2

I had a problem two weeks ago on March 28. At the time I didn't note the kernel version that was being used. How can I find out what kernel version I booted that day using journalctl -b?

3

On older installs journalctl doesn't keep boot history

Firstly only new installs will have boot history stored by journalctl as per this bug report. If you installed Ubuntu on or before January 2018, you need to turn on history for boot records. See this Q&A: Why does `journalctl --list-boots` only show the current boot?

Use --list-boots to get boot number

journalctl references boot history by boot number. Use --list-boots parameter and then use Home, End, PgUp PgDn or Up/Down arrow keys to scroll the list. When done press Q to quit:

$ journalctl --list-boots
-49 14b941ea19ec411cb20d6a78d3775084 Fri 2018-03-09 04:30:01 MST—Fri 2018-03-09 04:30:38 MST
-48 0d72b55a48e64a62b6b30ccd99dbd4ce Fri 2018-03-09 04:30:58 MST—Fri 2018-03-09 04:31:13 MST
-47 9194c4e5bae34f768b674d14c15aeca4 Fri 2018-03-09 04:31:54 MST—Fri 2018-03-09 17:11:27 MST
-46 faa606cbe44745eb934ec42952221295 Fri 2018-03-09 17:11:49 MST—Fri 2018-03-09 17:13:44 MST
-45 17d844d20fa74cf9963a309b6d4b5f3f Fri 2018-03-09 17:14:07 MST—Sat 2018-03-10 17:17:16 MST
-44 1442b6ef67c7449aaabe045723bf5c56 Sat 2018-03-10 17:17:36 MST—Sat 2018-03-10 20:01:56 MST
-43 0a41b59210b443fa94a3763b68f8cfe0 Sat 2018-03-10 20:02:22 MST—Sat 2018-03-10 20:02:52 MST
-42 716ce601e74141b1855164307898caa8 Sat 2018-03-10 20:03:11 MST—Sat 2018-03-10 20:03:46 MST
-41 826cafbae53d4c8e9ac96910a50d4e20 Sat 2018-03-10 20:04:11 MST—Sat 2018-03-10 20:11:25 MST
-40 6ec8cf675f274b18a6efb108a8f4d3be Sat 2018-03-10 20:11:46 MST—Sun 2018-03-11 09:02:55 MDT
-39 a0d8ab504037414885fb77c4366882fd Sun 2018-03-11 09:03:16 MDT—Mon 2018-03-12 19:10:39 MDT
-38 237e246e00fc45f998b77b1af535c3de Mon 2018-03-12 19:10:59 MDT—Wed 2018-03-14 05:54:02 MDT
-37 c5e1fd4eb0ec44178be6c15862f59151 Wed 2018-03-14 16:20:36 MDT—Thu 2018-03-15 19:01:51 MDT
-36 55692a80f3ce49fa81b3660dfaf97d27 Thu 2018-03-15 19:02:14 MDT—Sun 2018-03-18 14:28:06 MDT
-35 42745fa105dd4f42bf6f7de09b6a7fdc Sun 2018-03-18 14:28:26 MDT—Fri 2018-03-23 05:53:43 MDT
-34 778ac520ecf147fab9d467759f8a6340 Fri 2018-03-23 16:54:18 MDT—Sun 2018-03-25 07:40:11 MDT
-33 a219488717c744b5bce07f77440fa526 Sun 2018-03-25 07:41:25 MDT—Sun 2018-03-25 08:29:42 MDT
-32 4416533bb027475e8589a57d0802aefc Sun 2018-03-25 08:30:09 MDT—Sun 2018-03-25 17:33:54 MDT
-31 d13eb5e7796540199762f71624ce86a2 Sun 2018-03-25 17:34:21 MDT—Sun 2018-03-25 18:18:46 MDT
-30 d4631088004845e893d6298e68880263 Sun 2018-03-25 18:19:09 MDT—Sun 2018-03-25 18:19:35 MDT
-29 429ba89a0b0c429eb9ff362ed633c415 Sun 2018-03-25 18:20:05 MDT—Mon 2018-03-26 07:44:35 MDT
-28 a220a9b415a64d25957cd69927dd449c Mon 2018-03-26 07:45:03 MDT—Mon 2018-03-26 07:50:03 MDT
-27 5c1df993a3b540cd911e0b5d6918e9a1 Mon 2018-03-26 07:50:30 MDT—Mon 2018-03-26 11:15:48 MDT
-26 c28ff5b2f1c3453dbb1f095f9caf2dec Mon 2018-03-26 13:14:09 MDT—Mon 2018-03-26 13:22:42 MDT
-25 737d5b753e7b4a46b132968a00e02164 Mon 2018-03-26 13:23:08 MDT—Mon 2018-03-26 22:09:41 MDT
-24 394066eb2d9a43629f7b532d4ffb7dce Tue 2018-03-27 03:54:23 MDT—Tue 2018-03-27 16:46:32 MDT
-23 e9eef9c57f3543d09223d720290e0c4b Tue 2018-03-27 16:47:13 MDT—Tue 2018-03-27 17:16:48 MDT
-22 77c66f077f0e47a5b63f96d0e26fa9da Tue 2018-03-27 17:17:52 MDT—Fri 2018-03-30 11:17:30 MDT
-21 320ccedf358b4aba8b4786a1bec35b96 Fri 2018-03-30 11:52:18 MDT—Fri 2018-03-30 16:29:28 MDT
lines 7-35/56 63%

On the screen we see a boot on March 27th and then one on March 30th. So the boot we want is March 27th called boot number -22

List your boot records

Today's boot is -b, previous boot is -b-1, boot before that is -b-2, etc. We will use journalctl -b-22 to see the appropriate boot history:

-- Logs begin at Mon 2018-03-05 05:53:18 MST, end at Sun 2018-04-15 16:02:56 MDT. --
Mar 27 17:17:52 alien systemd-journald[295]: Runtime journal (/run/log/journal/) is 8.0M, max 75.8M, 67.8M free.
Mar 27 17:17:52 alien kernel: Linux version 4.14.23-041423-generic (kernel@kathleen) (gcc version 7.2.0 (Ubuntu 7.2.0-8ubuntu3.2)) #201802281111 
Mar 27 17:17:52 alien kernel: Command line: BOOT_IMAGE=/boot/vmlinuz-4.14.23-041423-generic root=UUID=f3f8e7bc-b337-4194-88b8-3a513f6be55b ro qui
Mar 27 17:17:52 alien kernel: KERNEL supported cpus:
Mar 27 17:17:52 alien kernel:   Intel GenuineIntel
Mar 27 17:17:52 alien kernel:   AMD AuthenticAMD
Mar 27 17:17:52 alien kernel:   Centaur CentaurHauls
Mar 27 17:17:52 alien kernel: x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
Mar 27 17:17:52 alien kernel: x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
Mar 27 17:17:52 alien kernel: x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
Mar 27 17:17:52 alien kernel: x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registers'
Mar 27 17:17:52 alien kernel: x86/fpu: Supporting XSAVE feature 0x010: 'MPX CSR'
Mar 27 17:17:52 alien kernel: x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
Mar 27 17:17:52 alien kernel: x86/fpu: xstate_offset[3]:  832, xstate_sizes[3]:   64
Mar 27 17:17:52 alien kernel: x86/fpu: xstate_offset[4]:  896, xstate_sizes[4]:   64
Mar 27 17:17:52 alien kernel: x86/fpu: Enabled xstate features 0x1f, context size is 960 bytes, using 'compacted' format.
Mar 27 17:17:52 alien kernel: e820: BIOS-provided physical RAM map:
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000000000000-0x0000000000057fff] usable
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000000058000-0x0000000000058fff] reserved
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000000059000-0x000000000009dfff] usable
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x000000000009e000-0x000000000009ffff] reserved
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000000100000-0x0000000030a5ffff] usable
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000030a60000-0x0000000030a71fff] reserved
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000030a72000-0x0000000030a89fff] usable
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000030a8a000-0x0000000030a8afff] ACPI NVS
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000030a8b000-0x0000000030ad4fff] reserved
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000030ad5000-0x0000000030b2dfff] usable
Mar 27 17:17:52 alien kernel: BIOS-e820: [mem 0x0000000030b2e000-0x0000000031099fff] reserved
lines 1-29

After the first few lines we see the Kernel version that was booted is: 4.14.23-041423-generic. When done press Q to quit.

More than kernel version is available

Of course there is alot more information in the logs available to you. If you wanted to know about say lightdm you could use:

$ journalctl -b-22 | grep lightdm
Mar 27 17:17:55 alien lightdm[1315]: PAM unable to dlopen(pam_kwallet.so): /lib/security/pam_kwallet.so: cannot open shared object file: No such file or directory
Mar 27 17:17:55 alien lightdm[1315]: PAM adding faulty module: pam_kwallet.so
Mar 27 17:17:55 alien lightdm[1315]: PAM unable to dlopen(pam_kwallet5.so): /lib/security/pam_kwallet5.so: cannot open shared object file: No such file or directory
Mar 27 17:17:55 alien lightdm[1315]: PAM adding faulty module: pam_kwallet5.so
Mar 27 17:17:55 alien lightdm[1315]: pam_unix(lightdm-greeter:session): session opened for user lightdm by (uid=0)
Mar 27 17:17:55 alien systemd[1]: Created slice User Slice of lightdm.
Mar 27 17:17:55 alien systemd-logind[802]: New session c1 of user lightdm.
Mar 27 17:17:55 alien systemd[1]: Started Session c1 of user lightdm.
Mar 27 17:17:55 alien systemd[1341]: pam_unix(systemd-user:session): session opened for user lightdm by (uid=0)
Mar 27 17:17:55 alien lightdm[1555]: PAM unable to dlopen(pam_kwallet.so): /lib/security/pam_kwallet.so: cannot open shared object file: No such file or directory
Mar 27 17:17:55 alien lightdm[1555]: PAM adding faulty module: pam_kwallet.so
Mar 27 17:17:55 alien lightdm[1555]: PAM unable to dlopen(pam_kwallet5.so): /lib/security/pam_kwallet5.so: cannot open shared object file: No such file or directory
Mar 27 17:17:55 alien lightdm[1555]: PAM adding faulty module: pam_kwallet5.so
Mar 27 17:17:55 alien lightdm[1555]: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "rick"
Mar 27 17:17:59 alien lightdm[1315]: pam_unix(lightdm-greeter:session): session closed for user lightdm
Mar 27 17:17:59 alien lightdm[1555]: pam_unix(lightdm:session): session opened for user rick by (uid=0)
Mar 27 17:19:55 alien systemd[1344]: pam_unix(systemd-user:session): session closed for user lightdm
Mar 27 17:19:55 alien systemd[1]: Removed slice User Slice of lightdm.
Mar 30 11:17:27 alien lightdm[1046]: ** (lightdm:1046): CRITICAL **: session_get_login1_session_id: assertion 'session != NULL' failed
Mar 30 11:17:28 alien lightdm[1046]: /etc/modprobe.d is not a file
Mar 30 11:17:28 alien lightdm[1046]: /etc/modprobe.d is not a file
Mar 30 11:17:28 alien lightdm[1046]: /etc/modprobe.d is not a file
Mar 30 11:17:28 alien lightdm[1046]: /etc/modprobe.d is not a file
Mar 30 11:17:28 alien lightdm[1046]: /etc/modprobe.d is not a file
Mar 30 11:17:28 alien lightdm[1046]: Error: can't open /lib/modules/4.14.23-041423-generic/updates/dkms
Mar 30 11:17:28 alien lightdm[1046]: Error: can't open /lib/modules/4.14.23-041423-generic/updates/dkms
Mar 30 11:17:28 alien lightdm[1046]: update-alternatives: error: no alternatives for x86_64-linux-gnu_gfxcore_conf

You might think "Mar 30" at the bottom is in error but it's a laptop that gets suspended and and still part of March 27, 2018 boot log.

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