Linux – Keyboard shortcuts in the virtual terminal

keyboard shortcutslinuxterminaltty

Is there any way of configuring keyboard shortcuts in the Linux virtual console?

For example, if I go to tty1 then press the key combination Ctrl+Alt+H, I would like the script /usr/bin/hello.sh to be executed.

Ideally, the shortcut would be available even before logging in (in which case it would be executed with the privileges of a user that I specify). I don't mind modifying the kernel either, if that's the only way of accomplishing this. Also, it doesn't have to be a shell script, it can also be a normal ELF binary or even a kernel module making system calls.

Example use cases

I'm in the console and browsing the web with something like links and I want to turn down the screen brightness. I press Fn+End, which happens to be the brightness down key and produces a single keycode, and a program runs which reduces the brightness by writing something in a /sys file.
I'm in a console text editor and listening to some music in the background that's being played by mpd. I press the ⏯ (play/pause) key, which again produces a single keycode, and that has the effect of executing a program which sends a signal to mpd to pause the current song.

Solution

Following dirkt's idea of using /dev/input, I created evd (event daemon) for this purpose. The application can be started in the background and will take over the keyboard wherever you are, including before login and within X.

Best Answer

Partial answer (because it's just an outline, and untested):

Write a demon which listens to whatever /dev/input device corresponds to your main keyboard (there are symlinks, look at them). Start that demon as the user you specify, using whatever init system you have (systemd, sysv, whatever).

The demon processes key events as defined in input-events-codes.h (or look at the source code of evtest). It has a state machine that recognizes your desired key sequences, and spawns whatever process you specify when such a sequence is complete.

This should be available before you login, and will always execute as the same user, no matter what user you are logged in at the virtual console. It will also execute under X, again as the same user.

Alternative, if you want to execute something in a shell: Use tmux or a similar program which can bind key sequences to actions. I suppose it should also be possible to automatically start tmux and attach to a new session whenever you log in at a virtual console, but I haven't looked into that.

This won't work before log in, but will also work in graphical terminal emulators that have keyboard focus, and will execute the script as the user which is logged in.

Related Solutions

Bash – What does this output from xev mean

When you say Function-F1, I assume you mean pressing that funny little Fn key in the corner of a laptop followed by the F1 key at the top left of the keyboard. And that your trying to map one of the extra keys like Sleep that some laptop keyboards have. It looks like that key is literally pressing multiple keys for you. First I see Win+L as one combo. On Windows, this shortcut locks the screen, but does not start the screensaver. Next, I see a mapping for XF86ScreenSaver which might mean that X is seeing some kind of sleep keycode. I'm guessing that that key on your laptop keyboard is physically sending out those three keycodes of Win + L + Sleep as a convenience for Windows users to quickly lock the screen and activate the screensaver. I'm not sure of a good way to filter those out. The Fn key on keyboards normally is not seen by the Linux Kernel. Instead, it changes which codes the keyboard tells the kernel. When I was trying to use a USB RF PowerPoint remote with OOo on Linux, I discovered that the button to start/stop the presentation was just a lame control to send out alternating F5 and ESC key codes. Other buttons were just as lame like sending out b to blank the screen.

How to print keyboard shortcuts in human-readable form

The short answer is no, I'm pretty sure no such program exists.

You could in principle build one; it would have to look at the readline configuration and at the terminal emulator (the kernel and hardware are not involved).

bind -P | grep 'can be found' in Bash lists the key bindings.

abort can be found on "\C-g", "\C-x\C-g", "\e\C-g".
accept-line can be found on "\C-j", "\C-m".

To have a more descriptive name for the command you'd need to parse the bash or readline documentation.

The correspondance between key sequences and keys is determined by the terminal (usually, the terminal emulator). It is often not readily available, and when it is the method to obtain it is entirely specific to the terminal.

You can come close in Emacs: start emacs -q -nw in a terminal, and press Ctrl+H, C (the describe-key-briefly command) then the key sequence (\e is Escape). This shows you the reconstructed function key, if any, and what the key does in Emacs. Readline's default bindings are strongly inspired by Emacs, so often the function in Emacs is similar to the function in readline, but not always. Example:

Ctrl+H C ESC [ A
<up> runs the command previous-line