I have used both uptime and cat /proc/uptime commands. I know the idle time in the later can be greater than running time (multiprocessors). However, when I add both running and idle times, and compare it to the uptime (5:30 hrs) they are not equal. I assume it's because the uptime returns the real time and the cat /proc/uptime returns processor(s) time.

Short things clear, I want to know the reason exactly and I want to know which one to choose, according to the topic.

Also, is it possible to get the processor(s) time in the last 24 hours?

NOTE: Please read this article to understand where this is going.

*Distributor ID: Ubuntu Description: Ubuntu 12.04.2 LTS Release: 12.04 Codename: precise *

  • The first field of /proc/uptime is the uptime in wall time. Meaning it's real time, not CPU time. If that doesn't match the value of uptime, then you've got some voodoo going on. Can you provide the output of each for us to look at?
    – phemmer
    Commented Jan 22, 2014 at 4:33
  • uptime: 21:24 ..cat \proc\uptime: 77098.74 103113.16. They are slightly different though..
    – Atieh
    Commented Jan 22, 2014 at 7:40

1 Answer 1


The uptime command reads the two values out of /proc/uptime, actually. The first value is the amount of time since the machine booted. The second value, however, is the sum of time that each cpu has not had a runnable process to schedule.

Check out fs/proc/uptime.c from the Linux kernel github and you'll see how the /proc/uptime file's content is generated.

So, to clearly answer, the amount of seconds+milliseconds since the computer booted is the first value in the output of cat /proc/uptime.

  • For example, a largely idle server I have access to has a /proc/uptime contents of "9843159.39 117450954.76". It has 12 CPUs. You'll notice that the second value divided by the first, equals 11.9322 which means I've had a little shy of 12 processors totally idle for the past 2,700 hours.
    – etherfish
    Commented Jan 21, 2014 at 17:33
  • No offense, but can you rephrase what you said? How could this answer my question?
    – Atieh
    Commented Jan 21, 2014 at 20:46
  • So the first value is equal to the 5:30 hrs..? So uptime command excludes the idle time when it gives me time in the format of 5:30?
    – Atieh
    Commented Jan 21, 2014 at 20:49

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