How to set the default shell?

If you want to set a shell as a the default for all users you can set the SHELL variable

$ sudo vi ~/.bashrc

export SHELL=/bin/bash

To change the default shell for a specific user you can either use the following command $ usermod -s /path/to/shell user_a

or, set the shell in the /etc/passwd file by changing the last column which represents the default shell for that user

user_a:x:1001:1001::/home/user_a:/bin/sh

Read more about the Linux shell