systems since 4.3BSD-Reno of 1990 (but is not part of POSIX). On Linux specifically, one can also read /proc/loadavg for this information. This file also provides instantaneous information on the number of processes in "R" state, the total number of processes, and the process ID of the most-recently-created process.

Systems calculate the load average as the exponentially damped/weighted moving average of the load number.