Make Debian default to caps lock as control system-wide by editing `/etc/default/keyboard`:
XKBOPTIONS="ctrl:nocaps"
See SETXKBMAP(1) and XKEYBOARD-CONFIG(7).
Show the current config with setxkbmap -query or setxkbmap -print -verbose 10.
Look at the available options in /usr/share/X11/xkb/rules/evdev.lst. Here are some I find useful:
ctrl:nocaps Caps Lock as Ctrl ctrl:lctrl_meta Left Ctrl as Meta shift:both_capslock Both Shift keys together toggle Caps Lock
or
caps:hyper Caps Lock as Hyper_L
or
altwin:swap_alt_win Alt is swapped with Win
Invoke options like:
setxkbmap -model pc105 -layout us -option \ -option ctrl:nocaps -option ctrl:lctrl_meta -option shift:both_capslock
(The empty -option argument resets things, so we start from a clean slate.)
There are some things setxkbmap can't do with simple options. For those things, create ~/.xkb/symbols/mysymbols and ~/.xkb/keymap/mykeymap.
Apply the custom keymap with: xkbcomp -I$HOME/.xkb ~/.xkb/keymap/mykeymap $DISPLAY.
Also see the man pages for xkeyboard-config and setxkbmap and keyboard.
N.B. I've found xkb to be a little flaky. For example, if I set keyboard options in my .xsession with xsetkbmap, those options get reset to defaults in the keyboard is unplugged/plugged. As an alternative, system-wide defaults can be set in /etc/defaults/keyboard.
xmodmap maps the KeyCodes of physical keys on your keyboard to what KeySym value get sent when you press that key.
Determine what key is what by running xev from a terminal. It will show you the keycode for any key you press. xmodmap -pk may also be useful; it prints the current key mappings. (Using xmodmap -pke gives the same information as a list of commands that can be fed back to xmodmap like xmodmap -e 'keycode 47 = semicolon colon semicolon colon'.)
Print the keycodes and keysym values for modifier keys like Caps Lock, Alt, and Control with xmodmap -pm. Changing modifier keys is slightly different from changing other keys, using clear, remove, and add.
Once you know what you want to do with xmodmap, either save your xmodmap commands to the beginning of your .xsession file if you only have one or two changes, or keep your entire key mapping in its own file by adding this to your.xsession:
if [ -s ~/.Xmodmap ]; then
xmodmap ~/.Xmodmap
fi
...and having xmodmap create a .Xmodmap file b:verbose set buftypeased on your current settings:
xmodmap -pke > ~/.Xmodmap