Programming Perl, 4th Edition

Signalling Process Groups

Processes (under Unix, at least) are organized into process groups, generally corresponding to an entire job. For example, when you fire off a single shell command that consists of a series of filter commands that pipe data from one to the other, those processes (and their child processes) all belong to the same process group. That process group has a number corresponding to the process number of the process group leader. If you send a signal to a positive process number, it just sends the signal to the process. But if you send a signal to a negative number, it sends that signal to every process whose process group number is the corresponding positive number—that is, the process number of the process group leader. (Conveniently for the process group leader, the process group ID is just $$.)

Suppose your program wants to send a hang-up signal to all child processes it started directly, plus any grandchildren started by those children, plus any great-grandchildren started by those grandchildren, and so on. To do this, your program first calls setpgrp(0,0) to become the leader of a new process group, and any processes it creates will be part of the new group. It doesn’t matter whether these processes were started manually via fork, automatically via piped opens, or as backgrounded jobs with system("cmd &"). Even if those processes had children of their own, sending a hang-up signal to your entire process group will find them all (except for processes that have ...