Read dmesg From Previous Session

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If your system uses journalctl then you can easily get the kernel messages (dmesg log) from prior shutdown/crash (in a dmesg -T format) through the following.

Options:

  • -k Show kernel messages.
  • -b <boot_number> How many reboots ago 0, -1, -2, etc.
  • -o short-precise Print in a dmesg -T format.
  • -p <priority> Filter by priority output (4 to filter out notice and info).

There is also an -o short and -o short-iso which gives you the date only, and the date-time in ISO format respectively.

Commands:

  • All boot cycles : journalctl -o short-precise -k -b all
  • Current boot : journalctl -o short-precise -k
  • Last boot : journalctl -o short-precise -k -b -1
  • Two boots prior : journalctl -o short-precise -k -b -2

And so on.

The amount of boots you can look back on can be viewed with the following.

journalctl --list-boot

The output of journalctl --list-boot looks like the following.

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-6 cc4333602fbd4bbabb0df2df9dd1f0d4 Sun 2016-11-13 08:32:58 JST—Thu 2016-11-17 07:53:59 JST
-5 85dc0d63e6a14b1b9a72424439f2bab4 Fri 2016-11-18 22:46:28 JST—Sat 2016-12-24 02:38:18 JST
-4 8abb8267e06b4c26a2466562f3422394 Sat 2016-12-24 08:10:28 JST—Sun 2017-02-12 12:31:20 JST
-3 a040f5e79a754b2a9055ac2598d430e8 Sun 2017-02-12 12:31:36 JST—Sat 2017-02-18 21:31:04 JST
-2 6c29e3b6f6a14f549f06749f9710e1f2 Sat 2017-02-18 21:31:15 JST—Sat 2017-02-18 22:36:08 JST
-1 42fd465eacd345f7b595069c7a5a14d0 Sat 2017-02-18 22:51:22 JST—Sat 2017-02-18 23:08:30 JST  
 0 26ea10b064ce4559808509dc7f162f07 Sat 2017-02-18 23:09:25 JST—Sun 2017-02-19 00:57:35 JST