Method 1:

Use ~/.profile .

For example, to change the default shell to bash, add the following lines to ~/.profile

# Change the default shell to bash
eval /bin/bash

Method 2:

use chsh. For example, to set the default shell to zsh.

rajulocal@hogwarts ~ % chsh -s /usr/bin/zsh 
Password:

echo $SHELL

• Which shell am I using?
• What is the difference between \$SHELL and \$0?