What Is the /run/user/$UID Directory?

1. Overview

There are different directories in Linux pseudo-filesystems that serve specific purposes. The /run/user/$UID path is one such directory that’s helpful in managing user-specific runtime data.

In this tutorial, we’ll look into the details of the /run/user/$UID directory. We’ll explore its purpose, its relationship to the /run path, and how it works in multi-user environments. Additionally, we’ll discuss the common files and subdirectories found within /run/user/$UID and the permissions associated with the structure.

2. The /run Directory

Before we look into the specifics, let’s understand the /run directory itself. Basically, the /run directory is a special filesystem in Linux used for storing temporary data that doesn’t need to persist across system reboots.

2.1. Purpose and History

Historically, temporary runtime data was scattered across different directories such as /var/run and /tmp. The introduction of the /run directory aimed to provide a standard location for storing such data. This makes it easier to manage and maintain the system.

2.2. Characteristics

The /run directory is distinguishable from other directories in the filesystem hierarchy.

To begin with, it’s mounted as a temporary filesystem (tmpfs) and resides in the system’s RAM. What this means is that the content of /run isn’t a permanent part of the secondary storage but rather exists only in volatile memory.

Consequently, any files or directories created within /run are automatically cleared when the system is rebooted.

Further, to prevent excessive consumption of RAM, the /run directory has a limited size. However, the exact size limit may vary depending on the system configuration but it’s typically a small portion of the total available memory.

2.3. Relation to /var/run Directory

The /run directory serves as a replacement for the older /var/run directory. Many paths that previously utilized /var/run now use /run instead. Therefore, to maintain compatibility with older software, /var/run is often symbolically linked to /run. As a result, this ensures that applications expecting to find temporary runtime data in /var/run can still function correctly.

3. The /run/user Directory

Within the /run directory, there’s a subdirectory called user. The /run/user path is dedicated to storing user-specific runtime data. Specifically, it contains subdirectories named after each user’s numerical user ID (UID).

3.1. Location and Naming Convention

The /run/user directory follows a specific naming convention. Notably, the UID is a non-negative integer value that the system uses to associate files, processes, and other resources with a certain user. For instance, if a user has a UID of 1000, their corresponding directory within /run/user would be /run/user/1000.

Thus, this naming convention lets us more easily identify and organize the user-specific runtime data.

3.2. Purpose

The primary purpose of the /run/user/$UID directories is to provide a dedicated location for processes running under a specific user’s context to store temporary files, sockets, and other runtime data.

Further, each user’s processes can access and modify files within their respective /run/user/$UID directory without interfering with the runtime data of other users.  This separation prevents conflicts that may arise when multiple users attempt to access or modify the same files.

4. Inside a /run/user/$UID Directory

Now that we understand the structure of the main /run/user directory, let’s explore what commonly resides within its dynamically-named subdirectories.

4.1. Common Files and Subdirectories

The contents of a /run/user/$UID directory can vary depending on the applications and services the user is engaging with. However, there are some common types of files and subdirectories that we can frequently encounter:

• Sockets: Many applications use sockets for inter-process communication (IPC). These sockets are often stored within the /run/user/$UID directory to ensure that they are accessible only to the user running the application.
• Temporary files: Applications may create temporary files within the /run/user/$UID directory to store intermediate data or cache information. These files are typically short-lived and are automatically cleaned up when the application exits or the user session ends.
• Runtime configuration files: Some applications may save runtime configuration files or settings within this directory. These files are specific to the user’s instance of the application and customize its behavior or store session-specific data.

Overall, the /run/user/$UID directory serves as the location for various types of user-specific runtime files.

4.2. Examples of Applications That Use It

Let’s see a few examples of user-level applications and desktop environments that use the /run/user/$UID directory for storing runtime data:

• Pulseaudio: Pulseaudio is a popular sound server that many Linux distributions use. It stores its socket file within the /run/user/$UID directory, thereby, enabling the user’s audio applications to communicate with the sound server.
• Systemd user instances: Systemd is the system and service manager most modern Linux distributions use. It creates a systems subdirectory within the /run/user/$UID directory. This subdirectory contains runtime data and sockets specific to the user’s systemd instance.
• Desktop environments: Desktop environments like GNOME and KDE utilize the /run/user/$UID directory to store session-specific data. This can include information about the user’s session, running applications, and other runtime settings.

These are just a few examples and many other applications and services may also make use of the /run/user/$UID directory for storing their runtime data.

4.3. Lifecycle of Files In /run/user/$UID

The lifecycle of files within the /run/user/$UID directory relates to the user session and the specific applications that create them.

When an application starts and requires temporary runtime storage, it may create files or directories within the /run/user/$UID directory. These files persist for the duration of the application’s execution or until the user session ends.

Upon termination of the application, it’s responsible for cleaning up its own files within the /run/user/$UID directory. If an application fails to clean up its files properly, they may remain in the directory until the user session ends.

When the user logs out or the session is terminated, the entire /run/user/$UID directory and its contents are typically removed. Thus, this ensures that the temporary runtime data doesn’t persist across user sessions and prevents the accumulation of stale files.

5. Permissions

By default, the user with the corresponding UID owns the /run/user/$UID directory. The directory permissions are 0700. We can display the permission of this directory using the stat command:

$ stat -c "%a %n" /run/user/*
700 /run/user/1000

As a result, -c “%a %n” displays the permissions in octal format and the file name.

Linux kernel typically enforces this per-user restriction making it effectively isolated even without additional user configuration. With these permissions, only the owner of the directory has the necessary privileges to read, write, and execute files within their own /run/user/$UID directory.

6. Conclusion

In this article, we explored the /run/user/$UID directory. We discussed its purpose and its relation to the /run directory. We also examined the common files and subdirectories found within this directory and the associated permissions.