Signaling Active Processes
Each process running on your system listens for signals, simple messages sent to the process either by the kernel or by a user. The messages are sent through inter-process communication. They are single-valued, in that they don’t contain strings or command-like constructs. Instead, signals are numeric integer messages, predefined and known by processes. Most have an implied action for the process to take. When a process receives a signal, it can (or may be forced to) take action.
For example, if you are executing a program from the command line that appears to hang, you may elect to type Ctrl-C to abort the program. This action actually sends an SIGINT (interrupt signal) to the process, telling it to stop running.
There are more than 32 signals defined for normal process use in Linux. Each signal has a name and a number (the number is sent to the process; the name is only for our convenience). Many signals are used by the kernel, and some are useful for users. Table 6-7 lists popular signals for interactive use.
Meaning and use
Hang up. This signal is sent automatically when you log out or disconnect a modem. It is also used by many daemons to cause the configuration file to be reread without stopping the daemon process. Useful for things like an httpd server that normally reads its configuration file only when the process is started. A SIGHUP signal will force it to reread the configuration ...