In Linux use:
For disk space usage | df -h
For directory space usage | du
Show memory and swap usage | free
du or Disk Usage. It walks through the directory tree and counts the sum, size of all files. It may not output exact information due to the possibility of unreadable files, hard-links in the specific directory tree.
df or Disk Free. Looks at disk used in file system. It returns results much faster that du but can only show info about the entire disk/partition.