Udevadm Commands
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Video Transcription
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>> Hey, Cybrarian, welcome back
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>> to the Linux plus course here at Cybrary.
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>> I'm your instructor, Rob Goelz.
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In today's lesson, we're going to be
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covering the udevadm command.
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Upon completion of this lesson,
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you are going to be able to understand this command,
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the udevadm command,
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you'll be able to explain when we
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might want to use this command,
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and recall the common options for udevadm.
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The udevadm utility is used to manage or gather
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device information and it allows
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you to send commands to udev.
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What are some examples of
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when you might want to use this command?
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Well, you might want to use udevadm
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to obtain information about a device.
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Maybe you want to uevents that are
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coming into the kernel or from the kernel.
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You might want to control the
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udevd daemon itself, udevd.
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Or you might want to test changes that you're making to
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rules before you reload them
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>> and put them on the system.
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>> Or once that's all good,
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you might want to go ahead
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>> and apply those rule changes.
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>> Those are all things you can do with udevadm.
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Here are some actual examples.
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If you're trying to get information
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about a device from the udev database,
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you could use udevadm info -q for query,
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and then you can specify a query type.
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Name, symlink, path, property, or all.
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If you want to perform real-time monitoring of
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the udev events or uevents that come in.
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you could do a udevadm monitor.
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If you want to test a change to
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a rule before you reload the role in the system,
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you can run udevadm test
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and then you specify the action,
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so --add for example,
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in the device file that you want to test.
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If you want to reload those rules
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once you verify that that
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change is good and basically is control udevd,
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you could do a udevadm control --reload,
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and that will reload those rules it's the udevd daemon.
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Then to verify the rule
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after the device has been created,
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you can run udevadm trigger.
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Let's take a look at some of these inaction
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with some demo time.
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Here, we are back in our Bundoo environment.
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Let's play with udevadm command.
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The first thing we could do is
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display some device options.
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We can do that with udevadm info -n, n is for name.
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We'll just pick on dev sda,
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which is one of our storage devices.
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Then we can see a bunch of
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good information about its path,
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and its name, and so on, and so forth.
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If we wanted to get
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properties related to this device only,
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we could run udevadm info q
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for query and we're going to query on
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the property of this device.
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In property, we're going to use the name,
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dev sda is the device we want to get information on.
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Then hit Enter, and there we go.
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We have information property
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just specific to this device.
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Now if we want to perform real-time monitoring of udev,
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let's clear our screen by hitting
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Control L or typing clear.
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To do this, we can run the command udevadm monitor
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and this will just print
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any received events for udevadm,
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the uevents that come in or
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any kernel uevents that are being sent.
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Nothing really going on in
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the system so we're not going to see anything here,
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but that's how we can monitor them in real-time,
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the equivalent of a tail f but for udevadm.
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Then if we had a rule change, we could test it.
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Now I don't really actually have
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any rural changes to test,
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but I have an existing device we can play around with.
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What I'll do is I'll do a udevadm test --action
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and our action in this case is going to be add.
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We'll mess with a character device dev char 890.
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If we liked the way
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this worked, if we were happy with it,
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if we're satisfied with it,
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we can reload the udevadm rules.
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We can say pseudo,
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udevadm control --reload,
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and that will reload the
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udevadm rules and pick up the changes.
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But with that, we reached the end of the lesson.
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In this lesson, we covered
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the purpose of the udevadm command.
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We talked about when you might want to use
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the udevadm command and some of
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the common udevadm options.
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Thanks so much for being here and I look
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forward to seeing you in the next lesson.
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