Even though SHELL=/bin/bash in /etc/default/useradd, useradd fails to set the default shell as bash for new users.

useradd-default-shell-ineffectual.sh

vagrant@local:~$ cat /etc/default/useradd | grep SHELL
# The SHELL variable specifies the default login shell on your
SHELL=/bin/bash
vagrant@local:~$ sudo useradd -D
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
CREATE_MAIL_SPOOL=no
vagrant@local:~$ sudo useradd testing123
vagrant@local:~$ sudo tail -n 1 /etc/passwd
testing123:x:2013:2013::/home/testing123: