Where are fsck results logged at boot time, after /forcefsck?

Possibly you are affected by this bug: "Does not log fsck invocations in /var/log/fsck/"

For Ubuntu 14.xx:

I found some fsck logs in /var/log/upstart/mountall.log.

For Ubuntu 16.04 and 18.04 root partitions

You're likely looking for /run/initramfs/fsck.log.

An fsck of the root filesystem necessarily happens before the root filesystem has been mounted as writable, so the filesystem check occurs early in the boot process while the system is still running from the initramfs. An fsck log is written to a RAM-backed filesystem (tmpfs) that is available for writing at this time, and it continues to be available after boot at /run/initramfs/fsck.log. This is volatile storage, so fsck logs are lost once the system reboots. It would be nice if these logs were copied to non-volatile storage after the root filesystem is mounted as writable, but this does not appear to be the case.

Here's an example:

$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 238.5G  0 disk 
├─sda1   8:1    0   512M  0 part /boot/efi
└─sda2   8:2    0   238G  0 part /

$ cat /run/initramfs/fsck.log 
Log of fsck -C -a -V -t ext4 /dev/sda2 
Fri Nov 30 22:35:21 2018

fsck from util-linux 2.31.1
[/sbin/fsck.ext4 (1) -- /dev/sda2] fsck.ext4 -a -C0 /dev/sda2 
/dev/sda2: clean, 653295/15597568 files, 6658147/62383360 blocks

Fri Nov 30 22:35:21 2018
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