Signal

An asynchronous message sent to a process, like SIGTERM or SIGKILL.

Signals are the kernel's way of tapping a process on the shoulder. Each has a number, a name and a default action: SIGTERM (15) politely requests termination, SIGHUP (1) traditionally means "reload your configuration", SIGINT (2) is what Ctrl+C sends.

Programs may install handlers to react gracefully: flush data, close files, reload config. Two signals cannot be caught or ignored: SIGKILL (9) and SIGSTOP, which is why kill -9 always works and why it never lets the program clean up.

See it in practice

$ kill -l | head -3

Lists the signal names the system knows. Sending one is kill -NAME PID, and the default is SIGTERM.

Where you'll meet it

kill

Terminate processes politely or forcibly, and understand what -9 really does.

jobs, & and nohup

Run things in the background, juggle them, and keep them alive after logout.

Process

A running program, with its own ID, memory and environment.

Learn these concepts by typing them: the interactive course builds them up lesson by lesson in a real simulated shell.