Some users may ask you to change the default shell to BASH as their used to be. Here are the steps:
Check Default Shell
Login as root, then check current default shell of the user.
# grep scott /etc/passwd
scott:x:100:10::/home/scott:/bin/sh
It's /bin/sh, the system shell.
Check BASH
Let's see where BASH is.
# which bash
/usr/bin/bash
Change Shell by usermod
Now we can change the default shell to BASH by the command usermod with -s option.
# usermod -s /usr/bin/bash scott
Or you might be interested in KSH.
# usermod -s /usr/bin/ksh scott
Check Default Shell Again
Let's check the result.
# grep scott /etc/passwd
scott:x:100:10::/home/scott:/usr/bin/bash
We have changed the default shell to BASH for the user.
In some Unix (AIX), you have to additionally make sure BASH is in the allowable list, if not, append it.
# vi /etc/security/login.cfg
...
shells = /bin/sh,/bin/bsh,/bin/csh,/bin/ksh,/bin/tsh,/bin/ksh93,/usr/bin/sh,/usr/bin/bsh,/usr/bin/csh,/usr/bin/ksh,/usr/bin/tsh,/usr/bin/ksh93,/usr/bin/rksh,/usr/bin/rksh93,/usr/sbin/uucp/uucico,/usr/sbin/sliplogin,/usr/sbin/snappd,/usr/bin/bash
Further reading: How to Change Primary Group of a User in Linux.