udev Rules

udev is a system of managing the entries in the /dev directory. The entries in /dev directories are special they are device files and not regular files. So if you have a uart to usb dongle plugged into your computer you will have one entry for it in the /dev directory.

How is udev going to help?

Well basically you will have some control over these files. Basically kernel will create them on the fly when you insert a device.

What if you want to do some thing when a particular device is plugged in your system? You could run a script as soon as you insert a usb to uart dongle. Or how about shutting down your pc as soon as some one inserts a pendrive in it!

So what you will have to do is create a udev rule for the device and add it to the system.

Sources for Learning udev

I found the following links useful for learning about the udev rules.

• This article on reactivated.net explains a lot about writing udev rule and you can take it as a starting point on writing your own custom udev rules. Some of the commands used in the article has to be changed as they have been changed after the article was written. For eg. udevinfo written in the tutorial would have to be replaced by udevadm info.
  These commands might not run as such but is gives you a good base. You can refer this link for knowing the recent udev commands.

• This email from Greg KH on lwn is fun to read. Also if you want to know more you can read a paper on udev by the developer who created it.. guess who? Here’s the paper on udev

Use Cases

Having a usb device to mount always at a common mount point, or having a driver loaded automatically when the device is inserted can be a use case for writing your own udev rule.

If you have to work on making a usb device to mount on a particular mount point you can also use the exsisting package USBmount

Also, if you interested in triggering a script based on device insertion or removal event you will have to make sure that the script is small. Because udev would be waiting for your script to complete execution and other events would be missed. This is rude! Also udev would kill your script after a timeout. The best way out of it is to have a background task – a systemd service do all the heavy lifting for you.

Additional Resources

1. Hotplugging using udev from bootlin.