In X11 jargon, the name for a key is called its keysym (short for:
key symbol). The keysym generated by Shift+Tab
ISO_Left_Tab. So, in theory, one could just assign the
ISO_Left_Tab keysym to any key (which you can do with the command
xmodmap) and this would do the trick.
Unfortunately, some applications (for instance: Firefox) explicitly
look for the Shift keypress, and this breaks it, because you cannot
assign two keysyms (Shift and Tab) to one single key.
Fortunately, Roger Pate's answer to this AskUbuntu question provides
a way around it: bind a shortcut to a program that will simulate the
simultaneous pressing of Shift and Tab.
Install the xvkbd package
From the System->Preferences->Keyboard shortcuts menu, create a
new shortcut, name it e.g. "Shift+Tab" and insert the following
sh -c "xvkbd -text '\S\[ISO_Left_Tab]'"
The purpose of this command is just to send a virtual Shift+Tab
keystroke to the currently focused window. (Consult the xvbkd man
page for more information.)
Bind this shortcut to any key you want. Note that, to bind a
shortcut to a modifier key (i.e., one that is only meaningful in
combination with another key, for instance Shift or Alt) you
need to deprive it of its modifier status. For example, before
binding right-Control, you need to issue this command in a
xmodmap -e 'remove control = Control_R'
(See this other AskUbuntu question for another way to bind
Note: the xmodmap line above will only have effect until next reboot/logout. To make it persistent, you would have to either put that line into the
~/.gnomerc file, or add
remove control = Control_R to the