How do I set fish as the default shell?

You can set fish as your default shell by running

chsh -s `which fish`

To do so for another user, just add the username at the end of the command, and use sudo.

Then log out and back in.

(To set it back to bash, you need to use Fish syntax of course, like this chsh -s (which bash))

usermod -s /usr/bin/fish username

Must be run as root though.

This will change the shell permanently for the specified user.

I just added the line fish to the end of my .bashrc.