This is a simple guide to check your Ubuntu system resource use & running programs with the built-in program System Monitor. To formerly Windows users, this is your Task Manager on Ubuntu. You will learn to read information in it and do end a process with it. 

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Processes

A program that runs (is residing) in memory is called process. It makes use of computer resources which are CPU load and memory space. Multiple programs that run in memory are called processes. To view processes, computer user makes use of a special program called process viewer, also called Task Manager (Windows) and System Monitor (Ubuntu).

Run System Monitor

To run System Monitor, open the start menu and type system monitor and click the System Monitor you find. Alternatively, press Alt+F2 to display the runner and type gnome-system-monitor followed by Enter to run it.

Understand Processes View

The first view you will see in System Monitor is called Processes View. It is a table consisted of rows and columns of active programs in your computer.

Understand Resources View

To view it, click Resources tab positioned at the middle of three buttons at the top. In this view, you see three sections of CPU - Memory - Network. CPU section displays moving graph(s) of processor loads from time to time along with the percentage(s). It will show individual cores of CPU you have as different colors. Memory section displays moving graphs and values in megabits (MiB) of memory use per capacity along with swap use per capacity. Network

Understand Filesystem View

To view it, click the File Systems tab beside Processes and Resources. This view tells hard disk drive available in the computer as well as its disk partitions, filesystem types, total capacity of each one, and how much each one is used in megabyte or gigabyte with percentage bar for each one. For example, picture below tells that in the computer there is one hard disk (sda) with two partitions (sda1 and sda3) formatted in FAT32 and EXT4 filesystems, where the Ubuntu root partition is sda3 ( / ) by capacity of 20GB with 10.6GB free.

Show All Processes

Sort Active Programs

Sorting by PID (process identifier) will sort processes by how new individual programs are started against the others. To sort this way, click the "ID" column at the top of table. In Ubuntu and almost all other modern GNU/Linuxes, the number 1 process is called systemd. Newer process will have bigger number. Newest process would have biggest number.

Sorting by %CPU will sort processes by how much load individual programs are having against the processor. To do so, click the "Memory" column. With this view, user can tell which programs burden the CPU the most. Picture below shows an example that three top processes by %CPU namely a game (Extreme Tux Racing, hence etr), a display controller program (Xorg), and a user interface (gnome-shell) put loads to computer CPU the most by 8.45%, 3.46%, and 2.7% respectively.

Sorting by Memory will sort processes by how much space individual programs are occupying in the memory. The space is represented in megabyte or MB unit. Picture below shows an example of three most memory occupying processes namely the web browser (firefox), the user interface (gnome-shell), and a child process of the web browser (Web Content) by 396MB, 158MB, and 130MB respectively.

End an Active Program

The main purpose of System Monitor is, aside from the other purposes, to end a certain process. To do so:

• find the active program by reading the list or searching,
• select the program name,
• right click it,
• select Kill,
• wait until the program does not appear anymore in the list, and
  
• program ended.

Relationship to Other Programs

System Monitor is a utility program and is in one group with other Ubuntu utilities such as Archive Manager, Disk Usage Analyzer (Baobab), and Disks. It is part of GNOME Applications -- apps created by The GNOME Project -- that built most of Ubuntu Desktop. As such, System Monitor is different to amusement programs --like Rhythmbox-- and productivity tools --like LibreOffice--.