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docs: env: bash-style default value operator :- #2568

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11 changes: 8 additions & 3 deletions administration/configuring-fluent-bit/classic-mode/variables.md
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Expand Up @@ -6,19 +6,24 @@ 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}`
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@patrick-stephens patrick-stephens May 8, 2026

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I don't think this is true, this is a bash shell syntax rather than anything with Fluent Bit itself.

Yes you can do this in bash outside of your Fluent Bit process but doing it in a Fluent Bit configuration file will not work - plus it won't work on targets like Windows either.

You could use it if you were setting them from the command line with a bash/posix shell but that's down to shell substitution.

e.g.

$ cat fluent-bit.conf
[INPUT]
  name dummy
  tag test
  dummy { "message": "${MY_TEST_VAR:-default}" }

[OUTPUT]
  name stdout
  match *
$ docker run --rm -it -v $PWD/fluent-bit.conf:/fluent-bit/etc/fluent-bit.conf:ro fluent/fluent-bit:5.0.5
...
[2026/05/08 09:16:43.720] [ warn] [env] variable ${MY_TEST_VAR:-default} is used but not set
[2026/05/08 09:16:44.720] [ warn] [env] variable ${MY_TEST_VAR:-default} is used but not set
[0] test: [[1778231803.720809223, {}], {"message"=>""}]
[0] test: [[1778231804.720746078, {}], {"message"=>""}]

As you can see it's actually using the whole string as the name of the variable rather than any default expansion and you end up with an empty string because of it.

I could run it (on a compatible shell that supports this syntax) like so but this is down to the shell itself and not Fluent Bit:

$ docker run --rm -it fluent/fluent-bit:5.0.5 -i dummy -p dummy="{ \"message\": \"${MY_TEST_VAR:-default}\"}" -o stdout -m '*'
...
[2026/05/08 09:20:10.480] [ info] [output:stdout:stdout.0] worker #0 started
[0] dummy.0: [[1778232010.720772049, {}], {"message"=>"default"}]
[0] dummy.0: [[1778232011.720769028, {}], {"message"=>"default"}]

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@benjaminmueggenburg-serato benjaminmueggenburg-serato May 8, 2026 •
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Oh my apologies, I should've linked the original PR in the description:
fluent/fluent-bit#11787

This is the PR for the updated docs if the changes were to be merged 😄

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@patrick-stephens patrick-stephens May 8, 2026

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Ah right, that makes more sense!

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@patrick-stephens patrick-stephens May 8, 2026

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Can you update the description to include the links and a bit of description?
Relying on AI for it all is not great in my experience :)

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@eschabell eschabell May 8, 2026

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@benjaminmueggenburg-serato same for me, see comment @patrick-stephens above.

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@benjaminmueggenburg-serato benjaminmueggenburg-serato May 10, 2026

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Thanks for your reviews! I've updated the description and resolved the linting/coderabbit suggestions.


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.

### 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 \(`fluent-bit.conf`\):
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12 changes: 12 additions & 0 deletions administration/configuring-fluent-bit/yaml/environment-variables-section.md
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Expand Up @@ -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
```

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benjaminmueggenburg-serato marked this conversation as resolved.
## 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.
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