macOS comes with a Bash version with GPLv2 license, it is a version back in 2007:

$ /bin/bash --version
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin18)
Copyright (C) 2007 Free Software Foundation, Inc.

To use newer version,

1. Install from brew:

   $ brew install bash

2. Whitelist it:

   $ sudo vim /etc/shells

   And add /usr/local/bin/bash to /etc/shells.

3. Finally set default shell:

   $ chsh -s /usr/local/bin/bash
   $ echo $BASH_VERSION
   $ sudo chsh -s /usr/local/bin/bash

Do not set $TERM in bashrc! It should be set by terminal emulator. However, to make prompt $PS1 display properly (italics) in tmux, it's best to use tmux-256color as tmux default-terminal, as in tmux.conf. And ncurses in macOS does not provide the right terminfo out-of-box, which creates problem for using Vim in tmux session. To enable tmux-256color terminfo support in macOS, see terminfo.

When installing using brew, use the cask version, because it has python/dyn and python3/dyn enabled. Otherwise you need to compile Vim.

I use a Python 3.8.6 version installed by pyenv. Its dynamic library (called framework in macOS) is linked in vimrc. To build CPython with framework support on macOS:

1. Uninstall binutils with brew temporarily. Usually instead of uninstalling we can do

   $ AR=/usr/bin/ar pyenv install ...

   but with option in next step this won't work. So we need to uninstall binutils and install it back afterwards.

2. Run following in Bash:

   $ PYTHON_CONFIGURE_OPTS="--enable-framework" pyenv install 3.8.6

I tried this with pyenv-virtualenv but no luck.

This command queries system appearance:

$ defaults read -g AppleInterfaceStyle

In my vimrc, function MacBackground() utilises it.

Per this pull of MacVim:

• An event OSAppearanceChanged is added. This event is emitted when MacVim changes its appearance.
• A vim variable v:os_appearance is added. This variable reflects the appearance setting.

In my vimrc, function MacGUIBackground() utilises it.