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Glossary

Short definitions for terms used across the docs.

Appliance concepts

  • Mode: One of the mutually-exclusive runtime states on the Pi: kiosk mode or Retro mode.
  • Fail-open: A non-fatal failure mode: if something external is unavailable (for example NFS or kiosk mode), the appliance continues operating using local data where possible.

Storage + NFS

  • NFS: Network File System; used here for optional ROM sync (NFS -> local) and optional save backups (local -> NFS).
  • Share layout: The expected directories at the NFS export root: roms/ and backups/.
  • Hostname component: The per-device directory under backups/<hostname>/ used to keep multiple kiosks separated.

Linux + systemd

  • systemd: The Linux init/service manager. Starts services at boot, restarts them on failure, and enforces ordering.
  • Unit: A systemd configuration file that describes something systemd manages (e.g. .service, .timer).
  • Service: A unit (.service) describing how to run a process (ExecStart, user, restart policy, dependencies).
  • Timer: A unit (.timer) that triggers a service on a schedule.
  • OnFailure: A systemd directive that starts another unit if the current one fails.
  • journald: The system logging daemon used by systemd.
  • journalctl: The command used to read logs from journald.

Display/session plumbing

  • Xorg: The X display server used here to render Chromium and RetroPie.
  • xinit: A helper that starts an X server + a program/script (used to launch the kiosk/retro session).
  • VT (virtual terminal): Linux text consoles (e.g. VT7/VT8). This project uses fixed VTs to keep modes isolated.
  • logind: The systemd component that manages user sessions and device access (seat ownership, input, display).

Hardware interfaces

  • evdev: Linux’s standard input event interface. Controllers appear as /dev/input/event* devices.
  • sysfs: Kernel-exposed files under /sys/ used to control hardware (LED triggers, backlight brightness, etc).
  • Backlight: The sysfs interface under /sys/class/backlight/<name>/ used to read/write screen brightness.

MQTT

  • MQTT: A lightweight pub/sub messaging protocol commonly used by dashboards/controllers and IoT devices.
  • Topic: The string namespace messages are published to (e.g. kiosk-retropie/led/act/set).
  • Topic prefix: A shared prefix (KIOSK_MQTT_TOPIC_PREFIX, default kiosk-retropie) so related topics group together.
  • Retained message: A message stored by the broker and delivered immediately to new subscribers (useful for “current state”).
  • State topic: A topic where the appliance publishes current state for UIs/automation.
  • Set/command topic: A topic where a dashboard/controller (or another client) publishes desired state.

Repo tooling

  • Devcontainer: A Docker image + configuration used to provide a consistent toolchain for development and CI.
  • Bats (Bash Automated Testing System): The test framework used for Bash scripts.
  • kcov: Coverage tool used here to measure Bash line coverage and enforce 100% coverage in CI.