process signal

some Unix process signals:

1, HUP, "hangup"; the controlling terminal disconnected, or the controlling process died.
2, INT, "interrupt"; the user typed CTRL+C.
9, KILL, "kill"; terminate immediately, no cleanup. Can't be trapped.
15, TERM, "terminate," but cleanup first. May be trapped. The default when using the kill command.

来源：https://my.oschina.net/redhands/blog/3107883