	CPU architecture of the client. Examples: `amd64`, `386`
consul.datacenter	The Consul datacenter of the client node if Consul found
cpu.numcores	Number of CPU cores on the client
driver.\	See the [task drivers](/docs/drivers/index.html) for attribute documentation
hostname	Hostname of the client
kernel.name	Kernel of the client. Examples: `linux`, `darwin`
kernel.version	Version of the client kernel. Examples: `3.19.0-25-generic`, `15.0.0`
platform.aws.ami-id	On EC2, the AMI ID of the client node
platform.aws.instance-type	On EC2, the instance type of the client node
os.name	Operating system of the client. Examples: `ubuntu`, `windows`, `darwin`
os.version	Version of the client OS

Environment Variables

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.

Variable	Description
${NOMAD_ALLOC_DIR}	The path to the shared `alloc/` directory. See [here](/docs/jobspec/environment.html#task_dir) for more information.
${NOMAD_TASK_DIR}	The path to the task `local/` directory. See [here](/docs/jobspec/environment.html#task_dir) for more information.
${NOMAD_MEMORY_LIMIT}	The memory limit in MBits for the task
${NOMAD_CPU_LIMIT}	The CPU limit in MHz for the task
${NOMAD_ADDR_"label"}>	The `ip:port` pair for the given port `label`. See [here](/docs/jobspec/networking.html) for more information.
${NOMAD_HOST_PORT_"label"}	The port on the host if port forwarding is being used for the port `label`. See [here](/docs/jobspec/networking.html#mapped_ports) for more information.
${NOMAD_META_"key"}	The metadata value given by `key` on the task's metadata
${"env_key"}	Interpret an environment variable with key `env_key` set on the task.