Switching to fish

If you wish to use fish as your default shell,
use the following command:

chsh -s /usr/local/bin/fish

Use the following command if fish isn’t already
added to /etc/shells to permit fish to be your
login shell:

echo /usr/local/bin/fish | sudo tee -a /etc/shells

To switch your default shell back, you can run
chsh -s /bin/bash (substituting /bin/bash
with /bin/tcsh or /bin/zsh as appropriate).