If you want to see your disk usage / disk space, simply run:
df -h
Output sample:
Filesystem Size Used Avail Use% Mounted on /dev/sda3 33G 3.9G 27G 13% /
If you want to list your existing hard disks and their partitions, run:
fdisk -l
Output sample:
Device Boot Start End Blocks Id System /dev/sda1 1 1306 10490413+ 27 Unknown /dev/sda2 * 1307 20110 151041024 7 HPFS/NTFS /dev/sda3 20111 24328 33881085 83 Linux /dev/sda4 24329 38913 117154012+ 5 Extended /dev/sda5 24329 24450 979933+ 82 Linux swap / Solaris /dev/sda6 24451 38913 116174016 83 Linux
To list all your mount points, run :
sudo mount -l