If a command is issued that can't be executed as a nor-
mal command, and the command is the name of a direc-
tory, perform the cd command to that directory.
Make cd push the old directory onto the directory
If the argument to a cd command (or an implied cd with
the AUTO_CD option set) is not a directory, and does
not begin with a slash, try to expand the expression as
if it were preceded by a `~'.
An array (colon-separated list) of directories specify-
ing the search path for the cd command.
The default argument for the cd command.