Config and Theme

Config and theme

How the settings file is stored on a NebulaOS printer. The fields themselves are on Configuration.

• NebulaScreen reads nebulascreen.json from the directory of its executable, /opt/nebulascreen/ — part of the read-only squashfs image.
• At boot, NebulaOS bind-mounts the persistent file /usr/data/nebulaos/nebulascreen/nebulascreen.json over /opt/nebulascreen/nebulascreen.json, and the same file onto config/nebulascreen/nebulascreen.json so Mainsail can show and edit it. Both boot slots (A/B) use the same data partition.
• The first time, the file is seeded from the image's default (or copied once from a GuppyScreen-era guppyconfig.json).
• Because the target is a single-file bind mount, a save cannot rename a new file over it. Config::save() first writes a complete copy to nebulascreen.json.saving next to the persistent file, then rewrites the config in place; at the next start an interrupted save is completed from that copy or dropped.
• If the file exists but cannot be read or parsed, Config::init() uses the built-in defaults instead of crashing (ThemeConfig::init() does the same for themes). Covered by make test-config-theme-parse-safety.

Themes are read-only files in /opt/nebulascreen/themes/; the chosen theme name is stored in the config (theme).

See also: Integration with NebulaOS.