The Sages debate whether the Torah’s use of the plural “doorposts” indicates that a doorway with only one post does not require a mezuzah. In practice, if there is a post on the right side, we affix a mezuzah, without a berachah. Here are some other typical cases of single doorposts which are topped by lintels:
If a doorway has one doorpost on the right side of the person entering the room, and on its left, the wall continues without a break, a mezuzah should be affixed on the right. This is true even if that doorpost is nothing more than the end of a wall.[A]
If there is a doorpost on the right, but no wall continuing on the left, no mezuzah is necessary.
If the sole doorpost is on the left, and the continuing wall is on the right, no mezuzah is necessary.[B]
However, if the right wall continues less than three tefachim of the left wall, they are deemed to be in alignment enough to require a mezuzah on the right post. Even so, no berachah is recited. [C]
 Shach Y.D. 287:1.
 Aruch HaShulchan 247:8; Shevet HaLevi 5:160; Agur B’ohalecha )20:8( cites Maharsham )4:111(, who rules that if there is no door, one may exempt this opening in a case of need.
 Agur B’ohalecha )20:10 and 23:5( rules that one may exempt this opening in a case of need.
 Aruch HaShulchan 287:8; Shevet HaLevi 5:160; Agur B’ohalecha 20:8.
 Da’as Kedoshim cited in Chovas HaDar 7 and Mezuzot Beitecha 287:1.