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My Linux sandbox boots into the text mode. During the boot process many lines are prefixed with a number between brackets ie. [40.000] Normally this number reaches upwards to 80. Recently I've noticed this number can be as short as 40 and today it was at a completely different magnitude and ended around 1200.

  1. What is it called?
  2. What does it represent?
  3. Any idea why a stable install without auto update would suddenly change this value?
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1 Answer

up vote 5 down vote accepted
  1. It is called timestamp.

  2. The numbers between brackets represent the number of seconds passed since your operating system started.

  3. Maybe today you saw a trace of an event that occurred 20 minutes after your system stared -for example, you might plugged in a thumb drive.

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It is not the number of seconds since the system started, it's the number of seconds since the kernel started logging. –  Chris Down Nov 28 '11 at 19:53
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Well no, actually it's approximately the number of seconds since the kernel image got uncompressed by the boot loader, and the start_kernel function in init/main.c was invoked. Satisfied? Of course an answer like that would be useless for someone who is not familiar with operating systems terminology and internals. Sometimes it is preferable to sacrifice precision for simplicity. –  dkaragasidis Nov 28 '11 at 20:14
    
@dkaragsidis ...which is the time since the kernel started logging. –  Chris Down Nov 28 '11 at 20:32
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@ChrisDown Yes. But what if the person asking is not aware of what kernel is and of its functions? Then you'd have to give him an abstraction of the reality. System (as in "operating system") seems as a good enough abstraction to me. I feel though that we're getting off topic with these comments. :) –  dkaragasidis Nov 28 '11 at 20:39
    
Oi! I know there exists a kernel, even if I don't know the functions in it! :P And yes. It makes complete sense. I have installed Luks encryption which pauses, usually, after time stamp 10 to ask for the passcode. Today I did step away before entering this code. Then the case of a lower than usual number may be caused by, say, a crash and the system not fully recovered yet booting only part way? –  xtian Nov 28 '11 at 22:26
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