useradd -b does not exist however useradd -D -b does and then its used as the following
Code:
useradd -D -b /home/dir
When invoked with the -D option, useradd will either display the current default values, or update the default values from the commandline.
and if you give the option -b /home/dir then /home/dir will be the new default homedir of the current user