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
