Since a process cannot send a signal directly to another process, it must ask the kernel to send the signal. Users (including the superuser) can send signals to other processes (programs) by using kill.

kill is really a bad name. Although it is used a lot to kill processes, it is really designed to send signals to processes. Therefore, kill is not really accurate.

The default signal sent is SIGTERM (#15) which can be handled, or ignored by the receiving process in order to prevent its death. It is preferable to use this signal to give the process a chance to clean up after itself. If this signal is ignored, the user can usually send a SIGKILL (#9) signal, which cannot be ignored, to kill the process.

We use kill to send signals (by either number or name):

$ kill 1234

$ kill -9 1234

$ kill -SIGTERM 1234