Macos – Map CapsLock key to ESC in Mac OS X

Aside from the "Modifier Keys" System Preferences section, I don't think an application can override the behaviour of the caps lock key..

I'm fairly sure the Caps Lock key event is not passed to applications, nor can they override the toggle effect.. Maybe you could do something whereby you monitor caps lock key state, and send alt when it changes.. but this is extremely convoluted, and slightly useless (it wouldn't work if you hold the key..)

Do you really need the caps lock key in other applications?

If so, perhaps you could write an AppleScript to toggle the Caps Lock key mapping? For example, in pseudo-code:

loop forever:
    if frontmost application == "Aquaemacs":
        keys.capslock = "Meta"
    else:
        keys.capslock = "Default

Shouldn't be difficult using AppleScript, the code from this Apple support thread should be a good start:

tell application "System Preferences"
    activate
    set current pane to pane "com.apple.preference.keyboard"
end tell

tell application "System Events"

    tell application process "System Preferences"
        get properties

        click button "Modifier Keys..." of tab group 1 of window "Keyboard & Mouse"
        tell sheet 1 of window "Keyboard & Mouse"
            click pop up button 1
            click menu item "No Action" of menu 1 of pop up button 1
            delay 3
            click button "OK"
        end tell
    end tell
end tell

Use the following: Send Hex Codes: 0x1B 0x05

• Escape is ASCII 27 (it's often written as octal \033 if your shell doesn't know \e).

• Ctrl-[Character] combinations result in the ASCII code corresponding to the character's position in the alphabet. Well-known examples are^H for ASCII 8 (Backspace), ^C for ASCII 3, End of Text, or ^D for ASCII 4, End of Transmission, and likewise, pressing ^E results in ASCII 5 — so that hex code needs to be sent instead.

You can even use this (I cannot imagine a real use case, but let's not care about that) to replace pressing Enter (Line Feed) by ^J, or pressing Tab by ^I. See here for more information about control characters.