The vmstat command prints the system virtual memory statistics for Linux and Unix systems. vmstat prints information about system processes, memory, swap, I/O blocks, interrupts and context switches and the CPU activity.

Load Average

Load average, reported by commands like top, uptime and w, is explained. Load average is an indication of whether the system resources (mainly the CPU) are adequately available for the processes (system load) that are running, runnable or in uninterruptible sleep states during the previous n minutes.

top Command in Linux

The top command is useful for monitoring a Linux or Unix system continuously for processes that take more system resources like the CPU time and the memory. top periodically updates the display showing the high resource consuming processes at the top.

Calendar Time Under Linux

Linux systems maintain a system clock which stores the calendar time. The is stored as seconds passed since January 1, 1970 00:00:00 UTC. Applications can query this system clock with the time and gettimeofday system calls. There are functions to convert the system calendar time to the familiar local time string.


anacron helps in running shell scripts once in a given number of days. Unlike cron, anacron does not require that the exact time running of the script be fixed a priori. For this reason, anacron is particularly relevant for home computers and laptops, which are not switched on all the time.


The cron daemon runs shell scripts for root and other users at predefined times. cron is the standard way for running programs periodically on Linux and other Unix-like systems.