From 468481140c1f3274777e0e4ae3cde8694b112707 Mon Sep 17 00:00:00 2001
From: benjaminmueggenburg-serato <benjamin.mueggenburg@serato.com>
Date: Mon, 11 May 2026 08:56:12 +1200
Subject: [PATCH] docs: env: bash-style default value operator :-

Signed-off-by: benjaminmueggenburg-serato <benjamin.mueggenburg@serato.com>
---
 .../configuring-fluent-bit/classic-mode/variables.md | 11 ++++++++---
 .../yaml/environment-variables-section.md            | 12 ++++++++++++
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/administration/configuring-fluent-bit/classic-mode/variables.md b/administration/configuring-fluent-bit/classic-mode/variables.md
index 1cfaf3e5b..1d16aeda5 100644
--- a/administration/configuring-fluent-bit/classic-mode/variables.md
+++ b/administration/configuring-fluent-bit/classic-mode/variables.md
@@ -6,12 +6,12 @@ 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}`
 
 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)
@@ -19,6 +19,11 @@ When Fluent Bit is running under [`systemd`](https://systemd.io/) (using the off
 
 These files are ignored if they don't exist.
 
+### Fallback behavior
+If the `${VARIABLE:-DEFAULT}` syntax is used, Fluent Bit will use the `DEFAULT` value if the primary 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** (`''`).
+
 ## Example
 
 Create the following configuration file \(`fluent-bit.conf`\):
diff --git a/administration/configuring-fluent-bit/yaml/environment-variables-section.md b/administration/configuring-fluent-bit/yaml/environment-variables-section.md
index 2553d9181..c3e80c672 100644
--- a/administration/configuring-fluent-bit/yaml/environment-variables-section.md
+++ b/administration/configuring-fluent-bit/yaml/environment-variables-section.md
@@ -65,6 +65,18 @@ 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
+
+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.