Some signals more commonly encountered than others.
When a process is sent this signal, it means the operating system intends to terminate the process immediately. This signal cannot be ignored or caught. This means that a process cannot stop the OS or a privileged program from killing it. Furthermore, the action is immediate.
When a process is sent this signal, it is paused. Like SIGKILL, a process cannot choose to ignore or catch this signal. Unlike SIGKILL, SIGSTOP is meant to suspend a process, not to terminate it. A suspected process can be resumed at a later time.
This signal is sent to a process when a user wants to INTterrupt it. This is what happens when control-C is pressed on the keyboard when a process is running.
Some programs may want to catch this signal if the default action of termination may lead to a loss of data. The handler of this signal should complete whatever action needs to be performed to save the data or complete a critical session, then shutdown properly by reverting to the default behavior of SIGINT and send itself the SIGINT signal.
SIGQUIT is similar to SIGINT, except the default action is terminate and core dump. The signal occurs when control-\ is pressed on the keyboard.
This is a signal that originated back in the old days of modems. When a modem detects that the connection has been hung up, this signal is generated and sent to the process that was using the modem as a console.
Although modern computers use NICs for communication, the concept still applies. This signal is generated when the terminal environment of a process is terminated. This happens when you close a PuTTY window when a process is running.
For daemons (processes that do not have any attached console), this signal usually means “you are requested to reload configurations from the configuration file”. Yes, the meaning of this signal is “overloaded” in C++ terms.
This signal is sent to a process when the attached terminal is resized. This mechanism is useful for text-mode full-screen programs, such as vim, top and etc.
The default behavior of a program is to ignore the signal. Most line based text programs can safely ignore this signal without any harm. However, a full-screen text-mode program can also catch this signal to perform necessary reconfiguration to utilize the new screen size.