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I know I can run dmesg to look at boot errors. However, my system is failing to boot right now. It hangs indefinitely while trying to initialize hard drives. However, I can boot up into safe mode.

When I boot up in safe mode and look at dmesg it does not contain old boot messages. It just contains the boot messages from the current boot-up into safe mode.

How do I save the dmesg log (or any log for that matter) during the failed boot process so that I can view it when I log in to diagnose via save mode?

dinkelk
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2 Answers2

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I'm not at a Ubuntu pc right now, but I'm trying to help out.

If I'm not mistaken boot logging is disabled by default.

In /etc/default/bootlogd set

BOOTLOGD_ENABLE=Yes

Now, each boot entries in /var/log/boot will be created in a file that's readable by root (so sudo is needed). If I'm not mistaken this log is rotated so you'll have older versions.

If that won't work or you're not in /etc/sudoers, you can also try to boot with a bootable usbdrive and check the bootlog after a failed boot or boot into safe mode and redirect dmesg into a file using the output redirection operator:

dmesg > bootmessages$(date +%Y%m%d).bak
Michael Cox
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You can use journalctl to look at previous boot logs, thusly: 

journalctl -b -1 -e

will look at the previous boot. By varying the number, you can see earlier boot logs. Also journalctl --list-boots is handy to show when boot #N took place.

You can also ask journalctl about the disk's log messages: 

jounalctl /dev/sda
waltinator
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