Define an alias in fish shell

Just use alias. Here's a basic example:

# Define alias in shell
alias rmi "rm -i"

# Define alias in config file ( `~/.config/fish/config.fish` )
alias rmi="rm -i"

# This is equivalent to entering the following function:
function rmi
    rm -i $argv
end

# Then, to save it across terminal sessions:
funcsave rmi

This last command creates the file ~/.config/fish/functions/rmi.fish.

Interested people might like to find out more about fish aliases in the official manual.

This is how I define a new function foo, run it, and save it persistently.

sthorne@pearl~> function foo
                    echo 'foo was here'
                end
sthorne@pearl~> foo
foo was here
sthorne@pearl~> funcsave foo