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I wanted to remap Ctrl-N/Ctrl-P to Up and Down keys. And I found this post and did something like this:
! add Mode_switch
keycode 66 = NoSymbol NoSymbol
keycode 66 = Mode_switch
keycode 57 = n N Down
keycode 33 = p P Up
And it kinda works. But there's one problem - I actually wanted Caps Lock to be Ctrl and also do that. I can't seem to find a way.
Your remappings of Ctrl+N and Ctrl+P in your last two lines are fine. The problem lies in the attempt to remap the Caps_Lock button (keycode 66 decimal, or 0x42 hex).
Caps Lock is one of the modifier keys, along with Shift and Control (Ctrl). X.org maintains a table of modifier mappings as well as the familiar key mappings. xmodmap -pm will show you your current modifier map. You'll need to update that map as well as the keymap, as I explain below:
The (Caps) Lock modifier, that gives the characteristic behaviour of locking capitals on, won't release the physical key for other purposes unless you explicitly tell it to. Personally, I always used to pry off my Caps Lock key with a screwdriver, it annoyed me so much, so I'm happy to clear it entirely, with clear Lock.
(I've also seen people enable changes to their Lock bindings by surrounding them with
remove MODIFIERNAME = KEYSYMNAME ... and
add MODIFIER = KEYSYMNAME. Check out man xmodmap for details.)
clear Lock
keycode 66 = Control_L
add Control = Control_L
After the Lock modifier has been told to release the Caps_Lock key, you're free to remap that key (keycode 66 in line 2 of the snippet) to one of the Ctrl keys - in this case I've chosen the left one, hence the _L, but it doesn't really matter which. Now the Caps_Lock key "thinks" it's a Ctrl key, but the modifiers table doesn't yet know that we want to use it as a modifier.
The last line in my listing declares that Control_L is a new Control modifier, and should therefore act like a Ctrl key. Look at the before/after comparison of the modifier maps (from xmodmap -pm) below:
Before: the Caps_Lock key belongs to the Lock modifier.
After: the keycode identifying the physical Caps_Lock key (66/0x42) now identifies as an additional left Control key, still with Caps_Lock's original keycode, and has now moved from the Lock modifier to the Control modifier in the modifier map. The original left Control key is still there, too.
