I do not recommend change of SHELL systemwide. Let’s make it safe if brew upgrade will go bad.

MacOS is using bash version 3 (until MacOS 10.14 as in 10.15 zsh will replace it). Or to be exact it’s using /bash/sh which is:

sh-3.2# sh --version
GNU bash, version 3.2.48(1)-release (x86_64-apple-darwin12)
Copyright (C) 2007 Free Software Foundation, Inc.

Newer bash (v5 as of right now) has usefull things like associative arrays:

declare -A array

And more you can read in NEWS file.

So to upgrade bash to version 4 on our osx we need brew (kinda obvious).

Then we need to install bash:

brew install bash

Next we need to add our bash to shells:

sudo bash -c "echo $(brew --prefix)/bin/bash >> /private/etc/shells"

And the last step is to change our shell in system preferences so go to:

system preferences -> Users & Groups (unlock pref pane) -> right click on your account Advanced Options… and change Login shell option to /usr/local/bin/bash: