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Linux Master 2· AssociateWeek 8 · Day 5~21 min read

Day 5 - Week 8 Comprehensive Review: The X Window Name Tag Sheet

📌 Key Takeaways

The side with the screen is the X server and the program is the X client. It is the opposite of the web.
The evolution order is XFree86 → X.org Server → Wayland.
The layers from the bottom are X server → Xlib · XCB → toolkit → application. XCB is the lowest level.
A name ending in DM is a display manager. This one rule can sort a good share of classification questions.
DISPLAY is host:display.screen, and local is :0.0.

One Sheet of Name Tags

This week is all about which slot a name goes into.

   Layer                     Names
   ─────────────────────────────────────────────────────────────
   X server impl.   XFree86 → X.org Server → Wayland
                    (XWayland is a compatibility layer)
 
   Low-level libs   Xlib · XCB          ← XCB is further down (lower level)
 
   Toolkits         GTK+ (C, GIMP Toolkit) · Qt (C++) · FLTK · Motif · Tk
 
   Window managers  KWin (KDE) · Mutter (GNOME) · Metacity (GNOME 2)
                    Xfwm (Xfce) · Openbox (LXDE) · Marco (MATE) · Muffin (Cinnamon)
                    Windowmaker · Afterstep · Enlightenment · Fluxbox · twm · FVWM
 
   Display managers XDM (default) · GDM (GNOME) · KDM (old KDE)
                    SDDM (KDE Plasma) · LightDM (Ubuntu · Xfce) · LXDM
 
   Desktop environments  GNOME · KDE · Xfce · LXDE · LXQt · MATE · Cinnamon · CDE
 
   File managers    Nautilus (GNOME) · Dolphin (KDE) · Thunar (Xfce) · PCManFM (LXDE)

Bundling Desktop Environments on One Line

Environment	Toolkit	Window manager	File manager	Display manager
GNOME	GTK+	Mutter	Nautilus	GDM
KDE	Qt	KWin	Dolphin	SDDM
Xfce	GTK+	Xfwm	Thunar	LightDM
LXDE	GTK+	Openbox	PCManFM	LXDM
CDE	Motif	—	—	—

Memorize each horizontal line as a whole. Even just being sure of the GNOME line and the KDE line solves most pairing questions.

Contrasting the Confusing Pairs

Item	A	B	Deciding criterion
X server vs X client	The side with the screen	The program	Who provides the resource
Xlib vs XCB	Traditional, heavy	Lower level, light	Distance from the X server
Library vs toolkit	Xlib · XCB	GTK+ · Qt · FLTK	Whether it provides widgets
GTK+ vs Qt	C, GNOME	C++, KDE	Language and camp
Xfce vs Xfwm	Desktop environment	Window manager	They differ by one letter
Window vs display manager	Windows	Login screen	When it operates
Window manager vs desktop environment	Windows only	Includes everything	Containment relationship
XFree86 vs X.org	Former	Current standard	Generation
X vs Wayland	Existing	Replacement protocol	Not a new version
`xhost` vs `xauth`	Per host, weak	Per user, strong	Authentication method
Runlevel 3 vs 5	Text	Graphical	Boot mode

Commonly Missed Points

The remote server is the X server → No. The computer in front of me is the X server.
Xlib is the lowest level → XCB is further down.
GTK+ is C++ → It is C. Qt is C++.
Xfce is a window manager → It is a desktop environment. The window manager is Xfwm.
Wayland is a new version of X → It is a replacement. The structure is different.
The X Window System is part of the kernel → No. It is an application, so you can do without it.
Using xhost + is convenient → It is a dangerous command that opens your screen to everyone.
DISPLAY=0:0 → The order is reversed. :0.0 — the colon comes first.
Runlevel 5 is text → 3 is text and 5 is graphical.
KDM is the current KDE display manager → It is SDDM now. KDM is the old one.

Values That Appear on the Exam as They Are

Value	What it is
1984 / MIT	The start of the X Window System
X11	The version still in use today
`:0.0`	The DISPLAY value for the local first screen
3 / 5	Text / graphical runlevels
`/etc/X11/xorg.conf`	The X server config file
`~/.Xauthority`	The file where magic cookies are stored
MIT-MAGIC-COOKIE-1	The authentication method of `xauth`

Abbreviation	Full name
XCB	X protocol C-language Binding
GTK+	GIMP Toolkit
FLTK	Fast Light Toolkit
CDE	Common Desktop Environment
GDM / KDM / XDM	GNOME / KDE / X Display Manager

Dissecting One Window by Layer

If you have memorized the name tags, this time take one window on the screen and point out who is responsible for which part. This is the case of launching gedit on GNOME and Kate on KDE.

What you see on the screen	Layer responsible	In GNOME	In KDE
The login screen right after boot	Display manager	GDM	SDDM
The top panel, menus, settings windows	Desktop environment	GNOME	KDE Plasma
The window's title bar, borders, close button	Window manager	Mutter	KWin
The menus, buttons, and text boxes inside the window	Toolkit	GTK+	Qt
The editor program itself	Application (X client)	gedit	Kate
Converting drawing requests into X protocol and sending them	Low-level library	Xlib · XCB	Xlib · XCB
Actually drawing on the screen and passing key input	X server	X.org Server	X.org Server

The window's border and the window's interior are drawn by different parties. The window manager draws the border and title bar, and the program draws the interior content using the toolkit. That is why, if you launch a Qt program on GNOME, the border has the GNOME look while only the buttons inside look different.
The bottom two lines are the same whether GNOME or KDE. The camps split from the toolkit upward.
So even if a program freezes and its interior locks up, you can still grab the border and move the window. That is because the window manager is a separate program and is still alive. At that point, click the window with xkill to close it.
Among recent GNOME programs, some even draw the title bar themselves. What the exam asks is the traditional division of roles in the table above.

From Power to Window — In Time Order

   Boot        graphical.target (runlevel 5)        ← if multi-user.target (3), a text login happens here
     │
   Display manager starts (GDM · SDDM · LightDM)
     │           launches the X server (X.org Server) as display :0   ← config /etc/X11/xorg.conf
     │           draws the login screen
     │
   Session starts    desktop environment = window manager + panel + file manager …
     │
   Program      the X client looks at DISPLAY=:0 and connects to the X server
     │           toolkit (GTK+ · Qt) → Xlib · XCB → X protocol
     ▼
   Window on the screen

If you booted into text, a single startx line does the job of launching the X server and starting the session. Since you have already logged in by text, you skip the login screen.
This week's four days are one slot each of this picture — Day 1 is the X server and client, Day 2 is libraries and toolkits, Day 3 is the three managers, and Day 4 is execution and DISPLAY.

Separating Names with the Same Initial

The people who make the choices gather names that begin with the same letter. Sort them out in advance.

Starting with X	What it is
XFree86	The old X server implementation
X.org Server	The current X server implementation
X11	The version of the X protocol
Xlib · XCB	Low-level libraries
XDM	Display manager
Xfce	Desktop environment
Xfwm	Xfce's window manager
XWayland	The compatibility layer that runs X programs on Wayland
`xterm` · `xclock` · `xeyes`	X clients (applications)
`xhost` · `xauth` · `xrandr` · `xkill` · `xdpyinfo`	X-related commands

Initial	Desktop environment	Window manager	Display manager	Toolkit	Other programs
G	GNOME	(Mutter)	GDM	GTK+	GIMP, gedit
K	KDE	KWin	KDM (now SDDM)	(Qt)	Konsole, Kate
L	LXDE, LXQt	(Openbox)	LXDM, LightDM	—	—
M	MATE	Mutter, Metacity, Marco, Muffin	—	Motif	—

In the G row, GNOME, GDM, GTK+, and GIMP are all different kinds — environment, login, toolkit, and application. If asked to pick "the same kind as GNOME", you must look in the KDE and Xfce side, not among the names starting with G.
In the M row, only MATE is a desktop environment, Motif is a toolkit, and the other four are window managers.

Practice Identifying — Picking the Odd One Out

Cover the answer and first say "what the other three have in common". Once the commonality comes out in words, the odd one becomes visible on its own.

Choices	The odd one	Commonality of the other three
`GTK+ · Qt · Motif · Xlib`	Xlib	Toolkits
The one providing widgets among `XCB · Xlib · FLTK`	FLTK	(The other two are low-level libraries)
`Mutter · KWin · Xfwm · SDDM`	SDDM	Window managers
`GDM · KDM · XDM · twm`	twm	Display managers
`GNOME · Xfce · LXDE · KDE` (by toolkit)	KDE	GTK+-based
The one not Qt-based among `KDE · LXQt · Xfce`	Xfce	(The other two are Qt-based)
`Nautilus · Dolphin · Thunar · Evince`	Evince	File managers (Evince is a document viewer)
`XFree86 · X.org Server · Wayland · Xlib`	Xlib	On the server/protocol side that handles the screen (Xlib is a client library)
`xhost · xauth · ssh -X · xrandr`	xrandr	Access and connection for remote screens (xrandr is resolution)
`Windowmaker · Afterstep · Fluxbox · Cinnamon`	Cinnamon	Standalone window managers

Practice Identifying — Reading Values

What is given	How to read it
`DISPLAY=:0`	Local, display 0, screen omitted (0) — same as `:0.0`
`DISPLAY=192.168.0.10:1.0`	The second X server of 192.168.0.10, first screen. TCP port 6001
`DISPLAY=localhost:10.0`	A session that came in with `ssh -X`. The fake display made by sshd
`access control disabled, clients can connect from any host`	Someone ran `xhost +` — to close it, `xhost -`
`Error: Can't open display:`	`DISPLAY` is empty
`systemctl get-default` → `multi-user.target`	Boots into text (runlevel 3). For graphics, use `startx`
`id:5:initdefault:`	A line in the old `/etc/inittab` saying to boot into graphical mode
`Section "Device"`	The graphics card section of `xorg.conf` (not the keyboard or mouse)

The Old and the Current

Area	Old	Current
X server implementation	XFree86	X.org Server (and the replacement protocol Wayland)
Low-level library	Xlib	XCB (Xlib is also used alongside)
X server config file	`XF86Config`	`xorg.conf`
GNOME's window manager	Metacity	Mutter
KDE's display manager	KDM	SDDM
Specifying the boot mode	Runlevel (`/etc/inittab`)	Target (`systemctl set-default`)
Remote display	`xhost` + `DISPLAY`	`ssh -X`

The exam also uses the old names as they are. The answer differs between when it asks for the "current standard" and when it asks for the "evolution order", so remember the two columns as a pair.

Next week is network concepts. We cover the OSI 7 layers, TCP/IP, and IP addresses and subnets.

📖 Glossary

X server / X client: The side with the screen and input devices / the program that requests that something be drawn on the screen.
XCB: A low-level library thinner than Xlib and closer to the X server.
Toolkit: A higher-level library that provides widgets such as buttons and menus. GTK+, Qt, etc.
Window manager / display manager / desktop environment: Handles windows / handles the login screen / the overall integrated environment.
DISPLAY: The environment variable that specifies the screen to output to.
X11 forwarding: The feature that carries X traffic over SSH's encrypted connection. Used as ssh -X.
Compositor: In Wayland, the program that handles both screen output and window management.
Wayland: A display server protocol newly designed to replace X.

📝 Practice Questions

Click a choice to reveal the answer and explanation.

Question 1

Which of the following sits lowest in the layers of the X Window System and communicates directly with the X server?

Question 2

Which of the following is not a display manager?

Question 3

Which window manager does the Xfce desktop environment use?

Question 4

Which runlevel corresponds to text mode in the X Window context?

Question 5

Which desktop environment was built on the Qt toolkit?

Question 6

Which method safely displays a graphical program from a remote computer on my screen through encrypted communication?

Official references

Linux Master 2 official exam syllabus(opens in a new tab)