How to Use Environmental Variables in Fluentd Config

Fluentd, starting from version v1.13.0, allows the integration of environment variables within its configuration using the syntax #{ENV['YOUR_ENV_VARIABLE']}.

For instance, if you've defined an environment variable SECRET_KEY in your system, it can be used in the Fluentd configuration file as follows:

some_field "#{ENV['SECRET_KEY']}"

Remember to enclose the syntax in double quotes, not single quotes.

This setup dynamically assigns the value of the SECRET_KEY environment variable to some_field.

To illustrate with a more specific example, consider a match block in Fluentd's configuration used for forwarding logs to Better Stack, which requires a source token like W8QhqAQ82MVn1nqgPZoe2T9G. The environment variable can be utilized in this context as shown:

<label /etc/fluent/fluentd.conf>
  <match docker.nginx.**>
    @type logtail
    @id output_nginx_logtail
    source_token "#{ENV['SOURCE_TOKEN']}"
    flush_interval 2 # in seconds
  </match>
</label>

Additionally, environment variables can be employed for setting tags. For example, using the FLUENTD_TAG environment variable to specify a tag would look like this:

<match "docker.#{ENV['FLUENTD_TAG']}">
  ...
</match>

For a comprehensive understanding of Fluentd and its capabilities in log data management, you can refer to this guide on collecting, processing, and shipping log data with Fluentd.

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