udev added

README.md

@@ -12,6 +12,7 @@
     * [Archlinux](./linux/arch/README.md)
         * [Installation instructions for UEFI + GPT](./linux/arch/installation.md)
 	* [Shell commands](./linux/shell_commands.md)
+    * [udev](./linux/udev.md)
 * [MySQL](./MySQL/README.md)
 * [ODroid](./odroid.md)
 * [OpenSSL](./OpenSSL.md)

linux/udev.md

@@ -0,0 +1,20 @@
+# udev
+
+## Start (and stop) a systemd daemon
+
+Create a udev rule to start the daemon:
+```
+% cat /etc/udev/rules.d/10-bluetooth-service.rules 
+ACTION=="add", SUBSYSTEM=="usb", ATTRS{idVendor}=="0a5c", ATTRS{idProduct}=="21e8", TAG+="systemd", ENV{SYSTEMD_WANTS}="bluetooth.service"
+```
+This one starts the bluetooth systemd service when a specific usb bluetooth dongle is added to the system.
+
+If you need to find `idVendor` and `idProduct` you can start `udevadm monitor --property` while plugging the usb device in and out.
+
+To stop the daemon again you need to edit the systemd service file of the daemon - in this case `systemctl edit bluetooth.service`:
+```
+[Unit]
+BindsTo=sys-subsystem-bluetooth-devices-hci0.device
+After=sys-subsystem-bluetooth-devices-hci0.device
+```
+The device name in this case is found by `systemctl list-units --all --full | grep "\.device"`.