Textbook of Linux

A signal is a software interrupt—a standardised, numbered message delivered asynchronously to a process. Signals are how Unix systems communicate events like "your child exited" (SIGCHLD), "the user pressed Ctrl+C" (SIGINT), "please terminate cleanly" (SIGTERM), "die now" (SIGKILL), "segmentation fault" (SIGSEGV), and "hang up" (SIGHUP). About 30 standard signals are defined, plus a range of real-time signals.

A process can install a handler for most signals, choose to ignore them, or let the default action happen (which is usually to terminate the process). Two signals cannot be caught, blocked, or ignored: SIGKILL (#9) and SIGSTOP (#19). They always take effect, which is why kill -9 is the hammer of last resort.

kill <PID>                  # default: SIGTERM
kill -9 <PID>               # SIGKILL
kill -HUP <PID>             # reload config (daemons' convention)
kill -l                      # list signal names and numbers
pkill -HUP nginx            # signal by name

Well-behaved daemons treat SIGTERM as "finish current work and exit cleanly" and SIGHUP as "reload your configuration without restarting". SIGKILL should be reserved for processes that refuse to die—it gives them no chance to clean up.