linux

Determine kernel version

uname -a

Determine the linux distro and version

cat /etc/issue
cat /etc/os-release
cat /etc/lsb-release
cat /proc/version

lsb_release -a
hostnamectl

Source: https://www.cyberciti.biz/faq/find-linux-distribution-name-version-number/

Display BIOS metadata -- no reboot needed

sudo dmidecode --type bios

Ubuntu (any Debian?) -- check for non-LTS OS upgrades

sudo vi /etc/update-manager/release-upgrades

Make sure the "Prompt" value is set to "normal" and not "lts"

Determine files within/installed by a package

apt-file list <package>   #NOTE: This works on any pkg, not just installed ones
dpkg -L <package>

Show the total size of the current directory (and all its subdirs)

Linux:
sudo du --total --summarize --human-readable

BSD / MacOS
sudo du -c -s -h

Determine locations of superblock backups

sudo mke2fs -n /dev/sda1

Needed to resolve "bad magic number in super-block" error

Reference: https://linuxroutes.com/resolve-bad-magic-number-in-super-block-in-error/

Mount a raid 1 (mirror) drive

sudo mdadm --assemble --run /dev/md0 /dev/<partition to be mounted>
sudo mount /dev/md0

I needed this when I lost one of my mirrored drives and wanted to make a backup of the one that still worked

Reference: https://serverfault.com/questions/383362/mount-unknown-filesystem-type-linux-raid-member

Show only local, hardware, mounted drives

mount | grep -e '^\/dev'

This will limit the output to lines that start with "/dev", to cut down on noise from tmpfs, sysfs, proc, snaps, etc

Change a user's default shell

sudo chsh -s /path/to/shell username

Source: https://superuser.com/questions/379725/how-do-i-change-a-users-default-shell-in-osx