xcape allows you to use a modifier key as another key when pressed and released on its own. Note that it is slightly slower than pressing the original key, because the pressed event does not occur until the key is released. The default behaviour is to generate the Escape key when Left Control is pressed and released on its own. (If you don't understand why anybody would want this, I'm guessing that Vim is not your favourite text editor ;)

First install the development dependencies. On Debian-based systems (including Ubuntu and Linux Mint), run:

$ sudo apt-get install git gcc make pkg-config libx11-dev libxtst-dev libxi-dev

On Fedora-based systems, run:

$ sudo dnf install git gcc make pkgconfig libX11-devel libXtst-devel libXi-devel

Then run:

$ git clone https://github.com/alols/xcape.git
$ cd xcape
$ make
$ sudo make install

$ xcape [-d] [-f] [-t <timeout ms>] [-e <map-expression>]

Debug mode. Does not fork into the background. Prints debug information.

Foreground mode. Does not fork into the background.

If you hold a key longer than this timeout, xcape will not generate a key event. Default is 500 ms.

The expression has the grammar 'ModKey=Key[|OtherKey][;NextExpression]'

The list of key names is found in the header file X11/keysymdef.h (remove the XK_ prefix). Note that due to limitations of X11 shifted keys must be specified as a shift key followed by the key to be pressed rather than the actual name of the character. For example to generate "{" the expression 'ModKey=Shift_L|bracketleft' could be used (assuming that you have a key with "{" above "[").

You can also specify keys in decimal (prefix #), octal (#0), or hexadecimal (#0x). They will be interpreted as keycodes unless no corresponding key name is found.

• This will make Left Shift generate Escape when pressed and released on its own, and Left Control generate Ctrl-O combination when pressed and released on its own.

  xcape -e 'Shift_L=Escape;Control_L=Control_L|O'
  

• In conjunction with xmodmap it is possible to make an ordinary key act as an extra modifier. First map the key to the modifier with xmodmap and then the modifier back to the key with xcape. However, this has several limitations: the key will not work as ordinary until it is released, and in particular, it may act as a modifier unintentionally if you type too fast. This is not a bug in xcape, but an unavoidable consequence of using these two tools together in this way. As an example, we can make the space bar work as an additional ctrl key when held (similar to Space2ctrl) with the following sequence of commands.

  # Map an unused modifier's keysym to the spacebar's keycode and make it a
  # control modifier. It needs to be an existing key so that emacs won't
  # spazz out when you press it. Hyper_L is a good candidate.
  spare_modifier="Hyper_L"
  xmodmap -e "keycode 65 = $spare_modifier"
  xmodmap -e "remove mod4 = $spare_modifier" # hyper_l is mod4 by default
  xmodmap -e "add Control = $spare_modifier"
  
  # Map space to an unused keycode (to keep it around for xcape to
  # use).
  xmodmap -e "keycode any = space"
  
  # Finally use xcape to cause the space bar to generate a space when tapped.
  xcape -e "$spare_modifier=space"
  

If you are in the habit of remapping keycodes to keysyms (eg, using xmodmap), there are two issues you may encounter.

1. You will need to restart xcape after every time you modify the mapping from keycodes to keysyms (eg, with xmodmap), or xcape will still use the old mapping.

2. The key you wish to send must have a defined keycode. So for example, with the default mapping Control_L=Escape, you still need an escape key defined in your xmodmap mapping. (I get around this by using 255, which my keyboard cannot send).

Find the latest version at https://github.com/alols/xcape

The author can be reached at albin dot olsson at gmail dot com