Df

The command df allows you to see how much disk space is left in your system:

Example:

tweedleburg:~ # df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1             917G  350G  521G  41% /
udev                  2.0G  304K  2.0G   1% /dev
/dev/sdb2             294G  167G  112G  60% /mnt/sdb2

Looking at the above, "-h" means "human-readable". 41% of your hard disk at / is in use, and 60% of your hard disk at /mnt/sdb2.