9.1. Introduction

We learned in the previous chapter that there are relationships between processes. First, every process has a parent process (the initial kernel-level process is usually its own parent). The parent is notified when the child terminates, and the parent can obtain the child's exit status. We also mentioned process groups when we described the waitpid function (Section 8.6) and how we can wait for any process in a process group to terminate.

In this chapter, we'll look at process groups in more detail and the concept of sessions that was introduced by POSIX.1. We'll also look at the relationship between the login shell that is invoked for us when we log in and all the processes that we start from our login shell.

It is impossible to describe these relationships without talking about signals, and to talk about signals, we need many of the concepts in this chapter. If you are unfamiliar with the UNIX System signal mechanism, you may want to skim through Chapter 10 at this point.