Fluent Bit supports the usage of environment variables in any value associated to a key when using a configuration file.
The variables are case sensitive and can be used in the following format:
When Fluent Bit starts, the configuration reader will detect any request for ${MY_VARIABLE} and will try to resolve its value.


Create the following configuration file (fluent-bit.conf):
Flush 1
Daemon Off
Log_Level info
Name cpu
Tag cpu.local
Match *
Open a terminal and set the environment variable:
$ export MY_OUTPUT=stdout
The above command set the 'stdout' value to the variable MY_OUTPUT.
Run Fluent Bit with the recently created configuration file:
$ bin/fluent-bit -c fluent-bit.conf
Fluent Bit v1.4.0
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
[2020/03/03 12:25:25] [ info] [engine] started
[0] cpu.local: [1491243925, {"cpu_p"=>1.750000, "user_p"=>1.750000, "system_p"=>0.000000, "cpu0.p_cpu"=>3.000000, "cpu0.p_user"=>2.000000, "cpu0.p_system"=>1.000000, "cpu1.p_cpu"=>0.000000, "cpu1.p_user"=>0.000000, "cpu1.p_system"=>0.000000, "cpu2.p_cpu"=>4.000000, "cpu2.p_user"=>4.000000, "cpu2.p_system"=>0.000000, "cpu3.p_cpu"=>1.000000, "cpu3.p_user"=>1.000000, "cpu3.p_system"=>0.000000}]
As you can see the service worked properly as the configuration was valid.