This book's focus is on the Linux system that normally lies underneath a user's interactive session. Eventually, the system and the user have to meet somewhere. You saw plenty of commands and shell features in Chapter 1 and Chapter 7, but there's still one little piece missing: the startup files that set defaults for the shell and other programs.
Most users do not pay close attention to their startup files. Over time, the files become cluttered, containing unnecessary environment variables and tests that can lead to annoying (or quite serious) problems. Worse still, operating system vendors usually do not consider the consequences of what they put in the default startup files, packing them full of items that may even confuse the user.
Keeping in tune with the shell-oriented theme of this book, this chapter focuses specifically on shell startup files. If you have ever received a Unix account in the past, it may have come with a bafflingly large array of startup files, sometimes called dot-files because they nearly always start with a dot (.). You may be surprised to learn that it really isn't necessary to have so many startup files.