chsh

chsh (man) change your login shell.

chsh -s /bin/bash 
chsh -s /bin/bash YOUR_USER 

chsh -s /bin/ksh
Changing shell for root.

chsh -s $(which fish)
Changing shell for YOURUSER.
chsh: /Users/user/brew/bin/fish: non-standard shell

echo $SHELL
sudo su -

Related

• Make fish your default shell in macOS

See also

• chsh
• Shell: Fish shell, sh, Bash, ksh, zsh, ash, PowerShell (Windows), /usr/bin/env, .hushlogin, dash, tcsh, Shell redirection,dquote, $SHELL, Google Cloud Shell, AWS CloudShell, explainshell.com, vmware_vm_shell, Nushell, Fig.io, direnv