Classic Shell Scripting

Summary

In this chapter, we have shown how to create, list, control, schedule, and delete processes, how to send signals to them, and how to trace their system calls. Because processes run in private address spaces, they cannot interfere with one another, and no special effort needs to be made to write programs that can run at the same time.

Processes can catch all but two of several dozen signals, and either ignore them or respond to them with any desired action. The two uncatchable signals, KILL and STOP, ensure that even badly misbehaving processes can be killed or suspended. Programs that need to perform cleanup actions, such as saving active files, resetting terminal modes, or removing locks, generally catch common signals; otherwise, most uncaught signals cause process termination. The trap command makes it easy to add simple signal handling to shell scripts.

Finally, we examined several different mechanisms for delaying or controlling process execution. Of these, sleep is the most useful for shell scripting, although the others all have their uses.