The command that displays disk space usage of file systems is `df`.
The name comes from “disk free,” and the command reports total, used, and available space for mounted file systems. It first appeared in Version 1 AT&T Unix in 1971 and became part of POSIX and the Single UNIX Specification.
`df` examines file-system-level capacity, not the size of individual directories. That distinction causes a common mix-up with `du`, which estimates how much space files and directories consume. `mount` displays or changes mounted file systems, while `fdisk` manages disk partition tables.
Useful options include `df -h` for human-readable units, `df -i` for inode usage, and `df -T` on GNU systems to show file-system types. By default, traditional Unix specifications use 512-byte blocks, although many Linux implementations display 1K blocks by default.