docs: env: bash-style default value operator :-

administration/configuring-fluent-bit/classic-mode/variables.md

@@ -6,9 +6,8 @@ Fluent Bit supports the usage of environment variables in any value associated t
 
 The variables are case sensitive and can be used in the following format:
 
-```text
-${MY_VARIABLE}
-```
+* **Standard:** `${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.
 
@@ -19,6 +18,18 @@ When Fluent Bit is running under [`systemd`](https://systemd.io/) (using the off
 
 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 \(`fluent-bit.conf`\):
@@ -53,4 +64,4 @@ $ 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}]
-```
+```

administration/configuring-fluent-bit/yaml/environment-variables-section.md

@@ -65,6 +65,22 @@ pipeline:
 
 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
+
+{% hint style="info" %}
+Minimum Fluent Bit version 5.0.6
+{% endhint %}
+
+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}`.
+
+```yaml
+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.