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.