You don't have to install powerline and fonts from github to get fish with powerline. The Ubuntu repo already has some packages. Just install them with apt.
apt install fish powerline
This pulls in fonts-powerline python-powerline python-psutil python3-powerline
add the following lines to ~/.config/fish/config.fish
set fish_function_path $fish_function_path "/usr/share/powerline/bindings/fish" source /usr/share/powerline/bindings/fish/powerline-setup.fish powerline-setup
and then start a new instance of fish by typing
Presumably if you have a different set up or distro, you can use grep/find to get the above information.
# these commands work with bash or fish cd / find -name fish 2>&1 | grep bindings # which returns ./usr/share/powerline/bindings/fish # and then find -name '*.fish' 2>&1 | grep -e 'powerline-setup' # returns ./usr/share/powerline/bindings/fish/powerline-setup.fish