# Variables

![](https://static.scarf.sh/a.png?x-pxid=1731c7b5-34c6-424f-bfc6-88c2aa71e81f)

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:

```
${MY_VARIABLE}
```

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`](https://systemd.io/) (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.

## Example

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

```yaml
[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:

```shell
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:

```shell
$ 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}]
```