On Linux, the qbittorrent-nox package includes a version of qBittorrent that does not require any GUI-related components to run, ideal to run in headless server environments. It can be controlled via a feature rich HTTP WebAPI and WebUI.
You can install it from a binary package repository such as your distro's repositories, the official Ubuntu PPAs, etc.
You can also compile it from source by toggling the flag that disables the GUI (refer to the relevant compilation guides on the wiki).
Simply execute qbittorrent-nox. At first execution, you should get prompted to accept the legal disclaimer. After that, qBittorrent will print out some useful information, such as the URL the WebAPI/WebUI can be accessed at, via a web browser, and the default credentials.
To quit, simply press Ctrl-c or send SIGINT or SIGTERM to the process.
After first start and having accepted the legal disclaimer, you can also use qbittorrent-nox -d to execute qbittorrent as a daemon in the background, instead of running it in the foreground.
You probably want to control the daemon, like setting it to start on system boot, for example. To learn how to do that with systemd on Ubuntu 15.04 and other modern releases of mainstream distros, check out the dedicated article.
Settings can be customized via the WebUI, WebAPI, or, if needed, manually editing the config file when qBitorrent is not running (`~/.config/qBittorrent/qBittorrent.conf).
The logs as well as other program data are located in ~/.local/share/data/qBittorrent.
Consult the man page or pass --help for more information.