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Written by Oleksandr Gavenko (AKA gavenkoa), compiled on 2017-01-30 from rev ccaa2f364422+.

X.

Standard.

See

http://www.tronche.com/gui/x/icccm/

Widget toolkit.

http://en.wikipedia.org/wiki/Widget_toolkit
Widget toolkit
http://en.wikipedia.org/wiki/List_of_widget_toolkits
List of widget toolkits

Keyboard's keys under X window.

scancodes
hardware-dependent codes generated by keystrokes and received by the X server.
keycodes
Codes sent by the X server to the client indicating which key was pressed. Keycodes by themselves do not indicate what the keystroke means; the client must request that information from the X server.
modifiers
Flags, such as shift and control, that the server sends with each keycode to the client. The client's interpretation of the keycode may depend on the state of the modifiers. For example, the state of the shift modifier determines whether an alphabetic character should be lowercase or uppercase. Because the X server encodes the states of the eight modifiers (shift, lock, control, mod1, mod2, mod3, mod4, and mod5) in a single byte of data, modifiers are often referred to as modifier bits.
keysyms
codes that specify the glyphs appearing on the keys. The X server also maintains a list of strings that describe the keysyms, such as a, B, and Control.

Find the keycode.

$ xev
$ xev | grep keysym --color
$ cat /usr/include/X11/keysymdef.h

Don't run under X:

$ showkey

To do same under X:

$ xev | sed -n 's=.*keycode \([0-9]*\).*=\1=p'

List key modifier (like shift/control/meta/mod2):

$ xmodmap -pm

Add keycode to modifier:

$ xmodmap -e "add shift = Shift_R"

Remove keycode from modifier:

$ xmodmap -e "remove shift = Shift_L"

Examining the current keymap table:

$ xmodmap -pk

Changing the keymap table:

$ xmodmap -e "keycode 34 = bracketleft braceleft"
$ xmodmap -e "keysym CapsLock = Control_L"

https://wiki.ubuntu.com/LaptopTesting/Keycodes
              Ubuntu Keycodes howto.
http://en.gentoo-wiki.com/wiki/Multimedia_Keys
              Gentoo Multimedia Keys howto.

List input devices.

$ xinput -list

You can use obtained ID in such command:

$ setxkbmap -device $ID -layout dvorak

Making screenshort.

$ sudo apt-get install scrot
$ scrot -d <sec> <file>

or:

$ sudo apt-get install imagemagic
$ import -quality 100 -pause <sec> -silent <file>

Input method.

Input method by setxkbmap.

$ setxkbmap ru

$ setxkbmap -rules xorg -model pc104 -layout "us,ru" -option "grp:rwin_toggle,grp_led:scroll" -variant ",winkeys"

Input method from xorg.conf.

Section "InputDevice"
    Identifier     "Keyboard0"
    Driver         "kbd"
    Option         "XkbModel" "pc105"
    Option         "XkbLayout" "us,ru"
    Option         "XkbVariant" ",winkeys"
    Option         "XkbOptions" "grp:rwin_switch,grp_led:scroll"
EndSection

.Xdefaults

The syntax of an Xdefaults file is as follows:

[client. | *][{restriction.} | *]resource: value

where:

client
The name of the application, some program allow change it by '-name' option. This element is optional (can be substituated with wildcard).
restriction
Class names or name of specific class instance. The classes names conventionally start with an upper-case letter.
resource
The name of the resource whose value is to be changed. Resources are typically lowercase with uppercase concatenation.
value
The actual value of the resource.
delimiters
A period (.) is used to signify each step down into the hierarchy. A colon (:) is used to separate the resource declaration from the actual value.

Comment start with '!' char and goes up to end of line or C-like /* */.

Use xprop utility to find classes and resources used by application.

To reread your .Xresources file, and throw away your old resources:

$ xrdb ~/.Xdefaults

To reread your .Xresources file, and keep your old resources:

$ xrdb -merge ~/.Xdefaults

Example:

*foreground: yellow
XClock*foreground: pink
Xman*topBox*foreground: blue

String value type.

This can be path specification like '/usr/bin/firefox'.

Colors value type.

For color names see '/usr/lib/X11/rgb.txt'. Also you can use hex representation #ffffff.

Font value type.

You can use either a full name, a wildcarded specification, or a font alias:

XTerm*Font: -adobe-courier-bold-r-normal--14-140-75-75-m-90-iso8859-1
XTerm*Font: *courier-bold-r*140*
XTerm*Font: 7x14

Geometry value type.

XCalc*geometry: 120x120-0-0 XClock*geometry: -50+100

Cursor names value type.

Cursor resources require the name of the file in /usr/include/X11/bitmaps that contains the cursor you want to use.

ScoTerm*pointerShape: gumby

Pixmaps value type.

Pixmaps are patterns, like bitmaps, that are used to texture or color an area on your display. Pixmap resources are specified like cursors or bitmaps.

Numebers value type.

XLogo*borderWidth: 10

Boolean value type.

Some resources require a boolean value, such as 'true' or 'false', 'yes' or 'no', or 'on' or 'off'.

Wildcard matching.

The asterisk can be used as a wildcard, making it easy to write a single rule that can be applied to many different applications or elements.

X utilities.

appres
List "application resources".
listres
List resources in widgets.
bitmap, atobm, bmtoa
Bitmap editor and converter utilities.
editres
Dynamic resource editor for X Toolkit applications.
iceauth
ICE authority file utility.
xauth
X authority file utility.
ico
Animate an icosahedron or other polyhedron.
luit
Locale and ISO 2022 support for Unicode terminals.
setxkbmap
Set the keyboard using the X Keyboard Extension.
x11perf
X11 server performance test program.
x11perfcomp
X11 server performance comparison program.
xcalc
scientific calculator desktop accessory that can emulate a TI-30 or an HP-10C.
xclock
Displays the time in analog or digital form.
xconsole
Monitor system console messages with X.
xdpyinfo
displaying information about an X server.
xev
"Event Tester" window.
xeyes
Showing two googly eyes which follow the cursor movements on the screen as if they were watching it.
xfontsel
Point and click selection of X11 font names.
xgamma
Alter a monitor's gamma correction through the X server.
xhost
Server access control program for X.
xinit
X Window System initializer.
startx
Initialize an X session.
xkill
Kill a client by its X resource.
xload
System load average display for X.
xlogo
X Window System logo.
xlsclients
listing information about the client applications running on a display.
xmag
Magnify parts of the screen.
xman
Manual page display program for the X Window System.
xmessage
Display a message or query in a window.
xmodmap
Utility for modifying keymaps and pointer button mappings in X.
xmore
Plain text display program for the X Window System.
xprop
Property displayer for X.
xrandr
Primitive command line interface to RandR extension.
xrdb
X server resource database utility.
xrefresh
Refresh all or part of an X screen.
xset
User preference utility for X.
xsetmode
Set the mode for an X Input device.
xsetpointer
Set an X Input device as the main pointer.
xsetroot
Root window parameter setting utility for X.
xsm
X Session Manager
resize
Set environment and terminal settings to current xterm window size.
xterm
Standard terminal emulator for the X Window System.
xvinfo
Print out X-Video extension adaptor information.
xwd, xwud
Useful and easy way to take screenshots. Saves in the XWD X Window Dump image data format.
xwininfo
Window information utility for X.

XText extension.

xdotool and xautomation utilities use XText for emulating user actions.

GLX info.

To determine whether 3D acceleration is working:

$ apt-get install mesa-utils
$ glxinfo | grep render

To see how many frames per second your video card is putting out:

$ glxgears -info

See:

https://wiki.debian.org/Mesa
Mesa is an open source 3D computer graphics library that provides a generic OpenGL implementation for rendering three-dimensional graphics on multiple platforms.

Running a secondary X server.

$ startx xterm -- :$N -depth 16

New server will be accessible at Alt+Ctrl+F$((N+6)):

:0  Alt+Ctrl+F7
:1  Alt+Ctrl+F8
...
:5  Alt+Ctrl+F12

In order to fix:

X: user not authorized to run the X server

run:

$ sudo dpkg-reconfigure x11-common

Running X server without display.

Run Xvfb and make screenshort of desktop:

$ apt-get install xvfb
$ Xvfb :1 -screen 0 1280x1024x24 &
$ xclock -display :1 &
$ import -display :1 -window root image.png
$ kill %1 %2

In order to run Xvfb permanently add entry to /etc/inittab:

xvfb:2:respawn:/usr/bin/Xvfb :1 -ac -screen 0 1280x1024x24

Running X server in nested windows.

$ sudo apt-get install xnest
$ startx xterm -- `command -v Xnest` :1 -geometry 800x600

Running X server under Windows

https://sourceforge.net/projects/vcxsrv/
VcXsrv Windows X Server, the xorg git sources compiled with Visual C++ 2012 Express Edition.
http://x.cygwin.com/
Cygwin's xwin. Cygwin/X is a port of the X Window System to the Cygwin API layer for the Microsoft Windows family of operating systems.
https://sourceforge.net/projects/xming/
Xming is the leading X Window System Server for Microsoft Windows 8/7/Vista/XP (+ server 2012/2008/2003). It is fully featured, small and fast, simple to install and because it is standalone native Microsoft Windows, easily made portable (not needing a machine-specific installation).