udev is a device manager for the Linux kernel, responsible for managing device nodes in the /dev directory. It operates in user space and provides dynamic creation and removal of device files as hardware is connected or disconnected. This guide explains the purpose of udev, how to use its commands, and provides examples to understand its functionality better.
What is udev?
udev is a part of the systemd system and service manager. It listens to kernel events (such as adding or removing devices) and performs appropriate actions, such as creating device nodes, loading drivers, or running user-defined scripts.
Key Features:
- Dynamic device node management in
/dev. - Triggering actions based on hardware events using rules.
- Supporting persistent device naming for stable identification.
How to Use udev
udev works in conjunction with the following utilities to manage and monitor device events:
udevadm
The main command-line interface for interacting with udev.
Syntax:
udevadm [command] [options]Main Subcommands:
info: Retrieve detailed information about devices.trigger: Simulate udev events for testing rules.monitor: Observe kernel events in real time.control: Manage the state of the udev daemon.
Examples of udev Command Usage
1. Viewing Device Information with info
The udevadm info command retrieves detailed information about a device.
udevadm info --query=all --name=/dev/sdaOutput:
P: /devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/block/sda
N: sda
S: disk/by-id/ata-VBOX_HARDDISK_VB12345678
S: disk/by-path/pci-0000:00:1f.2-ata-1
E: DEVNAME=/dev/sda
E: DEVTYPE=disk
E: ID_BUS=ata
E: ID_MODEL=VBOX_HARDDISK
...
--query=all: Displays all available information.--name=/dev/sda: Specifies the device to query.
2. Monitoring Events in Real Time
The udevadm monitor command allows monitoring of kernel and udev events in real time.
Example:
udevadm monitor --udev --propertyOutput:
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
UDEV [12345.67890] add /devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/block/sdb (block)
ACTION=add
DEVNAME=/dev/sdb
DEVTYPE=disk
...--udev: Displays onlyudevevents.--property: Displays event properties.
3. Testing and Triggering Events with trigger
Use the udevadm trigger command to test udev rules by simulating events.
Example:
udevadm trigger --action=add --subsystem-match=blockThis simulates an “add” action for all devices in the block subsystem, causing udev to process rules for those devices.
4. Managing the udev Daemon with control
The udevadm control command can modify the behavior of the udev daemon.
Reloading Rules:
udevadm control --reload This reloads the udev rules from the configuration directory (/etc/udev/rules.d/).
Stopping Event Handling:
udevadm control --stop-exec-queue This stops the execution of queued events, useful for maintenance.
Writing udev Rules
udev rules are stored in the /etc/udev/rules.d/ directory. Each rule specifies conditions and actions for device events.
Example Rule
Rule File: /etc/udev/rules.d/99-usb.rules
ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="1234", ATTR{idProduct}=="5678", RUN+="/usr/local/bin/custom-script.sh"Explanation:
ACTION=="add": Matches device addition events.SUBSYSTEM=="usb": Targets USB devices.ATTR{idVendor}: Matches the vendor ID of the device.ATTR{idProduct}: Matches the product ID of the device.RUN+="/usr/local/bin/custom-script.sh": Executes a script when the rule matches.
Example: Creating a Persistent Device Name
1. Identify the device information:
udevadm info --query=all --name=/dev/sda 2. Write a rule in /etc/udev/rules.d/10-persistent-disk.rules:
KERNEL=="sda", SUBSYSTEM=="block", SYMLINK+="mydisk"3. Reload the rules:
udevadm control --reload 4. Test the rule:
udevadm trigger --action=add --name=/dev/sdaThe disk /dev/sda will now have a persistent symlink /dev/mydisk.
Best Practices for Using udev
- Test Rules Carefully: Use
udevadm testto ensure rules behave as expected. - Backup Configuration Files: Always create backups before modifying rules.
- Monitor Events: Use
udevadm monitorto debug event handling.
