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When I press Ctrl+/ in a graphical terminal (e.g. xterm) I get "undo". However, in a virtual terminal (e.g. Ctrl+Alt+F1) I get "backspace".

In practice I run into this when I'm using emacs in a virtual terminal, probably debugging some X problem, and I try to use C+/ to undo but instead get a backspace.

I believe I have experienced this on multiple computers over the years, so I don't think it's very specific to my current setup (ubuntu).

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up vote 4 down vote accepted

When you press a key in xterm, it sends a character sequence that is determined by the translations in its X resources. Typically Ctrl+/ sends the single character ^_ (i.e. character number 31, like Ctrl+_). The shell interprets this control character as the undo command.

When you press the key in a Linux console, it sends a character sequence that is determined by the active keymap. The command loadkeys loads a keymap. The location of the default keymap depends on the distribution and on the installed packages; on recent Ubuntu distributions, the console keymap is derived from the default XKB keymap. Many default keymap assign Backspace (which is called Delete in this context: it's character 127) to Ctrl+/ (I don't know why); you can change this by using the following keymap fragment:

keycode  53 = slash            question        
        control keycode  53 = Control_underscore
        alt     keycode  53 = Meta_slash      
        shift   alt     keycode  53 = Meta_question   

If you have a file called /etc/console/boottime.kmap.gz, edit that. If you have a file /etc/default/keyboard, read it and follow the instructions.

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Thanks! I have /etc/default/keyboard. I'm using XKBOPTIONS there, and I don't want to maintain a full separate keymap, so I'm looking for a more modular solution. Loading the keymap fragment you give above with sudo loadkeys <file with keymap fragment> works for the current session, and I guess I could run that from an init.d script. – ntc2 Jul 20 '12 at 2:09
@enoksrd Right, you can use an init.d script, or even add loadkeys </etc/enoksrd/custom.kmap to /etc/rc.local. – Gilles Jul 20 '12 at 2:16

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