System Statistics Commands in this section will display statistics about the operating system, or a part of the operating system. du  [-abs] [path1 path2 ...pathN] du stands for disk usage. It will count the amount of disk space a given directory and all its subdirectories take up on the disk. du by itself will return a list of how much space every subdirectory of the current directory consumes, and, at the very bottom, how much space the current directory (plus all the previously counted subdirectories) use. If you give it an option parameter or two, it will count the amount of space used by those files or directories instead of the current one. The a flag will display a count for files, as well as directories. An option of b will display, instead of kilobytes (1024 characters), the total in bytes. One byte is the equivalent of one letter in a text document. And the s flag will just display the directories mentioned on the command-line and not their subdirectories. df  df is short for something, although this author isn't quite sure what. df summarizes the amount of disk space in use. For each filesystem (remember, different filesystems are either on different drives or partitions) it shows the total amount of disk space, the amount used, the amount available, and the total capacity of the filesystem that's used. One odd thing you might encounter is that it's possible for the capacity to go over 100%, or the used plus the available not to equal the total. This is because Unix reserves some space on each filesystem only for root. That way, if a user accidentally fills the disk, the system will still have a little room to keep on operating. For most people, df doesn't have any useful options. uptime  The uptime program does exactly what one would suspect. It prints the amount of time the system has been ``up''--the amount of time from the last Unix boot. uptime also gives the current time and the load average.  The load average is the average number of jobs waiting to run in a certain time period. uptime displays the load average for the last minute, five minutes, and ten minutes. A load average near zero indicates the system has been relatively idle. A load average near one indicates that the system has been almost fully utilized but nowhere near overtaxed. High load averages are the result of several programs being run simultaneously. Amazingly, uptime is one of the few Unix programs that have no options! who  who displays the current users of the system and when they logged in. If given the parameters am i (as in: who am i), it displays the current user. w  [-f] [username] The w program displays the current users of the system and what they're doing. (It basically combines the functionality of uptime  and who. The header of w is exactly the same as uptime, and each line shows a user, when the logged on (and how long they've been idle). JCPU is the total amount of CPU time used by that user, while PCPU the the total amount of CPU time used by their present task. If w is given the option f, it shows the remote system they logged in from, if any. The optional parameter restricts w to showing only the named user.

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