Explanations of All 31 Signals

• SIGSTOP: it is sent only by

  kill()

  ; It unconditionally stops a process and cannot be caught or ignored.
• SIGSYS: the kernel sends this signal to a process when it attempts to invoke an invalid system call, such as when a binary created on a newer system executes on an older system that doesn't support a syscall.
• SIGTERM: it is sent only by

  kill()

  ; it allows a user to gracefully terminate a process (the default action.)
• SIGTRAP: the kernel sends this to a process when it reaches a break point. Generally, debuggers catch this signal, and other processes ignore it.
• SIGTSTP: the kernel sends this to all the foreground group's processes when the user enters the suspend character (Ctrl+Z).
• SIGTTIN: it is sent to a process in the background when it attempts to read from its controlling terminal. The default action is to stop the process.