I've seen a few questions as to how to enter into bash while fish is the default shell. I actually want to do the opposite. I currently have fish as my default shell, but would like to return to bash as my default. However, I do not want to uninstall fish and would like the option to enter into a shell session with fish at my discretion. Any advice on how to do this?
You could change your default shell to bash by typing in the following line:
chsh -s /bin/bash
You will probably need to log out and log back in for the settings to take effect.
Then if you wanted to run fish, it would just be the command
fish at a bash prompt.
More information can be found at https://wiki.ubuntu.com/ChangingShells