For taking care of stubborn Linux applications, there are two very helpful commands: kill and killall. But how are they used? Are the complicated or are they simple? Let’s examine these two very handy commands.

Installation?

Oh no. These two commands are installed in all Linux distributions by default. To use them all you have to do is open up a terminal window and start blowing away applications. But which should you use?

Kill

The man page of the kill command states that kill will “send a signal to a process.” That sounds harmless enough. But what is the signal it sends? If reason stands, that signal would be one to terminate the process. Oddly enough TERM (short for terminate) is not the only signal kill can send. There is a fairly lengthy list of possible signals (all of them different) that can be had by issuing the command kill -l. You will find 62 different signals you can send with kill. The default is TERM which will terminate a process.

So, how do you use kill? Simple. You first must know the process ID of the process you want to kill. Let’s say good ol’ Firefox isn’t responding and you need to get rid of it. To do this you first need to find out what the PID of the currently running Firefox is. To do this you would issue a command like:

ps aux|grep firefox

Which would return something like:
jlwallen 18387  0.0  0.0   1832   556 ?        S    16:53   0:00 /bin/sh /usr/lib/firefox-3.6.12/firefox
jlwallen 18392  0.0  0.0   1832   572 ?        S    16:53   0:00 /bin/sh /usr/lib/firefox-3.6.12/run-mozilla.sh /usr/lib/firefox-3.6.12/firefox-bin
jlwallen 18396 26.0  2.2 201420 46492 ?        Dl   16:53   0:01 /usr/lib/firefox-3.6.12/firefox-bin
jlwallen 18413  0.0  0.0   3324   816 pts/0    S+   16:53   0:00 grep --color=auto firefox

The PID you are looking for will be the one associated with firefox-bin. In the example above, that PID is 18396. So to kill Firefox you would then issue the command kill 18396. That command would instantly kill Firefox.

There is an easier way!

killall

What if you don’t want to jump through the hoops of finding the PID of the process. If you know the name of the process you can, instead, make use of the killall command. Instead of killing a process by ID, killall kills a process by name. There are signals you can send with killall (either by name or number), but the one that is most often used is -9. So to kill Firefox with killall you would issue the command:

killall -9 firefox-bin

The obvious problem with this is you must know the exact name of the process you want to kill. So if you don’t know the name you can go back ps aux|grep firefox to get the exact name of the process.

Although you might think this a system of process management that should be avoided by the new user, you should consider that there may be times when knowing this process can really save your skin. So, although you may tend to shy away from the command line tools, this would be one of those that is good to know in a pinch.

The Commands

The first thing you have to understand is the commands you will be using. There are two commands that are useful: kill and killall. The kill command is used in conjunction with a process ID (PID) and the killall command is used in conjunction with command names. Your first reaction is most likely “How do I get the PID of an application?” Well, that will take another command.

Learn By Example

Let’s use Firefox as an example. Say, for some reason, Firefox has frozen and you can not get it to respond. Open up a terminal and issue the command killall firefox if that doesn’t kill Firefox then Firefox was started with a different command. To find out what that command is you could employ the ps command like so:

ps aux|grep firefox

The output would look something like:
[jlwallen ~]$ ps aux|grep firefox
jlwallen 10199  3.8 28.8 417396 223764 ? RNsl Feb23  77:22 /usr/lib/mozilla/firefox-bin -a firefox
jlwallen 13434  0.0  0.0   3796   688 pts/0  RN+  20:14   0:00 grep firefox

You see the line that includes firefox-bin? That is most like what you need to kill. So issue the command killall firefox-bin and the frozen Firefox will go away.

If you look up at the command output again you can see the output includes the PID of each command. The PID associated with firefox-bin would be 10199. You could issue the command kill 10199 and Firefox would close as well.

Frozen X

What if X Windows freezes and you know the culprit was, say, Evolution? You can’t get to a terminal window so what do you do? You have to go to a different virtual console. To do this you will hit the key combination Ctrl-Alt-F* (Where * is a number 1-9. This number will differ depending upon which virtual console X is running.) Once you are in a virtual consol you will have to log in as your user and then issue the kill command. You can then go back to the virtual console running X and, hopefully, it will be back under your control.

Final Thoughts

It’s not as user-friendly as either the GNOME or KDE tools, but this method is a very valuable method of process control. Some day you will need to have this method in your tool belt and, when you do, you’ll be glad you have it.