# Environment variables

The `env` section of YAML configuration files lets you define environment variables. These variables can then be used to dynamically replace values throughout your configuration using the `${VARIABLE_NAME}` syntax.

Values set in the `env` section are case-sensitive. However, as a best practice, Fluent Bit recommends using uppercase names for environment variables. The following example defines two variables, `FLUSH_INTERVAL` and `STDOUT_FMT`, which can be accessed in the configuration using `${FLUSH_INTERVAL}` and `${STDOUT_FMT}`:

```yaml
env:
  FLUSH_INTERVAL: 1
  STDOUT_FMT: 'json_lines'

service:
  flush: ${FLUSH_INTERVAL}
  log_level: info

pipeline:
  inputs:
    - name: random

  outputs:
    - name: stdout
      match: '*'
      format: ${STDOUT_FMT}
```

## Predefined variables

Fluent Bit supports the following predefined environment variables. You can reference these variables in configuration files without defining them in the `env` section.

| Name          | Description            |
| ------------- | ---------------------- |
| `${HOSTNAME}` | The system's hostname. |

## External variables

In addition to variables defined in the configuration file or the predefined ones, Fluent Bit can access system environment variables set in the user space. These external variables can be referenced in the configuration using the same `${VARIABLE_NAME}` pattern.

{% hint style="info" %}
Variables set in the `env` section can't be overridden by system environment variables.
{% endhint %}

For example, to set the `FLUSH_INTERVAL` system environment variable to `2` and use it in your configuration:

```shell
export FLUSH_INTERVAL=2
```

In the configuration file, you can then access this value as follows:

```yaml
service:
  flush: ${FLUSH_INTERVAL}
  log_level: info

pipeline:
  inputs:
    - name: random

  outputs:
    - name: stdout
      match: '*'
      format: json_lines
```

This approach lets you manage and override configuration values using environment variables, providing flexibility in various deployment environments.