Signals send an interrupt to a running program. When a process receives a signal, it responds by executing a signal handler. Compiled applications generate signals with the kill system call. A signal can be generated from the command line (or shell script) with the kill command. The trap command can be used in a script to handle received signals.

Each signal is identified by a name and an integer value. The SIGKILL (9) signal terminates a process immediately. The keystroke events Ctrl + C and Ctrl + Z send signals to abort or put the task in the background.