This directory contains system information files describing the system since it was booted. Generally, files in this directory should be cleared (removed or truncated as appropriate) at the beginning of the boot process.
Process identifier (PID) files, which were originally placed in /etc, are placed in /var/run. The naming convention for PID files is <program-name>.pid. For example, the crond PID file is named /var/run/crond.pid.
The internal format of PID files remains unchanged. The file should consist of the process identifier in ASCII-encoded decimal, followed by a newline character. For example, if crond was process number 25, /var/run/crond.pid would contain three characters: two, five, and newline.
Programs that read PID files should be somewhat flexible in what they accept; i.e., they should ignore extra whitespace, leading zeroes, absence of the trailing newline, or additional lines in the PID file. Programs that create PID files should use the simple specification located in the above paragraph.
The utmp file, which stores information about who is currently using the system, is located in this directory.
Programs that maintain transient UNIX-domain sockets should place them in this directory.