# Template Strings

Template Strings are strings which are dynamically replaced by Preview during a deploy, allowing you to inject environment-specific values into your deployment. 

Preview supports the use of Template Strings in both for all chart values and Kubernetes manifest files under the `./preview` directory.

For example, it is recommended to use the `PREVIEWHQ_HOSTNAME` as part of your application's Ingress specification:

```yaml
# .preview/chart-values/
ingress:
  enabled: true
  annotations:
    kubernetes.io/tls-acme: "true"
  path: /
  hosts:
    - host: ${PREVIEWHQ_HOSTNAME}
      paths:
        - /
  tls:
   - secretName: tls-secret
     hosts:
       - ${PREVIEWHQ_CLUSTER_HOSTNAME}
       - ${PREVIEWHQ_HOSTNAME}
```

There are 2 sets of Template Strings which can be utilized: one static set across all builds, and one set that is dynamically build based on image builds.

#### Static Template Strings

| Template String              | Description                                                     | Example                          |
| ---------------------------- | --------------------------------------------------------------- | -------------------------------- |
| `PREVIEWHQ_HOSTNAME`         | The hostname of the deployment, which will point to the root    | `my-pr-branch.nrmitchi.envs.dev` |
| `PREVIEWHQ_CLUSTER_HOSTNAME` | The hostname of the cluster which the deployment is deployed on | `nrmitchi.envs.dev`              |
| `PREVIEWHQ_NAMESPACE`        | The namespace which the deployment is deployed in to            | `my-pr-branch`                   |
| `PREVIEWHQ_REVISION`         | The git hash being deployed                                     |                                  |
| `PREVIEWHQ_IMAGE_REPOSITORY` | The URL of the Preview-provided Docker image repository that    |                                  |

#### Dynamic Template Strings

For each addition image specified in the `images` block of the configuration, Preview injects a `PREVIEWHQ_CUSTOM_IMAGE_TAG_{}` string, which is the Docker tag which was used for that custom build within the Docker image registry.

For example, if `images` is as follows:

```yaml
images:
  - name: frontend
    dockerfile: ./src/react/Dockerfile
    context: ./src/react
  - name: backend
    dockerfile: ./src/flask/Dockerfile
    context: ./src/flask
```

The the following additional Templates Strings are available:

* `PREVIEWHQ_CUSTOM_IMAGE_TAG_FRONTEND`
* `PREVIEWHQ_CUSTOM_IMAGE_TAG_BACKEND`