# Environment variables

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

Variables set in this section can't be overridden by system environment variables.

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 provides a set of predefined environment variables that can be used in your configuration:

| 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.

Variables set in the `env` section can't be overridden by system environment variables.

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.