First we need to install Homebrew. This is an OSX package manager that provides most of the functionality of a linux terminal to OSX. This can be done by giving the command below to your terminal

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Next we can install Fortune. This allows us to display the greetings when opening a terminal instance.

brew install fortune

We then need to install cowsay which will display the image of a cow together with the message.

sudo gem install cowsay

We then need to add this to the fish settings. This can be done by opening the settings file using VIM. Type in

vim ~/.config/fish/config.fish

We can check to see if fortune and cowsay is installed by typing in:

cowsay moo moooo

and

fortune

into our terminal.

Then we can insert the command below to have the greeting displayed. Press a to allow you to edit the file and then type in:

function fish_greeting
        fortune -a | cowsay
end

funcsave fish_greeting

Once you have edited the file press escape then :wq to save and quit.