This chapter is dedicated to a special feature of Bash: parameter expansion. Parameter expansion allows us to do many interesting things with variables, which we will cover extensively.

We will first discuss default values of variables, input checking, and variable length. In the second part of this chapter, we will look more closely at how we can manipulate variables. This includes replacing and removing patterns from text, modifying the case of variables, and using substrings.

The following commands will be introduced in this chapter: export and dirname.

The following topics will be covered in this chapter:

• Parameter expansion
• Variable manipulation