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Linux processes can receive signals and react to them.

That’s one way we can interact with running programs.

The kill program can send a variety of signals to a program.

It’s not just used to terminate a program, like the name would suggest, but that’s its main job.

We use it in this way:

kill <PID>

By default, this sends the TERM signal to the process id specified.

We can use flags to send other signals, including:

kill -HUP <PID>
kill -INT <PID>
kill -KILL <PID>
kill -TERM <PID>
kill -CONT <PID>
kill -STOP <PID>

HUP means hang up. It’s sent automatically when a terminal window that started a process is closed before terminating the process.

INT means interrupt, and it sends the same signal used when we press ctrl-C in the terminal, which usually terminates the process.

KILL is not sent to the process, but to the operating system kernel, which immediately stops and terminates the process.

TERM means terminate. The process will receive it and terminate itself. It’s the default signal sent by kill.

CONT means continue. It can be used to resume a stopped process.

STOP is not sent to the process, but to the operating system kernel, which immediately stops (but does not terminate) the process.

You might see numbers used instead, like kill -1 <PID>. In this case,

1 corresponds to HUP. 2 corresponds to INT. 9 corresponds to KILL. 15 corresponds to TERM. 18 corresponds to CONT. 15 corresponds to STOP.

This command works on Linux, macOS, WSL, and anywhere you have a UNIX environment