Using Shell Variables and Quoting
Before you begin to create scripts, let's look at two concepts that will become increasingly important as you deal with the complexity of the tasks at hand. These concepts are shell variables (and variable substitution) and quoting.
Variables are special words that, when expanded, are replaced by values that you've assigned to them. The shell provides a built-in process for creating and keeping track of variables, known as variable substitution. Variable substitution enables you to generalize commands that are included in shell scripts, making it possible for you to change the way a script acts (for example, the files it will affect) simply by changing the values of the variables it uses.
The use of variables ...