I found sometimes, the outputs of,
last -x reboot who -b uptime -s sudo journalctl --list-boots
Are showing different uptime since last booting which is annoying.
An example is,
last -x reboot # wtmp begins Wed Apr 1 10:29:40 2020 <-- this is actually not a reboot time. Explain later who -b # system boot 2020-03-02 19:19 uptime -s # 2020-03-02 19:19:54 sudo journalctl --list-boots # 0 46e3eb28ab51489ab8eb236d65a5086f Mon 2020-04-13 10:51:31 PDT—Mon 2020-04-13 15:46:43 PDT
Although this might be a better solution which always tells us the truth,
systemctl status docker.service | grep since # Active: active (running) since Thu 2020-04-02 15:56:23 PDT; 1 weeks 3 days ago
(Assuming dockerd shouldn't be manually rebooted in a production environment. Any stable service should also do the job.)
- system time is inaccurate (probably no hwclock, or out of sync)
- these commands are using the different data sources
- data sources were
logrotate-ed (old part has been deleted/renamed)
The 1st one is easy to be determined.
last -x reboot
At system bootstrap an entry would be written to
/var/log/wtmpwith the username
reboot, and at shutdown an entry would be written to
/var/log/wtmpwith the username
Finding the times that the system was rebooted was a matter of using the
last shutdowncommands to print out these entries.
Normally the output should be,
# shutdown system down ... <-- first the system shuts down # reboot system boot ... <-- afterwards the system boots
Although under unexpected shutdown like hard power off,
# reboot system boot ... <-- then we've a boot WITHOUT a prior shutdown
last outputs only a part of info if there is too many login info. To find all the info, firstly find out all the
wtmp logs by,
ls /var/log/wtmp* # /var/log/wtmp /var/log/wtmp.1
Then cat the old one,
last -f /var/log/wtmp.1 reboot shutdown # reboot system boot 4.15.0-88-generi Mon Mar 2 19:19 still running # reboot system boot 4.15.0-88-generi Mon Mar 2 19:14 still running # wtmp.1 begins Mon Mar 2 15:50:57 2020 <-- this is just when the file begins
See, the output here matches the
uptime -s and
who -b in the first example.
who -b && uptime -s
/run/utmpis a table of terminal login records, with special entries for bootup and shutdown. These entries are read by
who -b. They are written by programs such as
systemd-update-utmp, an analogue of the FreeBSD
utxcommand, which are run as parts of the startup and shutdown procedures. They are not run first or last, as the relevant services are not (and indeed cannot be) ordered absolutely first or last. There may be journal entries with the relevant boot ID that precede the time that
systemd-update-utmp rebootis run, and similar journal entries that postdate the time that
systemd-update-utmp shutdownis run.
systemd-journald), every journal record has a field named the
boot ID. You can see this with the
-o verbose optionto journalctl. A
boot IDis generated by the kernel at bootstrap, and
systemd-journaldapplies the current boot ID, taken from the kernel, to every log record as it is adding it to the journal.
To implement the
list-bootsfunctionality, journalctl scans the entire journal, reading the timestamps and boot IDs of every record, and noting the earliest and latest timestamps associated with each unique boot ID.
Thus if parts of the journal are purged, or conversely stick around overlong, the apparent boot and shutdown times reported by journalctl will differ wildly from the actual boot and shutdown times.
In sum, the
journald logs usually get purged (logrotate-ed). So it's not reliable.