df is a shell command for reporting the amount of available and consumed storage space on a file system.

Although df is short for disk free, the command is not limited to disk storage. It was developed during the long period of time when disk-based storage was the ubiquitous mass storage technology.

The Single Unix Specification (SUS) specifies that space is reported in blocks of 512 bytes, and that at a minimum, it reports the file system names and the amount of free space. Using 512-bytes as the unit of measure is a historical practice and maintains compatibility with ls and other commands. Notably, the file system need not be constrained to internally use 512-byte blocks. The -k option was added as a compromise measure. It was agreed by the standard developers that 512 bytes was the best default unit because of its complete historical consistency on System V (versus the mixed 512/1024-byte usage on BSD systems), and that a -k option to switch to 1024-byte units was a good compromise. Users who prefer the more logical 1024-byte quantity can use alias to map df to df -k without breaking many historical scripts relying on the 512-byte units.

The command first appeared in Version 1 AT&T Unix. The command is part of the X/Open Portability Guide since issue 2 of 1987. It was inherited into the first version of POSIX and the Single Unix Specification (SUS). The implementation of df bundled in GNU coreutils was written by Torbjorn Granlund, David MacKenzie, and Paul Eggert. The command is available for Windows via UnxUtils.

Any number of file parameters can be specified on the command line to select the storage for the file system containing the file.

SUS specifies options -k and -P but most implementations include additional options. Notable options include:

Use 1024-byte units, instead of the default 512-byte units, when writing space figures.

Output in portable format.