Authored Comments

lsblk is great for getting an overview of the physical devices mounted on the system, but it will miss any virtual filesystem mounts — most commonly, in-memory filesystems mounted as type 'tmpfs', of which modern Linux systems typically contain several.

/tmp and /dev/shm will be kept in memory for performance reasons on most systems (which is why /var/tmp should always be used instead of /tmp for large temporary files), and systemd installations will have /run and one or more /run/user/$UID mounted in memory as well.

df or df -h will provide a nicely-formatted listing of all mounts including virtual filesystems, or on distributions which provide it, I've come to prefer the output of di.