Another good program to get familiar with is top. This program is similar to ps but is usually started full screen and updates continuously with process information. This can help with programs that may infrequently cause problems and can be hard to see with ps. Overall system information is also presented, which makes a nice place to start looking for problems. Information such as total system CPU and memory resources, as well as the load average, is helpful by itself. Add to this a list of programs and their current status and individual statistics, and you can see why top is a commonly used tool. Don't forget pstree
And finally, another quick and easy tool for checking processes is pstree. This command will list current processes and their tree structure. When one process starts, it sometimes creates child processes of itself. You can easily see this when you run the command pstree:
$ pstree -cp 125
httpd(125)-+-httpd(126)
|-httpd(127)
| -httpd(129)
'-httpd(130)
Httpd is a good example because it will often spawn child processes. Here, we're looking at the tree for PID 125. If you need to stop httpd but don't want to kill all the individual children, go for the parent process. The pstree command can list trees for individual processes or all the processes on the system. Not only can this help you track down misbehaving processes, but it is also useful as a learning tool. You can learn a lot about Linux executing these commands and reading the associated man pages. Managing processes
Once you have used tools such as ps and top to monitor processes, you need to know how to manage them. You can do this with commands such as kill, killall, and renice. kill sends signals to running processes. The most obvious usage is to halt program execution. You will first need to get the PID for a running program (using ps aux, for example) and then use it in the command as follows: $ kill 125
Under normal circumstances, this should stop the process 125. Also note that you will either need to be the owner of the process or root to halt it. And sometimes, a process will not respond to just a simple kill command. You may have to try the following: $ kill -9 125
If the process is hung and not responding normally, you can try killing it with the -9 flag, as shown in the above example. Instead of sending a sigterm, as a normal kill command does, the -9 sends a sigkill (which forces the program to close). Other signals can be sent to either stop it or possibly restart it. You can see these by running the command kill --l. The command killall, while very much like kill, accepts arguments differently. Instead of passing it a PID, you can pass it a program name. All processes running with that program name will then be stopped. This applies to just the ones you own or to all of them if you are the root user. So running the command killall tcpdump will kill all instances of the program tcpdump. This is much more helpful when many processes are running under a single name. Be sure to watch what processes you stop, especially when you are root. Killing the wrong process could close your session or even halt the system. Get familiar with the standard running programs and how they use resources. Setting a baseline is invaluable in helping to isolate system problems. Remember when I mentioned earlier that you could change the priority of a running process? Well you can do this with the command renice. Changing priority tells the operating system to give a particular process more or less of its CPU time. The range of a process's "niceness" is from -20 to 20, with -20 being the highest priority. So to reduce the priority of httpd process 125, you could run: $ renice +20 125
You can do this on the fly to conserve system resources. The system will also do this naturally and can indicate if one program taking up more resources than it should. Speedier problem solving
The ability to monitor and control processes on your Linux system is essential. Programs such as ps, top, kill, and renice enable you to see what a process is doing and to control it. The more you know about what each process is up to, the easier it will be to pinpoint problems when they creep in. A system usually experiences problems such as slowness or instability for a reason, and using these tools should help you improve your ability to track down the causes.
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