# RAUC

> The update system of Home Assistant OS. It installs a new version of the whole system next to the running one, so a device can go back when an update does not work.

Source: https://contributors.home-assistant.io/glossary/rauc/

- In code: `.raucb`, `buildroot-external/ota/`
- Also: update bundle

Home Assistant OS has two system slots. [RAUC](https://rauc.io/) writes an update bundle to the slot that is not running, and the device boots into it. If that does not work, it goes back to the slot it came from. The Supervisor starts these updates. See [The update system](https://contributors.home-assistant.io/os/operating-system/update-system/).

## Related terms

- [Home Assistant OS](https://contributors.home-assistant.io/glossary/home-assistant-os/): The operating system made to run Home Assistant, with the Supervisor, apps, and backups. The recommended way to install Home Assistant.
- [Buildroot](https://contributors.home-assistant.io/glossary/buildroot/): The tool Home Assistant OS is built with. It builds a complete, small Linux system from source, instead of starting from a distribution like Ubuntu.
- [Supervisor](https://contributors.home-assistant.io/glossary/supervisor/): The part of Home Assistant OS that runs Home Assistant Core and apps, each in its own container, and handles their updates and backups.