Starting with the macOS Catalina beta, your Mac uses zsh as the default login shell and interactive shell. You can make zsh the default in earlier versions of macOS as well.
By default, your Mac uses either zsh or bash as the command-line interpreter for the login shell and interactive shell:
zsh (Z shell) is the default shell for all newly created user accounts, starting with the macOS Catalina beta, currently available only to members of the Apple Developer Program.
bash is the default shell in macOS Mojave and earlier.
zsh is highly compatible with the Bourne shell (sh) and mostly compatible with bash, with some differences. For more about zsh and its comprehensive command-line completion system, enter man zsh in Terminal.

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