Tokens reference

Tokens are placeholders in a check definition that the agent replaces with entity information before executing the check. You can use tokens to fine-tune check attributes (like alert thresholds) on a per-entity level while reusing the check definition.

When a check is scheduled to be executed by an agent, it first goes through a token substitution step. The agent replaces any tokens with matching attributes from the entity definition, and then the check is executed. Invalid templates or unmatched tokens return an error, which is logged and sent to the Sensu backend message transport. Checks with token-matching errors are not executed.

Token substitution is supported for check, hook, and dynamic runtime asset definitions. Only entity attributes are available for substitution. Token substitution is not available for event filters because filters already have access to the entity.

Available entity attributes will always have string values, such as labels and annotations.

Example: Token substitution for check thresholds

This example demonstrates a reusable disk usage check. The check command includes -w (warning) and -c (critical) arguments with default values for the thresholds (as percentages) for generating warning or critical events. The check will compare every subscribed entity’s disk space against the default threshold values to determine whether to generate a warning or critical event.

However, the check command also includes token substitution, which means you can add entity labels that correspond to the check commnand tokens to specify different warning and critical values for individual entities. Instead of creating a different check for every set of thresholds, you can use the same check to apply the defaults in most cases and the token-substituted values for specific entities.

Follow this example to set up a reusable check for disk usage:

  1. Add the Sensu disk usage check dynamic runtime asset, which includes the command you will need for your check:

    sensuctl asset add sensu/check-disk-usage:0.4.1

    You will see a response to confirm that the asset was added:

    fetching bonsai asset: sensu/check-disk-usage:0.4.1
    added asset: sensu/check-disk-usage:0.4.1
    You have successfully added the Sensu asset resource, but the asset will not get downloaded until
    it's invoked by another Sensu resource (ex. check). To add this runtime asset to the appropriate
    resource, populate the "runtime_assets" field with ["sensu/check-disk-usage].

  2. Create the check-disk-usage check:

    cat << EOF | sensuctl create
    type: CheckConfig
    api_version: core/v2
      name: check-disk-usage
      namespace: default
      check_hooks: []
      command: check-disk-usage -w {{index .labels "disk_warning" | default 80}} -c
        {{.labels.disk_critical | default 90}}
      env_vars: null
      handlers: []
      high_flap_threshold: 0
      interval: 10
      low_flap_threshold: 0
      output_metric_format: ""
      output_metric_handlers: null
      output_metric_tags: null
      proxy_entity_name: ""
      publish: true
      round_robin: false
      - sensu/check-disk-usage
      stdin: false
      subdue: null
      - system
      timeout: 0
      ttl: 0
    cat << EOF | sensuctl create
      "type": "CheckConfig",
      "api_version": "core/v2",
      "metadata": {
        "name": "check-disk-usage",
        "namespace": "default"
      "spec": {
        "check_hooks": [],
        "command": "check-disk-usage -w {{index .labels "disk_warning" | default 80}} -c {{.labels.disk_critical | default 90}}",
        "env_vars": null,
        "handlers": [],
        "high_flap_threshold": 0,
        "interval": 10,
        "low_flap_threshold": 0,
        "output_metric_format": "",
        "output_metric_handlers": null,
        "output_metric_tags": null,
        "proxy_entity_name": "",
        "publish": true,
        "round_robin": false,
        "runtime_assets": [
        "stdin": false,
        "subdue": null,
        "subscriptions": [
        "timeout": 0,
        "ttl": 0

    This check will run on every entity with the subscription system. According to the default values in the command, the check will generate a warning event at 80% disk usage and a critical event at 90% disk usage.

  3. To receive alerts at different thresholds for an existing entity with the system subscription, add disk_warning and disk_critical labels to the entity.

    Use sensuctl to open an existing entity in a text editor:

    sensuctl edit entity ENTITY_NAME

    And add the following labels in the entity metadata:

        disk_warning: "65"
        disk_critical: "75"

After you save your changes, the check-disk-usage check will substitute the disk_warning and disk_critical label values to generate events at 65% and 75% of disk usage, respectively, for this entity only. The check will continue to use the 80% and 90% default values for other subscribed entities.

Add a hook that uses token substitution

Now you have a resusable check that will send disk usage alerts at default or entity-specific thresholds. You may want to add a hook to list more details about disk usage for warning and critical events.

The hook in this example will list disk usage in human-readable format, with error messages filtered from the hook output. By default, the hook will list details for the top directory and the first layer of subdirectories. As with the check-disk-usage check, you can add a disk_usage_root label to individual entities to specify a different directory for the hook via token substitution.

  1. Add the hook definition:

    cat << EOF | sensuctl create
    type: HookConfig
    api_version: core/v2
      name: disk_usage_details
      namespace: default
      command: du -h --max-depth=1 -c {{index .labels "disk_usage_root" | default "/"}}  2>/dev/null
      runtime_assets: null
      stdin: false
      timeout: 60
    cat << EOF | sensuctl create
      "type": "HookConfig",
      "api_version": "core/v2",
      "metadata": {
        "name": "disk_usage_details",
        "namespace": "default"
      "spec": {
        "command": "du -h --max-depth=1 -c {{index .labels "disk_usage_root" | default \"/\"}}  2>/dev/null",
        "runtime_assets": null,
        "stdin": false,
        "timeout": 60

  2. Add the hook to the check-disk-usage check.

    Use sensuctl to open the check in a text editor:

    sensuctl edit check check-disk-usage

    Update the check definition to include the disk_usage_details hook for non-zero events:

      - non-zero:
        - disk_usage_details

  3. As with the disk usage check command, the hook command includes a token substitution option. To use a specific directory instead of the default for specific entities, edit the entity definition to add a disk_usage_root label and specify the directory:

    Use sensuctl to open the entity in a text editor:

    sensuctl edit entity ENTITY_NAME

    Add the disk_usage_root label with the desired substitute directory in the entity metadata:

        disk_usage_root: "/substitute-directory"

After you save your changes, for this entity, the hook will substitute the directory you specified for the disk_usage_root label to provide additional disk usage details for every non-zero event the check-disk-usage check generates.

Manage entity labels

You can use token substitution with any defined entity attributes, including custom labels. See the entity reference for information about managing entity labels for proxy entities and agent entities.

Manage dynamic runtime assets

You can use token substitution in the URLs of your your dynamic runtime asset definitions. Token substitution allows you to host your dynamic runtime assets at different URLs (such as at different datacenters) without duplicating your assets, as shown in the following example:

type: Asset
api_version: core/v2
  name: sensu-go-hello-world
  namespace: default
  - sha512: 07665fda5b7c75e15e4322820aa7ddb791cc9338e38444e976e601bc7d7970592e806a7b88733690a238b7325437d31f85e98ae2fe47b008ca09c86530da9600
    url: "{{ .labels.asset_url }}/sensu-go-hello-world-0.0.1.tar.gz"
  "type": "Asset",
  "api_version": "core/v2",
  "metadata": {
    "name": "sensu-go-hello-world",
    "namespace": "default"
  "spec": {
    "builds": [
        "sha512": "07665fda5b7c75e15e4322820aa7ddb791cc9338e38444e976e601bc7d7970592e806a7b88733690a238b7325437d31f85e98ae2fe47b008ca09c86530da9600",
        "url": "{{ .labels.asset_url }}/sensu-go-hello-world-0.0.1.tar.gz"

With this asset definition, which includes the .labels.asset_url token substitution, checks and hooks can include sensu-go-hello-world as a dynamic runtime assets and Sensu Go will use the token substitution for the agent’s entity. Handlers and mutators can also include sensu-go-hello-world as a dynamic runtime asset, but Sensu Go will use the token subtitution for the backend’s entity instead of the agent’s entity.

You can also use token substitution to customize dynamic runtime asset headers (for example, to include secure information for authentication).

NOTE: To maintain security, you cannot use token substitution for a dynamic runtime asset’s SHA512 value.

Token specification

Sensu Go uses the Go template package to implement token substitution. Use double curly braces around the token and a dot before the attribute to be substituted: {{ .system.hostname }}.

Token substitution syntax

Tokens are invoked by wrapping references to entity attributes and labels with double curly braces, such as {{ .name }} to substitute an entity’s name. Access nested Sensu entity attributes dot notation (for example, system.arch).

  • {{ .name }} would be replaced with the entity name attribute
  • {{ .labels.url }} would be replaced with a custom label called url
  • {{ .labels.disk_warning }} would be replaced with a custom label called disk_warning
  • {{ index .labels "disk_warning" }} would be replaced with a custom label called disk_warning
  • {{ index .labels "cpu.threshold" }} would be replaced with a custom label called cpu.threshold

NOTE: When an annotation or label name has a dot (e.g. cpu.threshold), you must use the template index function syntax to ensure correct processing because the dot notation is also used for object nesting.

Token substitution default values

If an attribute is not provided by the entity, a token’s default value will be substituted. Token default values are separated by a pipe character and the word “default” (| default). Use token default values to provide a fallback value for entities that are missing a specified token attribute.

For example, {{.labels.url | default ""}} would be replaced with a custom label called url. If no such attribute called url is included in the entity definition, the default (or fallback) value of will be used to substitute the token.

Token substitution with quoted strings

You can escape quotes to express quoted strings in token substitution templates as shown in the Go template package examples. For example, to provide "substitution" as a default value for entities that are missing the website attribute (including the quotation marks):

{{ | default "\"substitution\"" }}

Unmatched tokens

If a token is unmatched during check preparation, the agent check handler will return an error, and the check will not be executed. Unmatched token errors are similar to this example:

error: unmatched token: template: :1:22: executing "" at <.system.hostname>: map has no entry for key "System"

Check config token errors are logged by the agent and sent to Sensu backend message transport as check failures.

Token data type limitations

As part of the substitution process, Sensu converts all tokens to strings. This means that token substitution cannot be applied to any non-string values like numbers or Booleans, although it can be applied to strings that are nested inside objects and arrays.

For example, token substitution cannot be used for specifying a check interval because the interval attribute requires an integer value. Token substitution can be used for alerting thresholds because those values are included within the command string.