Logs – Understanding the Output of the ‘Last’ Command

lastlogs

Can anybody explain me what is the meaning of the last column of the output of the last command? I'm particularly interested in its meaning with respect to the reboot pseudo-user.

reboot   system boot  2.6.32-28-generi Sat Feb 12 08:31 - 18:09 (9+09:37)

What does that 9+09:37 mean?

Best Answer

reboot and shutdown are pseudo-users for system reboot and shutdown, respectively. That's the mechanism for logging that information, with kernel versions to same place, without creating any special formats for the wtmp binary file.

Quote from man wtmp:

The wtmp file records all logins and logouts.  Its format is exactly like utmp except that a null username indicates a logout on the associated terminal.  Furthermore, the terminal name ~ with username shutdown or reboot indicates a system shutdown or reboot and the pair of terminal names | / } logs the old/new system time when date(1) changes it.

wtmp binary file do not save other than timestamp for events. For example, last calculates additional things, such as login times.

reboot   system boot  2.6.32-28-generi Mon Feb 21 17:02 - 18:09  (01:07)    
...
user     pts/0        :0.0             Sat Feb 12 18:52 - 18:52  (00:00)    
user     tty7         :0               Sat Feb 12 18:52 - 20:53  (02:01)    
reboot   system boot  2.6.32-28-generi Sat Feb 12 08:31 - 18:09 (9+09:37)

The last column (in parentheses) is the length of event. For the user reboot, it's uptime.

After the latest reboot, time is current uptime. For earlier reboots, time is uptime after that reboot (so in the last line of my example it's uptime until the first line; there were no reboots in between). Number(s) before + means number of days. In the last line, it's 9 days, 9 hours and 37 minutes, and in the first line current uptime is 1 hour and 7 minutes.

Note, however, that this time is not always accurate — for example, after a system crash and unusual restart sequence. last calculates it as the time between it and next reboot/shutdown.

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