How do you make the fn keyboard button work like a Ctrl button?

Question

I'm used to having the Control button at the bottom left corner of the keyboard. It's a little annoying to be pressing Fn + Tab when I wanted a Control + Tab and the position of the Control button in the new Mac keyboard layout is just very uncomfortable for any finger to reach.

Does anybody know how you can make the Control and Fn buttons change function like how you can change the modifier keys in System Preferences?

Answer 1

1. Download and install Karabiner

2. Open Karabiner Elements and set the Target Device in the dropdown

3. Map fn to left_control and left_control to fn as below

   

Answer 2

Not sure which OS this started appearing in but in macOS 10.13, this can be customized in System Preferences > Keyboard > Modifier Keys:

Answer 3

This is my white whale. I can't use Emacs without the control key being on the left hand side. Recently I got a new MacBook so I had a chance to do this without Karabiner.

This is what worked for me to rebind the function key to control (for the built-in keyboard only), on macOS 10.15.6.

hidutil property --matching '{ "ProductID": 0x027e }' --set '{
  "UserKeyMapping": [
    {
      "HIDKeyboardModifierMappingDst": 30064771300,
      "HIDKeyboardModifierMappingSrc": 1095216660483
    },
    {
      "HIDKeyboardModifierMappingDst": 30064771296,
      "HIDKeyboardModifierMappingSrc": 280379760050179
    }
  ]
}'

Verify it with hidutil property --matching '{ "ProductID": 0x027e }' --get "UserKeyMapping".

How I found these keycodes

I've both seen the option to change the Function (fn) Key and not seen the option to change the Function (fn) Key on the same computer, using the same version of macOS. Take it with a grain of salt, but I can consistently get the variant with the Function (fn) Key option to appear if I:

1. Have an external keyboard attached, via a USB-Type C hub
2. Open the Keyboard → Keyboard → Modifier Keys
3. Lock the screen using the external keyboard
4. Walk away for some time (wait until the computer sleeps)
5. Wake the computer by pressing a key on the external keyboard and use Touch ID to log in
6. Now the Modifier Keys preference has the Function (fn) Key option. While in this state, change the behaviour of the function key using the drop down list (I chose ^ Control)
7. Copy ~/Library/Preferences/ByHost/.GlobalPreferences.${__UUID__}.plist (described here) somewhere else temporarily
8. Convert the binary PList to XML: plutil -convert xml1 .GlobalPreferences.${__UUID__}.plist

Contents of that file are:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>com.apple.keyboard.modifiermapping.1452-638-0</key>
    <array>
        <dict>
            <key>HIDKeyboardModifierMappingDst</key>
            <integer>30064771300</integer>
            <key>HIDKeyboardModifierMappingSrc</key>
            <integer>1095216660483</integer>
        </dict>
        <dict>
            <key>HIDKeyboardModifierMappingDst</key>
            <real>30064771296</real>
            <key>HIDKeyboardModifierMappingSrc</key>
            <integer>280379760050179</integer>
        </dict>
    </array>
</dict>
</plist>

This page shows up if you search for these HIDKeyboardModifierMappingSrc and says that 1095216660483 is for the left function key, 280379760050179 is for the right.

I thought I was going crazy so I took some screenshots for proof:

Modifier Keys preference pane without the Function (fn) Key option

Modifier Keys preference pane with the Function (fn) Key option

About ProductID

You can find the Product ID associated with your keyboard using the System Report button: Apple menu → System Report → Hardware → USB.

If you use the --matching parameter to hidutil using a specific Product ID, the changes will affect that piece of hardware only. For example, maybe you want to change the layout of the built-in keyboard, but not an external one.

Answer 4

you can use KeyRemap4MacBook

http://pqrs.org/macosx/keyremap4macbook/

you can find the included prepared settings list at this address:

http://pqrs.org/macosx/keyremap4macbook/list.html.en

as you can see there is the setting for Fn Key

Answer 5

I was happy, but then not as much. I just bought a Logitech K850 keyboard. Has a nice shape to help with wrist issues (for another time). Has keys from left as:

ctrl    fn    alt opt    cmd <butterfly>

Tried Karabiner but it didn't seem to work (tested by using ctrl-F in vi/vim). Then I found this document https://karabiner-elements.pqrs.org/docs/getting-started/features which lists a limitation.

Karabiner-Elements cannot modify the fn key on non Apple keyboards such as Logitech keyboards.

Not for Logitech. Ouch. Other suggestions? Back to best buy again? I'm really just looking for a mac layout keyboard with full size keys. I don't care about the num pad, etc. on right side. The K850 looked good except for this issue. Some gentle ergonomics help is good.

Answer 6

I'm a new Mac user so I don't know much about how this was solved before. But this seems to be fixed in MacOs Monterey 12.5 (MacBook Pro 16'' 2021). No third application required :)

Answer 7

You could also add this to private.xml:

<autogen>__KeyToKey__ KeyCode::FN, KeyCode::CONTROL_L</autogen>
<autogen>__KeyToKey__ KeyCode::CONTROL_L, KeyCode::FN</autogen>

See the source for the key code values and predefined settings.

Answer 8

In addition to trackpad gestures, Better Touch Tool allows you to remap individual keyboard combinations. I use it to remap fn ⇥ to ⌃ ⇥ and fn ⇧ ⇥ to ⌃ ⇧ ⇥.

Answer 9

Looks like the one way to make Karabiner work with Logitech keyboards is to uninstall Logi Options, but then:

• the keyboard will behave like standard Apple keyboard - the mapping of function keys will differ from pictograms (I guess these can be remapped with Karabiner)
• you will need to find another way to configure mouse buttons 3-5.

Answer 10

The hidutil-key-remapping-generator worked perfectly for me. You can access the tool online from hidutil key remapping generator for MacOS and copy the configuration to

~/Library/LaunchAgents/com.local.KeyRemapping.plist