Most of the counter-based tools on Linux systems read statistics from the /proc file system. /proc is a “pseudo” file system that exists only in kernel memory and is used primarily for querying various per-process as well as kernel statistics. The /proc file system is organized as a directory hierarchy, with the process (a unique integer value assigned to each process) appearing as a subdirectory below /proc. For example, the directory entry /proc/2155 would contain per-process statistics for the process with an ID of 2155. There are /proc entries for various kernel statistics as well. In both this chapter and Chapter 3, we provide programming projects where you will create and access the /proc file system