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- An important task of the kernel

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is to make sure that

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you can use your hardware.

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So how exactly does that work?

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Well, the kernel itself is small

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and it contains essential drivers only

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and additional drivers
are provided dynamically.

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And there's many ways
in which that happens.

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To start it, there is InitramFS

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the initial RAM file system,

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which is loaded while booting.

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Also there is systemd-udevd

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which takes care of loading drivers

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automatically and on demand.

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To access devices,

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device files in the
/dev directory are used.

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The kernel doesn't know
about device file names.

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It uses the device fellow major and minor

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to address the devices.

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Let me show you.

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So if I use ls - l on /dev/sda*

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/dev/sda* is obviously your hard drive.

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Then what I want you to be aware of

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is this 8,0 , this 8, 0.

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That's your major and your minor.

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And that is what the kernel

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really all knows about.

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Now in /proc there is this file

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with the name /proc/devices

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and in /proc/devices

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we have the block devices

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and what do we see?

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Block device 8 is sd.

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What does that mean?

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Well that means that

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if the block device major "8" is addressed

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then the kernel is
going to contact the sd,

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which is SCSI disk module

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and that is how these major minors

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allow the kernel to allocate

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the right, the right modules.

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You can also see that

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in a different way

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using lsblk, for instance.

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I like this overview because

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in lsblk you see not only
the major and the minor

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but on the left hand the device name

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and on the right hand,

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if the device is mounted,

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you can even see the mount point.

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And that's how major
and minor are are used.

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Now let's talk about hardware
initialization a bit.

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So to access hardware, the
Linux kernel uses drivers

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and these drivers are the kernel modules.

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Drivers can be provided
through the InitramFS

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or loaded dynamically
through systemd-udevd.

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And device valves in /dev provide

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user interface to the devices.

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Device files are mostly auto-generated

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and the device files correspond

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to the device nodes

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which are a major and a minor number

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that the kernel uses to identify devices.

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You can even create them yourselves

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using the mknod command.

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That's something that
was useful in the past.

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Nowadays they should never
be necessary anymore.

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Also involved in hardware management

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is a sysfs file system.

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So /sys is used to provide information

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about devices and their attributes.

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And /sys/devices is dedicated

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to device information.

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An easy interface to
query device information

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from /sys is udevadm.

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Use, for instance,

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udevadm info --query=all --name=/dev/sda

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and you will find all
the information stored

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in the sysfs file system
about a /dev/sda device.

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Now before I'm going to show you

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I want you to understand
systemd-udevd a bit.

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The Linux kernel initiates device loading

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and next sends out uevents

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to the udevd user space daemon.

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Well, the systemd-udevd user space daemon

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is what we should say

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because udevd is the
old name of the process.

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Nowadays it's integrated in systemd

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and the name of the process
really is systemd-udevd.

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systemd-uddevd catches the event

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and decides how to handle it

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based on attributes

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that it has received in the event.

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And it next reads its rules

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and acts based on these rules.

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It has default rules in
/usr/lib/udev/rules.d

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and custom rules can be stored

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in /etc/udev/rules.d.

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Now let me demonstrate
how to use udev rules.

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You can find the steps in this demo

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on the slide, just for your convenience.

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Let me show you.

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So to start with, I need udevamd monitor

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which is showing all the udev events

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that are going to happen.

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And now I'm plugging
in the USB thumb drive.

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So as you can see

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the USB thumb drive is now initialized

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and udevadm monitor is showing

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exactly what is happening.

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So it's a step-by-step procedure

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where we can see that the kernel

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has detected an event

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and then has pass the event through udev

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and based on that, the kernel can continue

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and down the road, everything is loaded

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that is, that is required.

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So here for instance you can see SCSI disc

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that is involved SCSI generic,

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that's about block devices.

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Then we can see that the
sdb block device is created

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and all of these are file names,

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file names that are relative
to the /sys file system.

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Now the big question is,

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"What exactly is going on

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when stuff like this is happening?"

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Well, that is determined
by the udev rules.

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So let's have a look at

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/lib/udev/rules.d

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/lib/udev/rules.d

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where we have multiple rules.

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We don't wanna see all 122.

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So let me just go for 60.

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And in 60 we have the persistent

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storage.rules

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and there we can see how
such a rule is composed.

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So it's, it's quite complex.

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Here we see if the kernel detects

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one of these loop devices,

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then it's going to take action.

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Now I don't want to go

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through all of the lines
in this example file.

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I want to create our own.

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Now how are we going to do that?

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Well, we need to work

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with the device that was just added.

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So which device is that?

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Well ls -l in /dev/disk

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is showing the different device names.

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lsblk is showing the
different device names

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from a different perspective.

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And what we need is sdb.

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In a udev rule

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you are going to use device attributes

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and to find these device attributes,

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udevadm info --query=all --name=dev/sdb

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udevadm info --query=all --name=dev/sdb

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It's showing all the information

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on this USB thumb drive.

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Now this is device-specific

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and you can use this
device-specific information

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in the udev rules.

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There's another way to display

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more or less the same

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information

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and

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that is

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- -attribute-walk.

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- -attribute-walk.

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That is giving detailed information

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about everything that was found

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on this USB thumb drive.

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And this information is the information

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that you can use

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in your

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udev rules.

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So that is what I wanna do now.

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I want to create a custom rule.

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So to make it easier

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to create a custom rule,

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I'm providing it as an example file

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in the course get repository.

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The name of the file's 50-custom.rules

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And in this custom rule
we define an action.

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So the action is "add"
on the subsystem "block"

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and the driver involved is USB-storage

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And if that is going to happen,

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then we are going to

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create a symbolic link.

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This symbolic link is created

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in the /dev directory.

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So it creates /dev/usb/

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We will see.

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And this is the power of udev.

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It allows you to run any arbitrary command

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and in this case the arbitrary command is

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/usr/bin/logger CUSTOMUDEV Added %k

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which is printing the message to udev.

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Now to have this work,

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I need to make sure that
we copy the custom rule

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to the appropriate location.

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And here we are going to
respect the convention

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that custom stuff is
in the /etc directory.

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/usr/lib is for the default stuff,

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custom files are in the /etc directory,

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in this case in /etc/udev/rules.d

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So, there we go.

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Just to make sure it's picked up,

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systemctl restart

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systemd-udevd

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systemd-udevd

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and now I'm going to

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disconnect the USB thumb drive

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and I'm going to reconnect it.

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So how can we verify?

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Well journalctl | grep CUSTOMUDEV.

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And there we can see, yay,

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CUSTOMUDEV Added sdb.

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That's a command

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that we have defined

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in the custom rule file.

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And that is how you can create

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your own udev rules.