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Creating a USB Symlink in /dev using /etc/udev/rules

Ubuntu 5

In this article, we will delve into the process of creating a USB symlink in the /dev directory using the /etc/udev/rules.d configuration file. This is a powerful tool for managing device files in Linux, giving you the ability to create custom device names, or symlinks, that persist across reboots.

Quick Answer

To create a USB symlink in the /dev directory using /etc/udev/rules, you need to create a udev rule in the /etc/udev/rules.d directory. The rule should specify the SUBSYSTEM=="usb" and SYMLINK+="myusb" attributes. After creating the rule, restart the udev service for the changes to take effect. Verify the symlink by listing the contents of the /dev directory.

Understanding the Basics

Before we dive into the process, it’s important to understand some basics. The /dev directory in Linux is a special directory that contains device files for all the hardware devices on a system. A symlink, or symbolic link, is a file that points to another file or directory.

udev is a device manager for the Linux kernel. It manages device nodes in the /dev directory. When hardware devices are added or removed from the system, udev is responsible for adding or removing the corresponding device nodes.

The /etc/udev/rules.d directory contains configuration files for udev rules. These rules specify actions to be taken by udev whenever a particular event occurs, such as a USB device being plugged in.

Creating a Udev Rule

To create a symlink for a USB device, we need to create a udev rule. This rule will tell udev to create a symlink whenever the USB device is plugged in.

First, create a new file in the /etc/udev/rules.d directory. The file name should end with .rules. For example:

sudo nano /etc/udev/rules.d/75-myusb.rules

In this command, sudo is used to run the command with root privileges. nano is a command-line text editor, and /etc/udev/rules.d/75-myusb.rules is the path to the new rules file. The 75- prefix determines the order in which the rule files are processed, with lower numbers being processed first.

In the new file, add the following line:

SUBSYSTEM=="usb", SYMLINK+="myusb"

This rule tells udev to create a symlink named myusb whenever a USB device is plugged in. The SUBSYSTEM=="usb" part of the rule matches any event on the USB subsystem, and the SYMLINK+="myusb" part of the rule tells udev to add a symlink.

Restarting Udev

After creating the new rule, you need to restart the udev service for the changes to take effect. You can do this by running the following command:

sudo /etc/init.d/udev restart

In this command, sudo is used to run the command with root privileges, /etc/init.d/udev is the path to the udev service script, and restart is the command to restart the service.

Verifying the Symlink

To check if the symlink was created, you can use the ls command to list the contents of the /dev directory and search for the symlink. For example:

ls -lF /dev | grep myusb

In this command, ls -lF /dev lists the contents of the /dev directory in long format, and grep myusb searches the output for myusb.

If the symlink was created successfully, it should appear in the output of this command.

Troubleshooting

If the symlink is not appearing, there could be a few reasons. First, make sure the udev rule file is correctly created and contains the correct rule. You can use a text editor to check the file.

If the symlink is still not appearing, you can try using a different naming convention for the udev rule file. Instead of using 75-myusb.rules, you can try using a higher number prefix like 90-myusb.rules. This ensures that your rule is processed after any other rules.

Additionally, the SUBSYSTEM=="usb" rule is quite generic and may match all USB devices. To make the rule more specific, you can include additional attributes like ATTRS{idVendor} and ATTRS{idProduct} to match a specific USB device. You can find these values by running the lsusb command and looking for your specific device.

Conclusion

Creating a USB symlink in /dev using /etc/udev/rules.d is a powerful way to manage device files in Linux. It gives you the ability to create custom device names, or symlinks, that persist across reboots. With a good understanding of the basics and a few simple commands, you can start creating your own udev rules and symlinks.

What is a symlink?

A symlink, or symbolic link, is a file that acts as a pointer to another file or directory. It allows you to create a custom name for a file or directory that points to the original file or directory.

What is the purpose of creating a USB symlink?

Creating a USB symlink allows you to define a custom name for a USB device in the /dev directory. This can be useful for easily identifying and accessing specific USB devices, especially when multiple devices are connected.

How do I create a udev rule for a USB symlink?

To create a udev rule for a USB symlink, you need to create a new file in the /etc/udev/rules.d directory with a .rules extension. In that file, specify the rule using the SUBSYSTEM=="usb" condition and the SYMLINK+="myusb" action, where myusb is the desired name for the symlink.

How do I restart the udev service?

To restart the udev service, you can use the command sudo /etc/init.d/udev restart. This command will restart the udev service and apply any changes made to the udev rules.

How can I verify if the symlink was created successfully?

You can use the ls -lF /dev | grep myusb command to check if the symlink was created successfully. This command lists the contents of the /dev directory in long format and searches for the symlink named myusb. If the symlink was created, it should appear in the output of this command.

What should I do if the symlink is not appearing?

If the symlink is not appearing, first ensure that the udev rule file is correctly created and contains the correct rule. Double-check the file using a text editor. If the symlink still does not appear, you can try using a different naming convention for the udev rule file or make the rule more specific by including additional attributes like ATTRS{idVendor} and ATTRS{idProduct} to match a specific USB device.

Can I create multiple USB symlinks using different udev rules?

Yes, you can create multiple USB symlinks using different udev rules. Each rule should have a unique name and specify the desired symlink name using the SYMLINK+="..." action. This allows you to create custom symlinks for different USB devices or device attributes.

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