How does udevd work

Viewed 6k times. That is basically my current understanding please correct me if im wrong and tell me extra info. Improve this question. What you understood about udev is correct I answered a summarized explanation about udev and many links to different posts — intika. Add a comment. Active Oldest Votes. The udev, as a whole, is divided into three parts: Library libudev that allows access to device information. Administrative command-line utility udevadm for diagnostics.

Improve this answer. Cbhihe it's from the linked Wikipedia page Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. When udev receives a device event, it matches its configured set of rules against various device attributes to identify the device.

Rules that match may provide additional device information to be stored in the udev database or to be used to create meaningful symlink names. All device information udev processes is stored in the udev database and sent out to possible event subscribers. Access to all stored data and the event sources is provided by the library libudev. All rules files are collectively sorted and processed in lexical order, regardless of the directories in which they live.

However, files with identical filenames replace each other. Rule files must have the extension. Every line in the rules file contains at least one key-value pair. Except for empty lines or lines beginning with " ", which are ignored. There are two kinds of keys: match and assignment. If all match keys match against their values, the rule gets applied and the assignment keys get the specified values assigned.

A matching rule may rename a network interface, add symlinks pointing to the device node, or run a specified program as part of the event handling. A rule consists of a comma-separated list of one or more key-operator-value expressions.

Each expression has a distinct effect, depending on the key and operator used. Compare for inequality. The specified key doesn't have the specified value, or the specified key is not present at all.

Assign a value to a key. Keys that represent a list are reset and only this single value is assigned. Values are written as double quoted strings, such as "string". Any other occurrences of a backslash followed by a character are not unescaped.

This can be useful for writing special characters when a kernel driver requires them. The following key names can be used to match against device properties.

Some of the keys also match against properties of the parent devices in sysfs, not only the device that has generated the event. If multiple keys that match a parent device are specified in a single rule, all these keys must match at one and the same parent device.

Match the name of a network interface. It can be used once the NAME key has been set in one of the preceding rules. Match the name of a symlink targeting the node. There may be multiple symlinks; only one needs to match. Match the driver name of the event device. Only set this key for devices which are bound to a driver at the time the event is generated. Trailing whitespace in the attribute values is ignored unless the specified match value itself contains trailing whitespace.

Search the devpath upwards for a device with matching sysfs attribute values. System's architecture. System's virtualization environment.

See systemd-detect-virt 1 for possible values. Execute a program to determine whether there is a match; the key is true if the program returns successfully. The device properties are made available to the executed program in the environment. This can only be used for very short-running foreground tasks.

For details, see RUN. Most of the fields support shell glob pattern matching and alternate patterns. This creates a symlink in the dev tree, so it does not interfere with the normal process of plugging in a device. This means that if you use a desktop environment that likes to auto-mount devices, you won't be causing problems for it. Once safely mounted, it uses rsync to back up the drive to my backup folder my actual script uses rdiff-backup, and yours can use whatever automated backup solution you prefer.

Udev is a very flexible system and enables you to define rules and functions in ways that few other systems dare provide users. Thanks for the heads-up, Lars. During writing, that test script got carved down from something that did more than just return a date. Great article. What I wanted to know, though, is the current relation between Udev and Systemd's. Are they a replacement to Udev's rules? Great question, Osqui.

However, udev is still important for two reasons: firstly, not all Linuxen ship with systemd, and secondly because there are some things not visible to Systemd that are visible to udev, so you would have to use udev tags to prompt Systemd to act. Given these two factors, I'm not clear on whether Systemd intends to eventually replace udev or just provide an additional means of achieving many of the same results. I don't always love udev I'm still not convinced it's actually able to reload its rules without a reboot but I'd hate to lose it entirely, since my Slackware systems don't run Systemd.

I use to tinker with arduinos and other usb programmed boards. Some of them need to be unplugged and plugged again in order to enter in 'flashing mode'. Then my IDE does not find the board I need to reconfigure it. I think it is something related to timeouts and reuse of old links, but I am not sure about how to fixit.

What I'd try not sure if it'll work because I haven't actually tested it is to create a udev rule that recognises arduinos by some fairly generic attribute. Create a rule such that when one is plugged in, it's assigned to some standard location of your choosing. Something like An introduction to Udev: The Linux subsystem for managing device events An introduction to Udev: The Linux subsystem for managing device events.

Create a script that triggers your computer to do a specific action when a specific device is plugged in. Image by :. Get the highlights in your inbox every week. A basic script The best way to work with udev is in small chunks. Open your favorite text editor and enter this simple script:! Topics Linux. About the author. He has worked in the film and computing industry, often at the same time. He is one of the maintainers of the Slackware-based multimedia production project Slackermedia.

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