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.
Using the free command, the free and used memory in a Linux or Unix system can be found.
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.
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.
The ps command is for the process status. ps gives the status of currently executing processes on a Linux or Unix system. There are multiple options for the ps command that help in getting the process information of interest.
The Coordinated Universal Time (UTC) is the universally accepted time standard. Computers around the world use it as the reference time. It is important that our computers have the correct UTC. The Network Time Protocol (NTP) helps us in synchronizing the time of our computers with the UTC maintained by timeservers.
Software systems often need to act based on time. The accuracy and precision of time maintained by a system is important. Alarms, sleep and high resolution timers provide a framework for application programs to carry out time-critical tasks.
As a process executes, it takes up the CPU time which is also called the execution time or the processor time. The times system call and the clock library function can be used by the process to find its execution time.
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.
The hardware real time clock tends to lose or gain time every day by a constant amount. This is called the systematic drift. hwclock program helps in calibrating and adjusting the hardware clock.
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.
Backing up a Drupal site involves copying the site's root directory structure and creating a dump of site's database. The backed up data can be used for restoring the site later on.
Moving a Drupal site to another host involves making a copy of site's root directory structure and creating a dump of the site's database. On an another host, the database is restored from the backup and the site's root directory tree structure is installed.