by habit

See: pro forma
References in classic literature ?
On the contrary, during one period of my life religion took a firm hold upon me; after I was twelve and until-until--why, I suppose until now, though I never thought much about it--just driven along by habit.
This study compared participants that completed the 42-day "Quit Smoking" program by Habit Changer against participants who used nicotine-replacement therapies or other alternatives to quit or reduce intake of cigarettes.
The Quit Smoking program is a 42-day program by Habit Changer that is a novel cognitive-behavioral therapy aimed at helping participants put an end to their smoking habit.
Each program developed by Habit Changer provides a 42-day challenge that helps participants create daily work plans with easy step-by-step action.