XDG_RUNTIME_DIR

The Linux file system is a hierarchical structure that organizes files and directories in a logical manner. At the root of this hierarchy sits the directory denoted by a single forward slash /. Everything within the Linux file system branches out from this root directory, creating a tree-like structure where each directory can contain files and other directories.

One of the foundational aspects of the Linux file system is its adherence to the Filesystem Hierarchy Standard (FHS). This standard defines the directory structure and the types of files that should be stored within each directory. The bootloader and kernel are stored in the /boot directory on my Pop!_OS.

Kernel space and user space are not directories in the traditional filesystem hierarchy. Kernel space refers to the area of memory where the kernel runs, separate from user space where the applications you use run.

/bin contains essential user command binaries, while /sbin holds system binaries for administrative tasks. This organization ensures consistency across different Linux distributions, making it easier for users and system administrators to navigate and manage the system. There’s also /lib, which includes libraries necessary for the binaries in /bin and /sbin. These libraries are vital for the system’s basic operations.

/etc is pivotal for system-wide configuration files. Here, you’ll find network configuration files, user account information and service configurations, among others. The importance of /etc cannot be overstated. It holds settings that control much of the system’s behavior at boot and during runtime.

Another critical directory is /var, where variable data files reside. This includes logs, spool files and temporary files that change over time. /var/log, for instance, is where system logs are stored, providing invaluable insights into system activities, errors and security events. This directory is crucial for system maintenance and troubleshooting.

For operational files, /proc is a pseudo-filesystem that does not take up actual disk space but provides an interface to kernel data structures. It provides a dynamic view into the system’s process and kernel information, with files appearing and disappearing as processes come and go. For instance, /proc/[pid] directories exist for each running process, providing detailed information about that process’s state. This isn’t a typical filesystem but an interface to kernel data structures.

Temporary files are managed in /tmp, which is cleared upon system reboot or periodically by the system. Applications store data in /tmp files that they only need during a session. /tmp files are not needed beyond a reboot.

/dev contains special files, related to hardware and system information, used to interface with device drivers. The /media directory is used for mounting removable media devices like USB drives or CDs, making their contents accessible within the file system. /mnt is used for temporarily mounting filesystems. Although less common now, it can still be used for manual mounts.

The /root directory is the home directory for the root user, distinct from /home where other users’ directories reside. The /home directory is where user-specific files are stored. Each user on the system has their own subdirectory under /home, where they have full control over their personal files, settings and configurations. This separation enhances privacy and security by isolating user data from system and shared directories.

The /opt directory is used for installation of add-on application software packages. This directory is useful when you install software manually or when software packages are installed in a way that doesn’t fit into the standard FHS layout. It’s a place for optional or add-on applications that are not part of the core distribution.

/usr is a large directory reserved for read-only user data; that is, data which doesn’t change during normal operation of the system. Here, you’ll find /usr/bin for user binaries, /usr/lib for shared libraries, and /usr/share for shared data like documentation and locale information. The /usr directory is essentially a secondary hierarchy for read-only data, complementing the system-wide directories under /. Software packages often place their data in /usr. This directory is split into subdirectories like /usr/bin, /usr/lib and /usr/share for different types of data.

This hierarchy ensures that each directory has a specific purpose, aiding in system management and maintenance. With the overview of the traditional Linux file system structure in mind, it’s important to discuss how modern Linux desktops manage user data and configurations more efficiently, leading us to the XDG Base Directory specification.

The XDG Base Directory Specification, part of the freedesktop.org initiative, aims to standardize the location of certain types of user-specific data. It defines several environment variables:

• XDG_CONFIG_HOME: Points to the base directory relative to which user-specific configuration files should be stored. By default, this is set to $HOME/.config.
• XDG_DATA_HOME: Specifies the base directory for user-specific data files. The default location is $HOME/.local/share.
• XDG_CACHE_HOME: Indicates where user-specific non-essential (cached) data should be stored. It defaults to $HOME/.cache.
• XDG_STATE_HOME: A location for system state data, introduced in a later version of the specification, defaults to $HOME/.local/state.
• XDG_RUNTIME_DIR: A location for runtime files like sockets and named pipes, which are not shared between processes or users.

These XDG directories help in keeping user data organized and accessible. They promote a cleaner, more predictable desktop environment by reducing clutter in the home directory. Applications that follow the XDG spec will store their configurations, data and cache in these designated locations rather than creating myriad dot directories in the user’s home folder.

For example, instead of having .config, .local/share, .cache, etc., scattered throughout the home directory, all these are neatly organized under a few well-defined directories. This not only helps in managing backups and migrations but also simplifies the task of system administrators who need to manage user profiles across different systems.

Moreover, the XDG spec encourages developers to respect these locations when writing software for Linux desktops. It leads to a more uniform user experience across different applications and distributions, reducing the learning curve for users and simplifying maintenance for developers.

However, not all applications or systems strictly adhere to the XDG standards due to legacy reasons or specific requirements. Hence, you might still encounter applications that use traditional dot directories or other custom locations for their data.

In practice, the Linux file system’s organization, combined with standards like XDG, provide a robust framework that supports both system administration and user experience. It allows for a system where files are not just stored but are placed with intent, making the system easier to navigate, maintain and secure.

This detailed structure of the Linux file system, coupled with modern specifications like XDG, showcases Linux’s commitment to order, efficiency and user-centric design, ensuring that even with the complexity of underlying systems, users and administrators can work with confidence and clarity.

XDG Runtime Files

XDG runtime files are managed through the XDG_RUNTIME_DIR environment variable as part of the XDG Base Directory Specification. XDG_RUNTIME_DIR provides a location for runtime files used by applications during their execution. These files are typically temporary and include:

• Unix domain sockets
• Named pipes (FIFOs)
• Runtime lock files
• PID files
• Other temporary runtime data

XDG_RUNTIME_DIR ensures that these files are not accessible by other users or processes outside the user’s session. The directory is created with permissions that allow only the user’s own processes to access it, typically with permissions like 0700 (read, write and execute for the owner only).

Each user session gets its own XDG_RUNTIME_DIR, which is cleared when the session ends, ensuring that session-specific data does not persist across logouts or system reboots. Having a dedicated space for these files reduces the overhead of creating and managing temporary files in less appropriate locations like /tmp, which might not offer the same level of isolation or security.

By default, XDG_RUNTIME_DIR is set to /run/user/$UID, where $UID is the user’s ID. This path ensures that each user has a unique directory, preventing conflicts and enhancing security. The directory is usually created by the system at login time and is automatically managed by session managers like systemd-logind. It ensures that the directory exists for the duration of the session and is cleaned up afterward. The path to this directory is stored in the XDG_RUNTIME_DIR environment variable, which applications can read to locate where to place or find runtime files.

Many modern applications, especially those adhering to newer Linux desktop standards, use XDG_RUNTIME_DIR for inter-process communication or for temporary storage of session data. For instance, a desktop environment might use it for session management or for running background services.

Even services that run in the context of a user’s session might use this directory for runtime data, like a user-specific instance of a daemon. Applications can use this directory to create lock files to manage shared resources or prevent race conditions between different instances of the same program.

Older or legacy software might not be aware of XDG_RUNTIME_DIR, sticking to older conventions like /tmp or creating files in other locations. This can lead to clutter or security issues if not managed properly. Users or system administrators can override the default location by setting XDG_RUNTIME_DIR to another path if necessary, although this is rarely needed unless in very specific scenarios or for testing purposes.

On systems using systemd, the creation and management of XDG_RUNTIME_DIR are well-integrated, providing a seamless experience for both users and developers. Other init systems might handle this differently, or it might fall to session managers or login scripts to manage this directory.

XDG_RUNTIME_DIR enhances the security, privacy and efficiency of runtime file management on Linux systems, aligning with modern practices of session isolation and data management. Applications use XDG_RUNTIME_DIR to help create a more orderly, secure and performant user environment.