5.3. Using kill to Control Processes

Linux and other Unix-like operating systems support a form of interprocess communication called signals. The kill command is used to send a signal to a running process. How a process responds to a signal, if it responds at all, depends on the specific signal sent and on the handler set by the process. If you are familiar with Unix signal handling, you will find that Apache adheres to the usual conventions, and you can probably skip this section. This section describes the use of kill in relation to Apache for readers who aren't accustomed to working with signals.

The name "kill" is a misnomer; it sounds as if the command is inherently destructive, but kill simply sends signals to programs. Only a few signals will actually kill the process by default. Most signals can be caught by the process, which may choose to either perform a specific action or ignore the signal. When a process is in a zombie or uninterruptible sleep( ) state, it might ignore any signals.