The Linux keyboard driver

do_self, commonly used for ordinary keys, just returns the given value, after possibly handling pending dead diacriticals.

do_fn, commonly used for function keys, returns the string func_table[value]. Strings can be assigned using loadkeys(1).

do_spec is used for special actions, not necessarily related to character input. It does spec_fn_table[value]();, where

static void_fnp spec_fn_table[] = {
do_null, enter, show_ptregs, show_mem,
show_state, send_intr, lastcons, caps_toggle,
num, hold, scroll_forw, scroll_back, boot_it,
caps_on, compose, SAK, decr_console,
incr_console, spawn_console, bare_num
 };

The associated actions (and their default key binding) are:

As long as no new releases of init and loadkeys have come out, you can play with this by using loadkeys and starting the program spawn_console:

% loadkeys >> EOF
 alt keycode 103 = 0x0212
 EOF
 % spawn_console &

Of course, if you put this in /etc/rc.local, you would probably want to start getty instead of bash.

do_pad, commonly used for the keypad keys. In keyboard application mode this produces some three-character string ESC O X (with variable X depending on the key), provided that Shift is not pressed simultaneously. Otherwise, when NumLock is on, we get the symbol printed on the key (0123456789.+ -/* and CR).

Finally, if NumLock is not on, the four arrow keys yield ESC [ X (with X=A, B, C, or Dp when not in cursor key mode, and ESC O X otherwise, while the remaining keys are treated as function keys, and yield the associated string. For the middle key (keypad-5) we find four possibilities:

If you think this is unnecessarily complicated, I agree. It is a messy combination of VT100 and DOS keyboard behavior. However, so far suggestions for change have met with too much resistance.

do_dead is used for “dead keys” that provide the following key with a diacritical. By default there are no dead keys. One may define keys producing a dead grave, acute, circumflex, tilde, or diaeresis. How a dead key combines with a following key is specified using the compose mechanism discussed above.

do_cons is used for switching consoles. By default the combinations (Ctrl-)AltL-Fn switch to virtual console n for n in the range 1-12, and AltR-Fn switches to console n+12 for these same n.

do_cur handles cursor keys. One gets either ESC [ X or ESC O X (with X one of A, B, C, or D) depending on the cursor key mode. (One sets or clears cursor key mode by sending ESC [ ? 1 h or ESC [ ? 1 l to the console.)

do_shift maintains the shift state (the up/down state of the modifier keys).

do_meta is commonly used for ordinary keys combined with AltL. If the keyboard is in metamode, this will yield a pair ESC x; otherwise x | 0x80 is produced, where x is the key pressed in both cases. (You can set or clear metamode using the tiny utility setmetamode(1).)

do_ascii is used to construct given codes: press AltL, type a decimal code on the keypad, and release AltL. This yields the character with the given code. In Unicode mode the same works in hexadecimal: press AltR, type a hexadecimal code on the keypad, possibly using the ordinary a, b, c, d, e, and f keys, and release AltR. This yields the Unicode symbol with the code given.

do_lock toggles the state of the corresponding modifier key lock. (Recall the line we saw above: shift_final = shift_state ^ kbd-<lockstate.) Thus, if you have your Cyrillic keys under combinations with AltR, you can use AltR together with other keys to get only a few Cyrillic symbols, but should type AltGr_Lock if you plan to type a longish Cyrillic text. (Note that the right Alt key, that I called AltR here, is usually known as AltGr.)

do_lowercase is used for the handling of CapsLock. Note that CapsLock is different from ShiftLock. With ShiftLock, a digit 4 will be turned into a dollar sign (for default keyboard layout), but CapsLock will only affect lower case letters, and turn them into the corresponding upper case letters. Type 11 is equivalent to type 0, with the added information that the symbol may be affected by CapsLock (and the resulting character is the one that would have resulted from pressing Shift).