# Dynamic configuration {#dynamic-configuration}

There are certain scenarios where you might need to dynamically configure your project at generation time. For example, you might want to change the name of the app, the bundle identifier, or the deployment target based on the environment where the project is being generated. Tuist supports that via environment variables, which can be accessed from the manifest files.

## Configuration through environment variables {#configuration-through-environment-variables}

Tuist allows passing configuration through environment variables that can be accessed from the manifest files. For example:

```bash
TUIST_APP_NAME=MyApp tuist generate
```

If you want to pass multiple environment variables just separate them with a space. For example:

```bash
TUIST_APP_NAME=MyApp TUIST_APP_LOCALE=pl tuist generate
```

## Reading the environment variables from manifests {#reading-the-environment-variables-from-manifests}

Variables can be accessed using the [`Environment`](https://projectdescription.tuist.dev/documentation/projectdescription/environment) type. Any variables following the convention `TUIST_XXX` defined in the environment or passed to Tuist when running commands will be accessible using the `Environment` type. The following example shows how we access the `TUIST_APP_NAME` variable:

```swift
func appName() -> String {
    if case let .string(environmentAppName) = Environment.appName {
        return environmentAppName
    } else {
        return "MyApp"
    }
}
```

Accessing variables returns an instance of type `Environment.Value?` which can take any of the following values:

| Case | Description |
| --- | --- |
| `.string(String)` | Used when the variable represents a string. |

You can also retrieve the string or boolean `Environment` variable using either of the helper methods defined below, these methods require a default value to be passed to ensure the user gets consistent results each time. This avoids the need to define the function appName() defined above.

::: code-group

```swift [String]
Environment.appName.getString(default: "TuistServer")
```

```swift [Boolean]
Environment.isCI.getBoolean(default: false)
```
<!-- -->
:::

## Forwarding additional environment variables to manifests {#forwarding-additional-environment-variables-to-manifests}

By default, only variables prefixed with `TUIST_`, plus `CI` and `DEVELOPER_DIR`, are forwarded to the subprocess that evaluates `Project.swift`, `Workspace.swift`, `Package.swift`, and the project description helpers. This keeps generation deterministic and prevents unrelated shell or CI variables from invalidating the manifest cache.

When a manifest needs to read variables that are not under your control (for example, a third-party `Package.swift` that already reads `OPENSWIFTUI_LIBRARY_TYPE`), declare them in `Tuist.swift` via `manifestEnvironment` on the generation options. Each entry is either a literal name or a name ending in `*` for prefix matching:

```swift
import ProjectDescription

let tuist = Tuist(
    project: .tuist(
        generationOptions: .options(
            manifestEnvironment: [
                "OPENSWIFTUI_*",
                "MY_VAR",
            ]
        )
    )
)
```

Listed variables participate in the manifest cache hash, so changes to their values invalidate the cache as expected. Variables not listed remain hidden from manifest evaluation.