macOS ships with an older version of the Bash shell, because newer versions use a license that makes it more difficult for Apple to integrate into their OS. In macOS Catalina, Apple changed the default shell to ZSH for this reason.

But by using Homebrew, you can install the latest version of Bash and use its features.

First, check your current Bash version:

$ echo $BASH_VERSION

If you see a version like the following, you're running a very old version:

3.2.57(1)-release

Ensure you have Homebrew installed, which you can do by following Installing Homebrew.

Then use Homebrew to install Bash:

$ brew install bash

To see where Homebrew installed Bash, run the following command:

$ ls "$(brew --prefix)/bin/bash" 

You'll see a result like this:

/usr/local/bin/bash

Next, add the path to Homebrew's version of Bash to the /etc/shells file. Display the contents of the file to ensure that it's not already listed:

$ cat /etc/shells

You'll see something like this:

# List of acceptable shells for chpass(1).
# Ftpd will not allow users to connect who are not using
# one of these shells.

/bin/bash
/bin/csh
/bin/ksh
/bin/sh
/bin/tcsh
/bin/zsh

Use the following command to add the newer version of Bash to the /etc/shells file:

$ echo "$(brew --prefix)/bin/bash" | sudo tee -a /etc/shells;

Since the file is owned by the administrator, you can't redirect text using standard redirection, but you can work around it by using tee to send output to a file as it's run with sudo. The -a flag appends text rather than overwriting the existing file.

Ensure it was added:

$ cat /etc/shells

The new shell will be listed at the end of the file:

# List of acceptable shells for chpass(1).
# Ftpd will not allow users to connect who are not using
# one of these shells.

/bin/bash
/bin/csh
/bin/ksh
/bin/sh
/bin/tcsh
/bin/zsh
/usr/local/bin/bash

Finally, use the chsh command to change the shell for your user:

$ chsh -s "$(brew --prefix)/bin/bash"

You'll be promped for your password. Enter it and your shell will change:

Changing shell for brianhogan.
Password for brianhogan:

The current shell doesn't change, so open a new Terminal and use the following command to check your Bash version:

$ echo $BASH_VERSION
5.0.11(1)-release

You now have the latest version installed.

To change back to the system-installed version of Bash, use chsh -s /bin/bash.

Conclusion

You've changed your shell to the latest version of Bash, and you know how to change it back to the default. You can use the same approach to install the latest version of ZSH, or other shells like Fish. Install the shell, add the path to its binary to /etc/shells, and use chsh to switch the shell.