If you add new users with command
useradd
then change /etc/default/useradd
You can also do it by running
Code:
useradd -D -s /bin/bash
See also
man useradd
If you add new users with command
adduser
then change /etc/adduser.conf
See also
man adduser
Regarding your script adduser.sh, alternatively you can set the shell case-by-case, by means of
Code:
useradd -s /bin/bash -c $2 -U $1