Top
The top command will display a Terminal version of the system processes and the amount of resources used by each process. It's surprisingly easy to view and understand, and it's dynamic so it'll update on the fly. Just press Ctrl + C to exit top when you're done.
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Htop
Htop is a further improvement to the top command, but it's not installed on most systems by default. To install it, use your command line package installer function (sudo apt-get install htop, for Debian based systems), then enter htop into the Terminal to see how good it really is. Again, use Ctrl + C to exit.
PS
The ps command lists all the running processes in much the same way as top, but with a little less detail. It's still a very good tool to use, though, and is worth getting to grips with using the following:
Code:
ps -A
After pressing Enter, you should get a thorough breakdown of your system processes. If the processes go off the bottom of the screen, then use the less extension command:
Code:
ps- A | less
And you can press Enter, for example, to scrutinize it line by line. And, you can even use the grep command to specify a particular process. Such as:
Code:
ps -A I grep cinnamon
And finally, by using the whatis command, you can see what the process is that's using up the resources:
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Code:
whatis cinnamon
Cool, isn't it?

Pstree


Pstree is a more visual version of the ps command and displays the process information in a tree fashion. It's a little more visually useful, but some still prefer the good old ps command.

Background Commands
When you enter a program name in the command line, by default it will execute it in the current open Terminal, and to enter more commands you’ll have to open up another Terminal. However, if you want to launch a program and still use the current Terminal without opening another, then do the following, using VLC as the example program:

Code:
vlc &
Pressing Enter will launch the program, and after pressing Enter a couple of times you'll be brought back to the command line, and the program will be running in the background. Clever, that.

Being Nice
The renice command is also extremely useful, in that it allows you to change the priority (or nice value) of the process being run. So a nice value of -19 is the highest priority a process can run in, whereas 19 is the lowest, and 0 is the default.

You'll need to provide the PID number of the process, which can be found using the ps command from earlier, and you'll need to enter it with elevated rights. So, if Firefox had a PID of 2957, then the command would be:

Code:
sudo renice -19 2957
to make Firefox run at the highest priority after entering your password.

Xkill


Xkill is a great tool and beats using the standard kill command if you're using the GUI. All you need to do is enter xkill into the Terminal, then left-click on the program window you want killing to stop its processes.

If you decide not to use xkill after entering it, then just right-click to stop it.

An Interesting Combo

As we've looked at before, you can execute two commands after each other by using the double ampersand in between the commands, such as sudo apt-get update && sudo apt-get upgrade. Provided the first command worked without any issues, then the second will kick in straight after it.

But if you want to delay the execution of a command (let's say Firefox) to perhaps display a message a user must first read, then you can use an interesting combination of commands:

Code:
sleep 20 && firefox &
This will run the sleep command for 20 seconds, then launch Firefox in the background while still leaving the Terminal free to use.

The Midnight Commander

For many years now, the Midnight Commander (which isn't as dodgy as it sounds) has been the graphical file manager of choice for many Linux command heroes. It's a great piece of software and harks back to the days of DOS and beyond with its 8-bitlike visuals.

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It is, though, an extremely powerful program and one that will make an excellent addition to your arsenal of Linux tools and utilities especially if you intend to use your Linux distro as a rescue boot device, for example. To get Midnight Commander, simply enter the following:

Code:
sudo apt-get install mc
Or whatever command line package installer you use. And once it's installed, issue the following command to get it up and running:

Code:
mc
It can take a little getting used to, but once you've got to grips with the Commander, it'll become your friend always (very cheesy, but how often do you get to mention the words Midnight Commander?).