Run another command before running the command the user wants to run

Sometimes an alias isn't powerful enough to easily do what you want, so here's a way without using them.

In some file that is sourced when your shell starts (e.g. .bashrc), add the following function:

ls () {
    echo "Hello world!"
    command ls "$@"
}

Unlike an alias, a function can recurse. That's why command ls is used instead of ls; it tells your shell to use the actual ls instead of the function you've just defined.

You must not forget to call ls:

alias ls='echo "Hello World!"; ls'