# BT.set_env

## Function

 writes to the BT runtime environment variable overlay.

## Syntax

```bt
BT.set_env(name, value)
```

## Parameters

| Parameters | Type | Required | Default value | Description |
| ------ | ------ | ------ | ------ | ------ |
| name | String | Yes | None | Environment variable name. |
| value | Any | No | '' | Environment variable value, converted to string before writing. |

## Return Value

| Type | Description |
| ------ | ------ |
| String/Empty | Returns the valid value before writing; returns empty if it does not exist originally. |

## Examples

```bt
BT.set_env('BT_MODE', 'dev')
result = BT.env('BT_MODE')

// Output: dev
print result
```

## Notes

- BT.set_env() only modifies the BT runtime overlay and does not call the system global environment variable writing.
- When the variable with the same name originally has a deletion mark, set_env will overwrite the deletion mark.