When you get to this part of the book, you'll have seen well over 50 example scripts. Many of these scripts have some shared components: input checking, error handling, and setting the current working directory have been used in multiple scripts. This code doesn't really change; perhaps the comments or echoes were slightly different, but in reality it's just duplicated code. Pair this with the problem of having to define functions at the top of your script (or, at the very least, before you start using them) and your maintainability is beginning to suffer. Luckily for us all, there is a great solution for this: creating your own function library!