The X Window System

The X Window System#

[!WARNING] This chapter is getting outdated since this is based on Debian 7.0 (Wheezy) released in 2013.

The X Window System on the Debian system is based on the source from X.Org.

Key packages#

There are a few (meta)packages provided to ease installation.

List of key (meta)packages for X Window

For the basics of X, refer to X(7) and the LDP XWindow-User-HOWTO.

Setting up desktop environment#

A desktop environment is usually a combination of a X window manager, a file manager, and a suite of compatible utility programs.

You can setup a full desktop environment such as GNOME, KDE, Xfce, or LXDE, from the aptitude under the task menu.

[!TIP] Task menu may be out of sync with the latest package transition state under Debian unstable/testing environment. In such situation, you need to deselect some (meta)packages listed under aptitude(8) task menu to avoid package conflicts. When deselecting (meta)packages, you must select certain packages providing their dependencies manually to avoid them deleted automatically.

You may alternatively setup a simple environment manually just with a X window manager such as Fluxbox.

See Window Managers for X for the guide to the X window manager and the desktop environment.

Debian menu#

Debian menu system provides a general interface for both text- and X-oriented programs with update-menus(1) from the menu package. Each package installs its menu data in the "/usr/share/menu/" directory. See "/usr/share/menu/README".

Freedesktop.org menu#

Each package which is compliant to Freedesktop.org's xdg menu system installs its menu data provided by "*.desktop" under "/usr/share/applications/". Modern desktop environments which are compliant to Freedesktop.org standard use these data to generate their menu using the xdg-utils package. See "/usr/share/doc/xdg-utils/README".

Debian menu from Freedesktop.org menu#

In order to access the traditional Debian menu from the Freedesktop.org menu compliant window manager environment such as GNOME and KDE, you must install the menu-xdg package.

The server/client relationship#

The X Window System is activated as a combination of the server and client programs. The meaning for the words server and client with respect to the words local and remote requires attention here.

List of server/client terminology

Modern X servers have the MIT Shared Memory Extension and communicate with their local X clients using the local shared memory. This bypasses the network transparent Xlib interprocess communication channel and gains performance for large images.

The X server#

See xorg(1) for X server information.

The (re)configuration of the X server#

The following (re)configures an X server.

# dpkg-reconfigure --priority=low x11-common

[!NOTE] Recent Linux kernels have good graphics and input device supports with DRM, KMS, and udev. X server is rewritten to use them. So "/etc/X11/xorg.conf" is usually not present on your system. These parameters are configured by the kernel. See "fb/modedb.txt" in the Linux kernel documentation.

For the large high resolution CRT monitor, it is a good idea to set the refresh rate as high as your monitor can handle (85 Hz is great, 75 Hz is OK) to reduce flicker. For the LCD monitor, slower standard refresh rate (60Hz) is usually fine due to its slow response.

[!NOTE] Be careful not to use too high refresh rate which may cause fatal hardware failure of your monitor system.

The connection methods to the X server#

There are several ways of getting the "X server" (display side) to accept connections from an "X client" (application side).

List of connection methods to the X server

[!WARNING] Do not use remote TCP/IP connection over unsecured network for X connection unless you have very good reason such as use of encryption. A remote TCP/IP socket connection without encryption is prone to the eavesdropping attack and is disabled by default on the Debian system. Use "ssh -X".

[!WARNING] Do not use XDMCP connection over unsecured network either. It sends data via UDP/IP without encryption and is prone to the eavesdropping attack.

[!TIP] LTSP stands for Linux Terminal Server Project.

Starting the X Window System#

The X Window System is usually started as an X session which is the combination of an X server and connecting X clients. For the normal desktop system, both of them are executed on a workstation.

The X session is started by one of the following.

• startx command started from the command line

• One of the X display manager daemon programs *dm started by systemd based on the dependency of "graphical.target".

[!TIP] The start up script for the display manager daemons checks the content of the "/etc/X11/default-display-manager" file before actually executing themselves. This ensures to have only one X display manager daemon program activated.

[!TIP] See ??? for initial environment variables of the X display manager.

Essentially, all these programs execute the "/etc/X11/Xsession" script. Then the "/etc/X11/Xsession" script performs run-parts(8) like action to execute scripts in the "/etc/X11/Xsession.d/" directory. This is essentially an execution of the first program which is found in the following order by the exec builtin command.

1. The script specified as the argument of "/etc/X11/Xsession" by the X display manager, if it is defined.

2. The "~/.xsession" or "~/.Xsession" script, if it is defined.

3. The "/usr/bin/x-session-manager" command, if it is defined.

4. The "/usr/bin/x-window-manager" command, if it is defined.

5. The "/usr/bin/x-terminal-emulator" command, if it is defined.

This process is affected by the content of "/etc/X11/Xsession.options". The exact programs to which these "/usr/bin/x-*" commands point, are determined by the Debian alternatives system and changed by "update-alternatives --config x-session-manager", etc.

See Xsession(5) for details.

Starting X session with gdm3#

gdm3(1) lets you select the session type (or desktop environment: Setting up desktop environment), and language (or locale: ???) of the X session from its menu. It keeps the selected default value in "~/.dmrc" as the following.

[Desktop]
Session=default
Language=ja_JP.UTF-8

Customizing the X session (classic method)#

On a system where "/etc/X11/Xsession.options" contains a line "allow-user-xsession" without preceding "#" characters, any user who defines "~/.xsession" or "~/.Xsession" is able to customize the action of "/etc/X11/Xsession" by completely overriding the system code. The last command in the "~/.xsession" file should use form of "exec some-window/session-manager" to start your favorite X window/session managers.

If this feature is used, the selection of the display (or login) manager (DM), session manager or window manager (WM) by the system utility is ignored.

Customizing the X session (new method)#

Here are new methods to customize the X session without completely overriding the system code as above.

• The display manager gdm3 can select a specific session and set it as the argument of "/etc/X11/Xsession".

• "/etc/profile", "~/.profile", "/etc/xprofile", and "~/.xprofile" files are executed as a part of gdm3 start up process.

• The "~/.xsessionrc" file is executed as a part of start up process. (desktop independent)

• "#allow-user-xsession" in "/etc/X11/Xsession.options" does not restrict execution of the "~/.xsessionrc" file.

• The "~/.gnomerc" file is executed as a part of start up process. (GNOME desktop only)

The selection of the display (or login) manager (DM), session manager or window manager (WM) by the system utility is respected.

These configuration files should not have "exec …" nor "exit" in them.

Connecting a remote X client via SSH#

The use of "ssh -X" enables a secure connection from a local X server to a remote application server.

Set "X11Forwarding" entries to "yes" in "/etc/ssh/sshd_config" of the remote host, if you want to avoid "-X" command-line option.

Start the X server on the local host.

Open an xterm in the local host.

Run ssh(1) to establish a connection with the remote site as the following.

localname @ localhost $ ssh -q -X loginname@remotehost.domain
Password:

Run an X application command, e.g. "gimp", on the remote site as the following.

loginname @ remotehost $ gimp &

This method can display the output from a remote X client as if it were locally connected through a local UNIX domain socket.

Secure X terminal via the Internet#

Secure X terminal via the Internet, which displays remotely run entire X desktop environment, can easily achieved by using specialized package such as ldm. Your local machine becomes a secure thin client to the remote application server connected via SSH.

Fonts in the X Window#

Fontconfig 2.0 was created to provide a distribution independent library for configuring and customizing font access in 2002. Debian after squeeze uses Fontconfig 2.0 for its font configuration.

Font supports on X Window System can be summarized as follows.

• Legacy X server side font support system

• The original core X11 font system provides backward compatibility for older version of X client applications.

• The original core X11 fonts are installed on the X server.

• Modern X client side font support system

• The modern X system supports all fonts listed below (Basic fonts, Additional fonts, and CJK fonts) with advanced features such as anti-aliasing.

• Xft 2.0 connects modern X applications such as ones from GNOME, KDE, and LibreOffice with FreeType 2.0 library.

• FreeType 2.0 provides font rasterization library.

• Fontconfig provides resolution of the font specification for Xft 2.0. See fonts.conf(5) for its configuration.

• All modern X applications using Xft 2.0 can talk to modern X server using the X Rendering Extension.

• The X Rendering Extension moves font access and glyph image generation from the X server to the X client.

Table of packages to support X Window font systems

You can check font configuration information by the following.

• "xset q" for core X11 font path

• "fc-match" for fontconfig font default

• "fc-list" for available fontconfig fonts

[!TIP] "The Penguin and Unicode" is a good overview of modern X Window System. Other documentations at http://unifont.org/ should provide good information on Unicode fonts, Unicode-enabled software, internationalization, and Unicode usability issues on free/libre/open source (FLOSS) operating systems.

Basic fonts#

There are 2 major types of computer fonts.

• Bitmap fonts (good for low resolution rasterization)

• Outline/stroke fonts (good for high resolution rasterization)

While scaling of bitmap fonts causes jugged image, scaling of outline/stroke fonts produces smooth image.

Bitmap fonts on the Debian system are usually provided by compressed X11 pcf bitmap font files having their file extension ".pcf.gz".

Outline fonts on the Debian system are provided by the following.

• PostScript Type 1 font files having their file extension ".pfb" (binary font file) and ".afm" (font metrics file).

• TrueType (or OpenType) font files usually having their file extension ".ttf".

[!TIP] OpenType is intended to supersede both TrueType and PostScript Type 1.

Table of corresponding PostScript Type 1 fonts

Table of corresponding TrueType fonts

[!TIP] DejaVu fonts are based on and superset of Bitstream Vera fonts.

Additional fonts#

aptitude(8) helps you find additional fonts easily.

• The short package list under "Tasks" → "Localization"

• The filtered flat package list of font data with regex on debtag: "~Gmade-of::data:font"

• The filtered flat package list of the BDF (bitmap) font packages with regex on package name: "~nxfonts-"

• The filtered flat package list of the TrueType (outline) font packages with regex on package name: "~nttf-|~nfonts-"

Since Free fonts are sometimes limited, installing or sharing some commercial TrueType fonts is an option for a Debian users. In order to make this process easy for the user, some convenience packages have been created.

• mathematica-fonts

• fonts-mscorefonts-installer

You'll have a really good selection of TrueType fonts at the expense of contaminating your Free system with non-Free fonts.

CJK fonts#

Here are some key points focused on fonts of CJK characters.

Table of key words used in CJK font names to indicate font types

Font name such as "VL PGothic" with "P" is a proportional font which corresponds to the fixed width "VL Gothic" font.

For example, Shift_JIS code table comprises 7070 characters. They can be grouped as the following.

• JIS X 0201 single-byte characters (191 characters, a.k.a. half-width characters)

• JIS X 0208 double-byte characters (6879 characters, a.k.a. full-width characters)

Double-byte characters occupy double width on console terminals which uses CJK fixed width fonts. In order to cope with such situation, Hanzi Bitmap Font (HBF) File with file extension ".hbf" may be deployed for fonts containing single-byte and double-byte characters.

In order to save space for TrueType font files, TrueType font collection file with file extension ".ttc" may be used.

In order to cover complicated code space of characters, CID keyed PostScript Type 1 font is used with CMap files starting themselves with "%!PS-Adobe-3.0 Resource-CMap". This is rarely used for normal X display but used for PDF rendering etc. (see X utility applications).

[!TIP] The multiple glyphs are expected for some Unicode code points due to Han unification. One of the most annoying ones are "U+3001 IDEOGRAPHIC COMMA" and "U+3002 IDEOGRAPHIC FULL STOP" whose character positions differ among CJK countries. Configuring priority of Japanese centric fonts over Chinese ones using "~/.fonts.conf" should give peace of minds to Japanese.

X applications#

X office applications#

Here is a list of basic office applications (LO is LibreOffice).

List of basic X office applications

X utility applications#

Here is a list of basic utility applications which caught my eyes.

List of basic X utility applications

[!CAUTION] The poppler-data package (previously non-free, see ???) needs to be installed for evince and okular to display CJK PDF documents using Cmap data (CJK fonts).

[!NOTE] Installing softwares such as scribus (KDE) on GNOME desktop environment are quite acceptable since corresponding functionality is not available under GNOME desktop environment. But installing too many packages with duplicated functionalities clutter your menu.

The X trivia#

Clipboard#

The X selection using 3 mouse buttons is the native clipboard feature of X (see ???).

[!TIP] Shift-Insert can work as the equivalent of the middle-mouse-button click.

List of basic X selection programs

The modern Desktop Environments (GNOME, KDE, …) offer different clipboard system for the cut, copy, and paste using the left mouse button and keys (CTRL-X, CRTL-C, and CTRL-V).

Keymaps and pointer button mappings in X#

xmodmap(1) is a utility for modifying keymaps and pointer button mappings in the X Window System. To get the keycode, run xev(1) in the X and press keys. To get the meaning of keysym, look into the MACRO definition in "/usr/include/X11/keysymdef.h" file (x11proto-core-dev package). All "#define" statements in this file are named as "XK_" prepended to keysym names.

Classic X clients#

Most traditional X client programs, such as xterm(1), can be started with a set of standard command line options to specify geometry, font, and display.

They also use the X resource database to configure their appearance. The system-wide defaults of X resources are stored in "/etc/X11/Xresources/*" and application defaults of them are stored in "/etc/X11/app-defaults/*". Use these settings as the starting points.

The "~/.Xresources" file is used to store user resource specifications. This file is automatically merged into the default X resources upon login. To make changes to these settings and make them effective immediately, merge them into the database using the following command.

$ xrdb -merge ~/.Xresources

See x(7) and xrdb(1).

The X terminal emulator — xterm#

Learn everything about xterm(1) at http://dickey.his.com/xterm/xterm.faq.html.

Running X clients as root#

[!WARNING] Never start the X display/session manager under the root account by typing in root to the prompt of the display manager such as gdm3 because it is considered unsafe (insecure), even when you plan to perform administrative activities. The entire X architecture is considered insecure if run as root. You must always use the lowest privilege level possible, like a regular user account.

Easy ways to run a particular X client, e.g. "foo" as root is to use sudo(8) etc. as the following.

$ sudo foo &

$ sudo -s
# foo &

$ ssh -X root@localhost
# foo &

[!CAUTION] Use of ssh(1) just for this purpose as above is waste of resource.

In order for the X client to connect to the X server, please note the following.

• Values of the old user's "$XAUTHORITY" and "$DISPLAY" environment variables must be copied to the new user's ones.

• The file pointed by value of the "$XAUTHORITY" environment variable must be readable by the new user.