9. Disk usage

No disk can hold an infinite amount of data. These commands report on how much disk storage is in use or available. (This has nothing much to do with how much main memory, i.e., RAM, a program is using when it runs; for that, you want ps or pstat or swap or some such command.)

9.1 df: Report filesystem disk space usage		Report filesystem disk space usage.
9.2 du: Estimate file space usage		Estimate file space usage.
9.3 sync: Synchronize data on disk with memory		Synchronize memory and disk.