Strange output at end of terminal session in OS X El Capitan
Terminal saves a bunch of history so that when you open it again, it all looks like it did when you quit it.  The messages you are seeing reflect that saving of data.  You can see exactly what it is doing by taking a peek inside /etc/bashrc_Apple_Terminal:

grep "truncating history files" /etc/* 2&gt; /dev/null
/etc/bashrc_Apple_Terminal:     echo -n 'truncating history files...'