It's pretty easy, first add the fish shell repository:
sudo apt-add-repository ppa:fish-shell/release-3
Then just updated your repos:
sudo apt-get update
Now we can install it:
sudo apt-get install fish
One more command and we are done:
chsh -s /usr/bin/fish
This last command will switch your default shell to fish. To see the changes close your terminal ...
These shells do not use the same configuration files. Normally the PATH environment variable is set in /etc/profile and ~/.profile which are files read by bash (and dash) but by neither of fish or zsh.
But you can manually execute your profile files (this sets PATH and does other things as well)
For fish you can use the script provided here: https://...
pantheon-terminal supports its process completion notifications for fish! To enable them, you need to put the following line either in /etc/fish/config.fish to enable them for the entire system or in ~/.config/fish/config.fish to enable them just for your user:
source /usr/share/pantheon-terminal/enable-fish-completion-notifications 2>/dev/null; or true