bash - wrong key sequence bindings with control+alt+space

Question

I am trying to install some key sequence bindings but I have trouble. My shell is bash, my terminal is gnome-terminal, and my system is Ubuntu 14.04 in graphic mode. Edit : my keyboard is french azerty with the numeric pad and ctrl,fn,super,alt.

Edit: a bash guide, readline chapter

I want to add contol+alt+space, but this exact sequence does not work while similar other sequences work fine. My goal is to run shell-expand-line readline but i tried upcase-word to compare.

--Let's analyze my goal's installation effect.

This is my ~/.inputrc added (ctrl+meta+space):

"\e\C- ": shell-expand-line

This is the new result of bind -p :

"\e\C-@": shell-expand-line

Of course I started a couple terminals, one before installation and one after every installation in order to compare thoroughly.

--Some experiments to ensure control+alt works in place of control+meta.

The system should emulate Meta with Alt and bash should emulate Meta with Escape, both using the byte '\033'. I tried both Alt and Esc to conclude it works fine. I tested some ctrl+meta+letter sequences to be sure.

By the way, the individual samples of Ctrl+Alt are simple : expanding an isolated tilde '~' (shell-expand-line) or changing case in random lowercase words (upcase-word) with ctrl+meta+e or ctrl+meta+v. Ctrl+meta+e is already binding to shell-expand-line so i just mixed the bindings with the useless ctrl+meta+v.

--Some more intricate observations...

I read '^@' illustrates 'NUL' character. Why this escape?

I tested xev and pressed ctrl+alt+space : the character reported is 'NUL' character.

I saw alt+space usually open the window menu of gnome-terminal while alt enables menus, of course without control key.

A conflict seems to come from the system, but I am a newbie. I just read most of the bash manual (especially readline chapter) and one forum advice about xev.

--Question

Is there more to say or to correct? Why the sequences are rewritten? How can I make my ctrl+alt+space work with bash and gnome-terminal ?

Edit : I found that '@' is transformed in 'NUL' because of a bitwise mask applied when escaping, but i do not see any reason why Alt+space gives 'NUL'.

Answer 1

The terminal emulator (Gnome-terminal, in your case) converts key combinations like Ctrl+Alt+Space (which it receives as the Space keysym with the Ctrl and meta modifiers are) into sequences of bytes. Text characters like a and space are sent as is; keys that don't correspond to characters are sent as control characters when there is one (e.g. byte 13 = carriage return = Return, byte 9 = tabulation = Tab) and as escape sequences beginning with byte 27 = escape if there isn't. Alt+character is sent as escape followed by that character (for all intents and purposes, in typical configurations nowadays, treat “Alt” and “Meta” as synonymous — I won't go into that topic in this post). See also How do keyboard input and text output work?

Ctrl+character is sent as a control character if there is one (e.g. Ctrl+A or Ctrl+Shift+A → byte 1 = ^A). There is no control-space character, but for historical reasons, the key combination Ctrl+Space is usually transmitted as byte 0 = ^@.

You can see what key sequence the terminal sends by pressing Ctrl+V on the bash command line then the key or key combination you're interested in. Ctrl+V tells bash to insert the next character, whatever it is, instead of interpreting it as a command.

It would be logical to transmit Ctrl+Alt+Space as the two-byte sequence 27, 0 = escape, ^@. Unfortunately for you, Gnome-terminal doesn't do this, it sends 27, 32 = escape, space instead. Key sequences sent by Gnome-terminal cannot be configured (you'd need to patch the VTE library). So if you want to be able to use Ctrl+Alt+Space in the terminal, you have several possibilities:

• Use a different terminal emulator. There are about 40 in Ubuntu.
• Edit the source code of the VTE library to make it send a different escape sequence. You may want to suggest this as an enhancement.
• Maybe use LD_PRELOAD to patch calls from Gnome-terminal to libvte. I haven't checked to see if it's practical, and anyway it would be cumbersome — if you care that much, use a different terminal emulator.
• Configure your window manager or an external program to change Ctrl+Alt+Space into Esc Ctrl+Space when a Gnome-terminal window is active. This is not easy to do without disrupting other applications in most environments.

In short, if you want to use this binding, you'll need to use a different terminal emulator, such as xterm (where escape sequences for key combinations are fully configurable).

Answer 2

Your gnome-terminal (actually the underlying vte-0.34) emits the wrong sequence for ctrl+Alt+space. The bug (https://bugzilla.gnome.org/show_bug.cgi?id=710349) was fixed in vte-0.36.

If you're not afraid of hacking a little bit and you're able to safely revert things in case of trouble, you can try to install vte-0.36 on your Ubuntu 14.04. You'll get many other fixes and improments along with this one. You might find a PPA or a package in Gnome3 staging, or compile it for yourself. Upgrading to this version of vte doesn't require touching any other software components. A complete restart of gnome-terminal is required (close all the windows).