For anyone looking for how to do this in macOS Catalina or above (10.15+ incl. Big Sur 11.0) which has deprecated bash in favour of zsh, here is my .zshrc file:
parse_git_branch() {
git branch 2> /dev/null | sed -n -e 's/^\* \(.*\)/[\1]/p'
}
COLOR_DEF='%f'
COLOR_USR='%F{243}'
COLOR_DIR='%F{197}'
COLOR_GIT='%F{39}'
NEWLINE=$'\n'
setopt PROMPT_SUBST
export PROMPT='${COLOR_USR}%n@%M ${COLOR_DIR}%d ${COLOR_GIT}$(parse_git_branch)${COLOR_DEF}${NEWLINE}%% '
If you don’t like the colours I have used, replace the 243/197/39 values with the colour codes as defined here:
https://misc.flogisoft.com/bash/tip_colors_and_formatting