Where does Plex media server store its cache/settings?

On Ubuntu/Debian settings and library stored in /var/lib/plexmediaserver/....

The PlexMediaScanner (cli-commands) can be found under /usr/lib/plexmediaserver/

In general, the location for the various Linux versions of Plex Media Server will be found under:

$PLEX_HOME/Library/Application Support/Plex Media Server/

Just cd to this path and all settings should be there. Usually this path should be under your home directory. In Ubuntu you can check where the path is by running:

cat /etc/default/plexmediaserver | grep  PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR