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. The env section supports two forms: a map form for static key-value pairs and an extended list form for advanced use cases like file-backed secrets with automatic refresh.

Values set in the env section are case-sensitive. However, as a best practice, Fluent Bit recommends using uppercase names for environment variables.

Map form

The map form defines variables as key-value pairs. The following example defines two variables, FLUSH_INTERVAL and STDOUT_FMT, which can be accessed in the configuration using ${FLUSH_INTERVAL} and ${STDOUT_FMT}:

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}

Extended list form

The env section also accepts a YAML list of entries. Each entry is a mapping with the following properties:

Property	Required	Description
name	Yes	The name of the environment variable.
value	No	An explicit value for the variable. Mutually exclusive with uri.
uri	No	A URI pointing to the variable's value source. Currently supports file:// URIs (for example, file:///run/secrets/my_secret). The file contents are read and used as the variable's value.
refresh_interval	No	How often (in seconds) to re-read the value from the uri source. Only meaningful when uri is set. Default is 0 (read once at startup).

Either value or uri must be provided for each entry.

When uri and refresh_interval are both set, Fluent Bit re-reads the file at the specified interval. This enables dynamic configuration values, such as rotating secrets, without restarting Fluent Bit. If refresh_interval is 0 or omitted, the file is read only once at startup.

env:
  - name: DB_PASSWORD
    uri: file:///run/secrets/db_password
    refresh_interval: 60
  - name: REGION
    value: us-east-1

service:
  flush: 1
  log_level: info

pipeline:
  inputs:
    - name: random

  outputs:
    - name: stdout
      match: '*'

In this example, ${DB_PASSWORD} is read from the file /run/secrets/db_password and refreshed every 60 seconds. ${REGION} is set to the static value us-east-1.

Default values

Minimum Fluent Bit version 5.0.6

Fluent Bit supports setting a default value for an environment variable if the variable isn't defined or is set to an empty string ('').

The syntax for a default value is ${VARIABLE_NAME:-DEFAULT_VALUE}.

service:
  flush: ${FLUSH_INTERVAL:-5}
  log_level: info

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.

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:

export FLUSH_INTERVAL=2

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

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.