uptime - Tell how long the system has been running.

No options:

upjustin@Justin:~$ uptime
 10:45:10 up 13 min,  2 users,  load average: 0.45, 0.89, 0.7

uptime gives a one line display of the following information.

• The current time,
• how long the system has been

  running,

• how many users are currently logged on,
• the system load averages for the past 1, 5, and 15 minutes.

NOTE:

System load averages is the average number of processes that are either in a runnable or uninterruptable state.

• A process in a runnable state is either using the CPU or

  waiting to use the CPU. (vmstat - proc - r)

• A process in uninterruptable state

  is waiting for some I/O access, eg waiting for disk. (vmstat - proc - b)

The averages are taken over the three time intervals.

Load averages are not normalized for the number of CPUs in a system , so a load average of 1 means a single CPU system is loaded all the time while on a 4 CPU system it means it was idle 75% of the time.

load average 一般来说不能大于系统cpu的个数：

• 长期大于cpu数目说明cpu繁忙，负载很高。
• 偶尔大于cpu数目可能是正常的
• 小于cpu数目说明是cpu空闲。

uptime 的输出是w命令输出的第一行：

justin@Justin:~$ w
 10:53:38 up 22 min,  2 users,  load average: 0.07, 0.49, 0.66
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
justin   tty7     :0               10:34   21:56   1:08   0.25s gnome-session
justin   pts/1    :0.0             10:45    0.00s  0.75s  0.03s w

FILEs：

/var/run/utmp information about who is currently logged on

-V: show versions

version:

justin@Justin:~$ uptime -V
procps version 3.2.8