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“I’m gonna create a prepare_bootloader hook [in core] so we can refactor u-boot [into an extension]”

The extensions framework allows the board/family developers, extension authors, and users to extend the Armbian build system without overloading the core with specific functionality.

It’s a simple framework, written in Bash, that works based on function naming conventions. It provides the core and the extensions with tracing and debugging, (error control,?) inline documentation and very simple dependency resolution.


  • The “core” is everything that’s in lib/ directory plus and some others. It’s the spine of the build system.
  • An “extension” is a separate Bash source file that contains exclusively functions. Extensions live in extensions/ or userpatches/extensions/ directory, but could one day be in a separate repository too.
  • An “extension method” (a.k.a “hook”) is used by the core to call extensions, via call_extension_method(). This will discover all enabled extension methods implementations, order them, and call them one by one. - The Armbian core already has quite a few of these in strategic spots. - More are coming as they’re identified.
  • An “extension method implementation” is a function that will be called when it’s extension method is called. It can be defined in extensions, but also in board config, family config, user config, etc.


Core calls extensions

The Armbian core build system has an extension method called run_after_build, also known as the “run_after_build hook”. You can find it in lib/ around line 546.

# in lib/
call_extension_method "run_after_build" [...]
Extension method implementation

Consider the following function:

function run_after_build__say_congratulations() { 
  echo "Congrats, the build is finished!"

Such a function is an “extension method implementation” called say_congratulations for the extension method run_after_build.

Extension file

A file userpatches/extensions/ containing the above function is an “extension” called be-festive.

Using it

An user of the build system can enable that extension by adding a call to enable_extension "be-festive" on his configuration file, or by passing ENABLE_EXTENSIONS=be-festive as a parameter to the build.

Naming conventions and ordering

An extension method implementation is just a Bash function that follows the pattern run_after_build__say_congratulations where

  • run_after_build is the name of the extension method.
  • __ is a marker/separator – very important – two underscores, not one, not three.
  • say_congratulations is the name of the extension method implementation, and should be unique.

The system will “magically” compose a single run_after_build() function, based on all the hook functions that begin with run_after_build__.

Hook functions will be sorted by their numerical value; hook functions that do not begin with a number will receive 500_ prefix automatically.

So the examples run_after_build__do_this and run_after_build__500_do_this are equivalent, and will run

  • sooner than run_after_build__900_do_smth_else
  • later than run_after_build__300_do_even_another_thing

What is an extension?

A extension is Bash source file that contains exclusively:

  • function definitions: - extension method implementation definitions (with __ separator) - other internal functions (for structure and clarity if needed)
  • calls to enable_extension "another-extension" at the top of the file. - that’s a very simple dependency system, one extension can enable another.

Specifically, extension files should not contain any code outside of functions – they should do nothing when sourced.

Extensions can be official Armbian fragments and live in /extensions, or can be user-specific in /userpatches/extensions.

An extension could be implemented in any of the following file/dir structures:

  • /extensions/ - an official, single-file extension.
  • /userpatches/extensions/ - a user-specific, single-file extension.
  • /extensions/our-dir-ext/ - an official, directory-based extension.
  • /userpatches/extensions/my-dir-ext/ - a user-specific, directory-based extensions.

The official extensions can be used by boards, family includes, etc, while the user-specific extensions can only be used by userpatches code or via ENABLE_EXTENSIONS=my-ext,my-dir-ext build parameter.

Single-file vs Directory-based

They’re the same, except:

  • Directory-based extensions will be passed a ${EXTENSION_DIR} environment variable.
  • That is useful if there are other files/assets that belong together with that extension. An example would be a template file, some configuration file, or other static asset that is directly related to the extension.
  • Using directory-based extensions and ${EXTENSION_DIR} allows for easy moving and PR’ing of user extensions.