CHAPTER 2
Patterns and Regular
Expressions

This chapter is a bit of a digression; if you are comfortable with patterns and regular expressions, you can just skip ahead to Chapter 3, where I begin the discussion of shell syntax. However, if you are unfamiliar with patterns and regular expressions, this material turns out to be very important for understanding and illustrating the coming examples. Furthermore, you will have to learn it to be an effective shell programmer, so if you haven't learned it before, start early.

Shell programming is heavily dependent on string processing. The term string is used generically to refer to any sequence of characters; typical examples of strings might be a line of input or a single argument to a command. Users enter responses to prompts, file names are generated, and commands produce output. Recurring throughout this is the need to determine whether a given string conforms to a given pattern; this process is called pattern matching. The shell has a fair amount of built-in pattern matching functionality (especially if you are comfortable with relying on POSIX shell features). Pattern matching is not unique to the shell; other programs, such as find, use the same pattern-matching rules. A special variant of shell pattern matching, called globbing, is used to expand file name patterns into groups of matching names. The distinction between globbing and pattern matching is a bit vague; many people call all patterns globs and use the term file globbing for the special case of matching file names. The shell manual pages, however, tend to call pathname expansion globbing.