how to make sysfs changes persistent in centos 7 (systemd)

Trying to fix up the fn keys on my apple keyboard on CentOS 7, I’ve set

$ cat /etc/modprobe.d/hid_apple.conf
options hid_apple fnmode=2

and yet after a reboot

$ cat /sys/module/hid_apple/parameters/fnmode
1

Suggestions on the internet include running update-initramfs, which doesn’t seem to exist on Centos 7, and doing the “echo 2 >> /sys/module/hid_apple/parameters/fnmode” in /etc/rc.local, which of course doesn’t exist at all any more under systemd.

What’s the right way to persist that setting?

Answer

There are 3 ways in which you can achieve this:

1. rc.local (Still works, remember to chmod +x after adding your lines)
2. systemd
3. udev rules (My own preferred)

With systemd:

# /etc/systemd/system/hid_apple_fnmode_set.service 
[Unit]
Description=Set Apple keyboard fn mode
After=multi-user.target

[Service]
ExecStart=/usr/bin/bash -c '/usr/bin/echo 2 > /sys/module/hid_apple/parameters/fnmode'

[Install]
WantedBy=graphical.target

Followed by this to make the service run at boot.

sudo systemctl enable hid_apple_fnmode_set.service

With udev rules:

# /etc/udev/rules.d/99-hid_apple.rules
SUBSYSTEM=="module", DRIVER=="hid_apple", ATTR{parameters/fnmode}="2"

The systemd script and udev rules are put together with some wild guesses, might take some tweaking to work. The following commands can help adjust and debug the udev rule:

udevadm info --attribute-walk --path=/module/hid_apple

udevadm test /sys/module/hid_apple/