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Variables

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:

  • Standard: ${MY_VARIABLE}
  • With Default Value: ${MY_VARIABLE:-default_value} (v5.0.6+)

When Fluent Bit starts, the configuration reader will detect any request for ${MY_VARIABLE} and will try to resolve its value.

When Fluent Bit is running under systemd (using the official packages), environment variables can be set in the following files:

  • /etc/default/fluent-bit (Debian based system)
  • /etc/sysconfig/fluent-bit (Others)

These files are ignored if they don't exist.

Fallback behavior

{% hint style="info" %} Minimum Fluent Bit version 5.0.6 {% endhint %}

If the ${VARIABLE:-DEFAULT} syntax is used, Fluent Bit will use the DEFAULT value if the variable meets either of the following conditions:

  1. The variable isn't defined (unset).
  2. The variable is defined but set to an empty string ('').

Nesting isn't supported. You can't use the ${} syntax within another ${} definition. ${MY_VAR:-${OTHER_VAR}} would be invalid.

Example

Create the following configuration file (fluent-bit.conf):

[SERVICE]
  Flush        1
  Daemon       Off
  Log_Level    info

[INPUT]
  Name cpu
  Tag  cpu.local

[OUTPUT]
  Name  ${MY_OUTPUT}
  Match *

Open a terminal and set the environment variable:

export MY_OUTPUT=stdout

The previous command sets the stdout value to the variable MY_OUTPUT.

Run Fluent Bit with the recently created configuration file:

$ bin/fluent-bit -c fluent-bit.conf

...
[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}]