[#templates]
= Templates in actions

OliveTin uses https://pkg.go.dev/text/template[Go text/template] syntax in action fields such as `shell`, `shellAfterCompleted`, entity directory titles, and `enabledExpression`. Template placeholders are written as `{{ ... }}`.

In OliveTin 3k, use dotted names for template context variables:

* `{{ .Arguments.NAME }}` — argument values (see xref:args/env.adoc[Environment variables])
* `{{ .CurrentEntity.property }}` — entity properties (see xref:entities/intro.adoc[Entities])
* `{{ .Env.VAR_NAME }}` — process environment (see xref:advanced_configuration/config_envs.adoc#using-env-in-template-replacements[Using .Env in template replacements])
* `{{ .OliveTin.Build.Version }}` and related build/runtime fields

In OliveTin 2k, argument and execution-request placeholders used the shorter form (for example, `{{ message }}` instead of `{{ .Arguments.message }}`).

[#json-encoding]
== JSON encoding with `Json`

The `Json` template function encodes a value as a JSON string. Pipe a template value to it when you need structured data in a command — for example, passing argument or entity state to a script or HTTP client that expects JSON.

[source,yaml]
----
actions:
  - title: curl my knx thing
    shell: curl --json '{{ .Arguments | Json }}' https://knx.example.com/v1/group/global_on/write
    entity: light
    arguments:
      - name: value
        default: "true"
----

After template substitution, `{{ .Arguments | Json }}` becomes a JSON object containing all argument names and values for that execution (including execution-request variables such as `ot_username` and `ot_executionTrackingId`).

=== Examples

Encode a single argument value:

[source,yaml]
----
shell: echo {{ .Arguments.value | Json }}
----

If `value` is `hello`, the substituted command is `echo "hello"`.

Encode an entity field:

[source,yaml]
----
shell: curl -d {{ .CurrentEntity.foo.bar | Json }}
----

If `foo.bar` is the string `baz`, the substituted command is `curl -d "baz"`.

Encode a nested entity object:

[source,yaml]
----
shell: curl --json -d {{ .CurrentEntity.payload | Json }}
----

If `payload` is `{on: true}`, the substituted command is `curl --json -d {"on":true}`.

=== Notes

. `Json` uses Go's `encoding/json` package. Strings, numbers, booleans, objects, and arrays are encoded according to normal JSON rules.
. Argument values in templates are strings (`map[string]string`). A checkbox or boolean argument therefore appears in JSON as a string (for example, `"true"`), not a JSON boolean.
. If the piped value is missing or nil, `Json` produces `null`.
. When embedding JSON in a shell command, quote the substitution if the JSON may contain spaces or shell metacharacters. Prefer single-quoted YAML strings around the template when possible, as shown in the curl example above.
. For HTTP request bodies, pass one JSON-encoded value (or build the JSON structure you need in one template expression). Piping several values with spaces between them does not produce a single valid JSON document.

See link:https://github.com/OliveTin/OliveTin/issues/829[GitHub issue #829] for the original feature request.
