You may have just upgraded to macOS Catalina 10.15 and noticed that EVERY. SINGLE. TIME. YOU. OPEN. THE. TERMINAL. You get a warning about changing the default shell to zsh instead of bash. There’s an Apple help page about it, but in case you just wanted to google the answer and find it here, the secret is to add the following to your ~/.bash_profile.

export BASH_SILENCE_DEPRECATION_WARNING=1