Which command is used to immediately stop running a program?

The command that is used to immediately stop running a program is "kill" followed by a signal. In Unix-like operating systems, the kill command is utilized to send signals to processes, instructing them to terminate. By default, if no signal is specified, the kill command sends the SIGTERM signal, which requests a process to terminate gracefully, but it can also send the SIGKILL signal, which forces the immediate termination of a process without cleanup. This is especially useful if a program is unresponsive.

Other answers may refer to various forms of stopping processes or shutting down systems but do not specifically fulfill the requirement of immediately halting a program like the kill command does. The stop command and the halt command typically refer to pausing processes or shutting down the system rather than directly terminating a currently running program. Meanwhile, terminate is not a specific command in most Unix-like systems and is more indicative of an action rather than an executable command. Therefore, "kill signal" stands out as the correct choice for immediately stopping a running program.