In the process of upgraded my office desktop to Fedora 40, I wound up needing to turn off the X server's CapsLock modifier. For people with a normal keyboard setup, this is simple; to turn off the CapsLock modifier, you tap the CapsLock key. However, I turn CapsLock into another Ctrl key (and then I make heavy use of tapping CapsLock to start dmenu (also)), which leaves the regular CapsLock functionality unavailable to me under normal circumstances. Since I don't have a CapsLock key, you might wonder how the CapsLock modifier got turned on in the first place.

The answer is that sometimes I have a CapsLock key after all. I turn CapsLock into Ctrl with setxkbmap settings, and apparently some Fedora packages clears these keyboard mapping settings when they're updated. Since upgrading to a new Fedora release updates all of these packages, my 'Ctrl' key resets to CapsLock during the process and I don't necessarily notice immediately. Because I expect my input settings to get cleared, I have a script to re-establish them, which I run when I notice my special Ctrl key handling isn't working. What happened this time around was that I noticed that my keyboard settings had been cleared when CapsLock didn't work as Ctrl, then reflexively invoked the script. Of course at this point I had tapped CapsLock, which turned on the CapsLock modifier, and then when the script reset CapsLock to be Ctrl, I no longer had a key that I could use to turn CapsLock off.

(Actually dealing with this situation was made more complicated by how I could now only type upper case letters in shells, browser windows, and so on. Fortunately I had a phone to do Internet searches on, and I could switch to another Linux virtual console, which had CapsLock off, and access the X server with 'export DISPLAY=:0' so I could run commands that talked to it.)

There are two solutions I wound up with, the narrow one and the general one. The narrow solution is to use xdotool to artificially send a CapsLock key down/up event with this:

xdotool key Caps_Lock

This will toggle the state of the CapsLock modifier in the X server, which will turn CapsLock off if it's currently on, as it was for me. This key down/up event works even if you have the CapsLock key remapped at the time, as I did, and you can run it from another virtual console with 'DISPLAY=:0 xdotool key Caps_Lock' (although you may need to vary the :0 bit). Or you can put it in a script called 'RESET-CAPSLOCK' so you can type its name with CapsLock active.

(Possibly I should give my 'reset-input' script an all-caps alias. It's also accessible from a window manager menu, but modifiers can make those inaccessible too.)

However, I'd like something to clear the CapsLock modifier that I can put in my 're-establish my keyboard settings' script, and since this xdotool trick only toggles the setting it's not suitable. Fortunately you can clear modifier states from an X client; unfortunately, as far as I know there's no canned 'capslockx' program the way there is a numlockx (which people have and use for good reasons). Fortunately, the same AskUbuntu question and answer that I got the xdotool invocation from also had a working Python program (you want the one from this answer by diegogs. For assorted reasons, I'm putting my current version of that Python program here:

#!/usr/bin/python
from ctypes import *

class Display(Structure):
  """ opaque struct """

X11 = cdll.LoadLibrary("libX11.so.6")
X11.XOpenDisplay.restype = POINTER(Display)

display = X11.XOpenDisplay(c_int(0))
X11.XkbLockModifiers(display, c_uint(0x0100), c_uint(2), c_uint(0))
X11.XCloseDisplay(display)

(There is also a C version in the question and answers, but you obviously have to compile it.)

In theory there is probably some way to reset the setxkbmap settings state so that CapsLock is a CapsLock key again (after all, package updates do it), which would have let me directly turn off CapsLock. In practice I couldn't find out how to do this in my flailing Internet searches so I went with the answer I could find. In retrospect I could might also have been able to reset settings by unplugging and replugging my USB keyboard or plugging in a second keyboard, and we do have random USB keyboards sitting around in the office.