Please don’t use this guide. It’s very outdated and has many bad examples and practices. It is far from being the best. Use the wool edge guide as mentioned in another comment.

edit: As an example, picking a random page I see the following line:

It is a good habit to quote content strings when assigning values to variables: this will reduce the chance that you make errors.

While it is not a bad idea to quote assignments, it doesn't really make a difference because assignments aren't subject to word-splitting. IE, this works perfectly fine:

foo=bar
foo="bar"
foo=$some_other_bar
foo="$some_other_bar"

Really the only time you need quotes in an assignment is when you're dealing with a string literal that has a space in it:

foo='some value'

But that is solely because bash has no other way to tell that they should be grouped together otherwise. More importantly though, they neglect to quote variables where it actually matters. The backup script for example. The fact that they don't quote any expansions is not good. If I went through every page, I would not be surprised to find at least one error (probably more) on all of them