Suppose you enter ps -f and see the following response:

UID        PID   PPID  C   STIME      TTY     TIME   COMMAND

chris      65     1    0   11:40:11   tty01   0:06   -bash

chris      71    65   61   11:42:01   tty01   0:14   total_updt inventory

chris     231    65   80   11:46:02   tty01   0:00   ps -f

chris     187    53   60   15:32:01   tty02 123:45   crunch stats

chris      53     1    0   15:31:34   tty02   1:06   -bash

To kill process 187, normally you would enter kill 187. If you then enter ps -f again and see that the process is still there, you know the process is set up to ignore the kill command. To kill it unconditionally, enter kill -9 187. When you enter ps -f again, you see that the process is no longer around.

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CAUTION:  
A disadvantage to using this unconditional version of the kill command is that kill -9 doesn’t allow a process to finish what it’s doing before it terminates the process. If you use kill -9 with a program that’s updating a file, you could lose the updated material or the entire file.

Use the powerful kill -9 command responsibly. In most cases, you don’t need the -9 option; the kill command, issued without arguments, stops most processes.

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Termination of All Background Processes

To kill all background jobs, enter kill 0. Commands that run in the background sometimes initiate more than one process; tracking down all the PID numbers associated with the process you want to kill can be tedious. Because kill 0 terminates all processes started by the current shell, it’s a faster and less tedious way to terminate processes. Enter the jobs command to see what commands are running in the background for the current shell.

From Here…

This chapter presented the commands you need to manage multiple processes. You saw that you run multiple processes whenever you put jobs in the background with the ampersand (&) or when you use pipes. You can schedule jobs at a specific time with the at command, at a time the system feels is appropriate with the batch command, and at regularly scheduled times with cron and crontab. For more information, see the following:

•  The chapters in Part III, “Managing the File System,” discuss how to monitor and maintain your Linux system. System administration isn’t an easy topic to learn and, in fact, requires a hands-on learning approach. This part of the book provides you with a basic understanding of the concepts and the tasks required of a systems administrator (often called a sys admin).

•  Chapter 18, “Understanding Linux Shells,” provides the shell-specific information to program scripts to start, stop, and monitor processes on your Linux system.

In addition, check out the man pages for the various commands discussed in this chapter.