Day 3 - Window Manager · Display Manager · Desktop Environment

📌 Key Takeaways

• The three do different jobs — the window manager handles windows, the display manager handles the login screen, and the desktop environment is the whole bundle.
• Window managers: KWin · Mutter · Metacity · Xfwm · Windowmaker · Afterstep · Enlightenment · Fluxbox · twm
• Display managers: XDM · GDM · KDM · SDDM · LightDM
• Desktop environments: GNOME · KDE · Xfce · LXDE · MATE · Cinnamon · CDE
• Questions that mix the names and ask you to pick the "one of a different kind" are repeatedly asked.

Where the Three Sit

The desktop environment contains the window manager. The fact that there is a containment relationship is the key to telling the three apart.

The Order in Which Things Appear After Power-On

   Boot complete (graphical.target)
        │
        ▼
   ① Display manager starts     launches the X server and draws the login screen      ← GDM · SDDM · LightDM
        │   enter ID and password, choose the session type
        ▼
   ② Session starts             the desktop environment comes up                       ← GNOME · KDE · Xfce
        │
        ├── window manager      window borders and placement                          ← Mutter · KWin · Xfwm
        ├── panel · desktop background
        └── file manager etc.   programs the user opens                               ← Nautilus · Dolphin · Thunar
        │
        ▼
   ③ Logout                     the session ends and returns to the login screen of ①

• The display manager and desktop environment can be chosen separately. It is possible to select a session on the login screen, enter through GDM, and use KDE. The "belongs to" column in the tables is just the default pairing.
• A window manager can be used alone without a desktop environment. Conversely, a desktop environment cannot exist without a window manager. Running only Fluxbox or twm on a light system is the former case.
• Because of the name "display manager", it is easy to mistake it for something that manages monitor resolution. What it does is login.

Window Managers

They handle drawing window borders, moving, resizing, and minimizing.

• twm (Tab Window Manager) is the simplest window manager, included with X by default.
• Windowmaker · Afterstep · Fluxbox · Enlightenment are window managers used standalone that do not belong to a desktop environment. The names look unfamiliar, so they are easily used as choices.

Names with a Clue and Names Without

• Of the four starting with M, three are window managers and only MATE is a desktop environment. In a set of choices like Metacity · Mutter · Marco · MATE, the odd one out is MATE.
• Combining windows into one screen while producing translucency, shadows, and window-switching effects is called compositing. Compiz was famous for these effects, and now KWin and Mutter do it themselves.

Display Managers

They handle the graphical login screen that appears after boot. When login succeeds, they start the session.

• The first letter of the name is a hint — GDM is GNOME, KDM is KDE, and XDM is X's default.
• If you do not use a display manager, you log in by text and then launch X with startx.

• A display manager does three things — launches the X server, authenticates the user, and starts the chosen session.
• The protocol used to bring up the login screen of a remote computer on my X server is XDMCP (X Display Manager Control Protocol). As the name says, it pairs with the display manager.
• You can check which one is running now with systemctl status display-manager.
• You turn on only one display manager at a time. Even if you install two, only one comes up at boot.

Desktop Environments

They bundle a panel, file manager, settings tools, and default applications on top of the window manager and provide one integrated environment.

• GNOME and KDE are the two major camps. The key pairing is that GNOME is based on GTK+ and KDE on Qt.
• Xfce and LXDE are lightweight environments, used on old computers.
• CDE (Common Desktop Environment) is the standard desktop of traditional commercial Unix and is Motif-based.

• If asked about the lightness order, generally LXDE and Xfce are on the light side and GNOME and KDE are on the heavy side.
• Grouped by toolkit, the GTK+ side is GNOME, Xfce, LXDE, MATE, and Cinnamon, and the Qt side is just two: KDE and LXQt. If asked to pick a "Qt-based desktop environment", it comes from these two.

Programs That Come with the Desktop Environment

A desktop environment has a set of default programs beyond the file manager. Questions that give a purpose and ask you to pick the program come from this area.

• Distinguish "viewing" from "editing". eog and Gwenview are programs for viewing images, and GIMP is a program for editing. Evince and Okular are programs for viewing PDFs.
• Pair the four of LibreOffice as Writer–documents, Calc–spreadsheet calculation, Impress–presentations, Draw–drawings.
• These programs are X clients. They are neither window managers nor display managers, so if they are mixed into a classification question, place them in the "application" column.

Questions That Mix and Ask You to Pick

"Which of the following is of a different kind from the other three?" KWin · Xfce · Windowmaker · Afterstep → It is Xfce. The other three are window managers, and only Xfce is a desktop environment.

• The rule that a name ending in DM is a display manager is the easiest clue.
• Do not confuse Xfce and Xfwm. Xfce is the desktop environment and Xfwm is its window manager. They differ by one letter, so they appear together in choices.

The Order of Sorting

When you get four names, sift them in the order below.

   ① Does it end in DM?               → display manager   (XDM · GDM · KDM · SDDM · LightDM · LXDM)
   ② Does it end in wm · Win · box?   → window manager    (twm · FVWM · Xfwm · KWin · Openbox · Fluxbox)
   ③ Is it GNOME · KDE · Xfce · LXDE · LXQt · MATE · Cinnamon · CDE?   → desktop environment
   ④ Among the rest, Mutter · Metacity · Marco · Muffin · Windowmaker · Afterstep · Enlightenment · Compiz
                                      → window manager (names without a clue)
   ⑤ Nautilus · Dolphin · Thunar · PCManFM · Caja · Nemo            → file manager

• There is a trick of gathering names that begin with the same initial into one set of choices. They separate the four starting with K, Xfce, Xfwm, and XDM starting with X, and GNOME, GDM, and GTK+ starting with G.
• LXDE and LXDM also differ by one letter. Ending in E means Environment, and ending in M means Manager.

Tomorrow we cover how to actually run X and bring up a remote screen.

📖 Glossary

• Window manager: The program that handles window borders and moving and resizing.
• Display manager: The program that provides the graphical login screen and starts the session.
• Desktop environment: An integrated environment bundling a panel, file manager, and applications on top of a window manager.
• KWin / Mutter / Xfwm: The window managers of KDE, GNOME, and Xfce respectively.
• GDM / SDDM / LightDM: The display managers used by GNOME, KDE Plasma, and Ubuntu respectively.
• twm: The simplest window manager, included with X by default.
• CDE: A Motif-based desktop environment of traditional commercial Unix.
• File manager: A program that handles files and folders graphically. Nautilus, Dolphin, Thunar, etc.