4.3.5.7. Interrupting your processes

As a non-privileged user, you can only influence your own processes. We already saw how you can display processes and filter out processes that belong to a particular user, and what possible restrictions can occur. When you see that one of your processes is eating too much of the system's resources, there are two things that you can do:

1. Make the process use less resources without interrupting it;

2. Stop the process altogether.

In the case that you want the process to continue to run, but you also want to give the other processes on the system a chance, you can renice the process. Appart from using the nice or renice commands, top is an easy way of spotting the troublesome process(es) and reducing priority.

Identify the process in the "NI" column, it will most likely have a negative priority. Type r and enter the process ID of the process that you want to renice. Then enter the nice value, for instance "20". That means that from now on, this process will take 1/5 of the CPU cycles at the most.

Examples of processes that you want to keep on running are emulators, virtual machines, compilers and so on.

If you want to stop a process because it hangs or is going totally berserk in the way of I/O consumption, file creation or use of other system resources, use the kill command. If you have the opportunity, first try to kill the process softly, sending it the SIGTERM signal. This is an instruction to terminate whatever it is doing, according to procedures as described in the code of the program:

joe:~> ps -ef | grep mozilla
joe    25822    1  0 Mar11 ?    00:34:04 /usr/lib/mozilla-1.4.1/mozilla-

joe:~> kill -15 25822

In the example above, user joe stopped his Mozilla browser because it hung.

Some processes are a little bit harder to get rid of. If you have the time, you might want to send them the SIGINT signal to interrupt them. If that does not do the trick either, use the strongest signal, SIGKILL. In the example below, joe stops a Mozilla that is frozen:

joe:~> ps -ef | grep mozilla
joe    25915    1  0 Mar11 ?    00:15:06 /usr/lib/mozilla-1.4.1/mozilla-

joe:~> kill -9 25915

joe:~> ps -ef | grep 25915
joe     2634 32273 0 18:09 pts/4   00:00:00 grep 25915

In such cases, you might want to check that the process is really dead, using the grep filter again on the PID. If this only returns the grep process, you can be sure that you succeeded in stopping the process.

Among processes that are hard to kill is your shell. And that is a good thing: if they would be easy to kill, you woud loose your shell every time you type Ctrl-C on the command line accidentally, since this is equivalent to sending a SIGINT.

	UNIX without pipes is almost unthinkable
	The usage of pipes (|) for using output of one command as input of another is explained in the next chapter, Chapter 5.

In a graphical environment, the xkill program is very easy to use. Just type the name of the command, followed by an Enter and select the window of the application that you want to stop. It is rather dangerous because it sends a SIGKILL by default, so only use it when an application hangs.