When I boot up Ubuntu and I don't have the graphical boot screen (Plymouth) enabled I see a bunch of text saying if an action was [OK] or it [FAILED].
Since this process is a bit quick I can't fully read everything, but I would like to know what happens exactly.
I think there must be a log file somewhere where all this is written down.
But where?
To see information from your most recent boot, run
journalctl -b
To see the details that were logged during a previous boot, just in case you need to compare them with your most recent, run
journalctl --list-boots
You will then see results similar to the output shown below, but will need to page to the bottom of the output to see the most recent.
-2 5d45a92d9d5a42d8ae1c8b454f7e1314 Sun 2018-08-19 02:29:21 PDT—Sun 2018-08-19 02:36:26 PDT
-1 d5b8ebc236734ee69c48c3809ae452b1 Sun 2018-08-19 02:43:06 PDT—Sun 2018-08-19 07:20:54 PDT
0 6f844406186e4e31ba0375a7bc736242 Sun 2018-08-19 10:43:31 PDT—Sun 2018-08-19 18:54:51 PDT
At the very bottom of the output, you will see a line that starts with 0, which will represent your most recently logged boot info, the same info you saw when you ran journalctl -b.
To see the data that was logged during the previous boot, copy the identifier from the line that starts with -1 and run the following
journalctl --boot=d5b8ebc236734ee69c48c3809ae452b1
To go back further in time, just change to -2, -3, -4, etc.
The log is in /var/log/boot.log. I set up a startup command on my system when I log in that opens a terminal and does "more /var/log/boot.log so each time I log in, I can check if there are any issues. If you only want to see failed lines, you can make it read:
grep FAILED /var/log/boot.log : more
which will show only lines with the word FAILED in them.
Note that /var/log/boot.log only shows the log of the startup commands, not the complete boot prior to that point.
The log file in question is /var/log/boot.log. This contains in particular the messages you are seeking.
journalctl -b. From the docs: "Show messages from a specific boot. [...] The argument may be empty, in which case logs for the current boot will be shown."