Is there a command-line BitTorrent client preinstalled on OS X?
No, not pre-installed as far as I know.



Try Deluge.


  It has been designed using the client server model with a daemon process that handles all the bittorrent activity. The Deluge daemon is able to run on headless machines with the user-interfaces being able to connect remotely from any platform.


Source: About Deluge

Alternatively, try rTorrent, aria2, or TorrentFlux.

For simple downloading of .torrent or magnet torrents, aria2 might be your best option.



Setting up aria2:


Install aria2 per the official install instructions.
aria2 can accept .torrent links and magnet links:

aria2c http://example.org/mylinux.torrent
aria2c 'magnet:?xt=urn:btih:248D0A1CD08284299DE78D5C1ED359BB46717D8C'





Setting up Deluge console access:


Install Deluge per the official install instructions.
Run the daemon and the console process, then connect locally.

deluged
deluge-console
connect 127.0.0.1:{{DELUGEDPORT}}

Add a torrent per this syntax:

add [-p &lt;save-location&gt;] &lt;torrent-file&gt; [&lt;torrent-file&gt; ...]

Use --help flag for more help on commands.


Setting up Deluge remote access:


Install Deluge per the official install instructions.
Create the configuration files by starting/stopping the daemon:

deluged
pill deluged

Add authentication with the syntax &lt;username&gt;:&lt;password&gt;:&lt;level&gt;:

echo "alice:MyC0mpL3xPass:10" &gt;&gt; ~/.config/deluge/auth


About authentication levels
Allow connections beyond localhost.

deluge-console "config -s allow_remote True"
deluge-console "config allow_remote"

Start the daemon.

deluged



Setting up rTorrent:


Install rTorrent per the official download &amp; install instructions.
Run rtorrent to begin the interactive access.

Use -h flag for more info on using rTorrent.