Varrun

Varun, commonly written as varrun in some documentation, refers to the conventional directory /var/run on Unix-like systems. It is used to store transient, runtime data needed by system services and daemons while the system is running. Data in varrun is non-persistent and typically cleared at reboot.

Location and barriers: Historically, /var/run was a standard directory under /var for runtime information. In many modern systems, /var/run is implemented as a symbolic link to /run, a separate filesystem mounted early in the boot process. /run is typically a tmpfs (RAM-based) filesystem, which makes runtime data fast to access and automatically cleared on shutdown or reboot. Some systems retain a distinct /var/run path alongside /run for compatibility, but the common arrangement now is a link from /var/run to /run.

Contents: varrun usually contains process identification files (PID files), inter-process communication sockets, and temporary lock files. Examples include files like /var/run/sshd.pid, /var/run/docker.sock (a Unix domain socket), and various service-specific files such as database or daemon state indicators. These files enable services to coordinate with each other and detect running instances.

Management and security: Access to varrun data is typically restricted to the root user and specific services. PID files help prevent multiple instances of a daemon from running simultaneously, while sockets facilitate local IPC. Because the data is ephemeral, it is often automatically cleaned up on reboot or via tmpfiles management in systemd-based systems.

Relation to standards: The Filesystem Hierarchy Standard defines /var/run as a location for volatile, non-persistent runtime data. The introduction of /run as a dedicated runtime location has influenced modern distributions, which commonly implement /var/run as a symlink to /run.