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Packages available in AUR
Plex Media Server (PMS)
Install the package from AUR: AUR.
To start plex, run sudo systemctl start plexmediaserver. Use sudo systemctl enable plexmediaserver to have it start on startup. Then go to (http://localhost:32400/manage) to set up plex. If you want to add media folders in your home directory you may get a permissions issue. chmod 775 ~/ and usermod -a -G users plex to allow plex to access your files.
Plex Home Theater
You can build either plex package from source, or use the unofficial alucryd repository providing both regular AUR and nightlies of AUR.