For those upgrading from an earlier macOS version; you may have seen the message:

The default interactive shell is now zsh.
To update your account to use zsh, please run `chsh -s /bin/zsh`.

Now you can switch to zshell, and it is probably not a bad idea to do so.
Keep in mind though that there will be some minor issues with your shell script.

So if you want to make bash your default shell again, execute this in Terminal:

chsh -s /bin/bash

if you want to use zshell (zsh) then execute this in Terminal:

chsh -s /bin/zsh