(abbreviation for disk free) is a standard Unix command used to display the amount of available disk space for file systems on which the invoking user has appropriate read access. is typically implemented using the statfs or statvfs system calls. History for Unix-like systems 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. It first appeared in Version 1 AT&T Unix. The version of bundled in GNU coreutils was written by Torbjorn Granlund, David MacKenzie, and Paul Eggert. The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities. Usage The Single UNIX Specification specifications for are: df [-k] [-P|-t] [-del] [file...] Use 1024-byte units, instead of the default 512-byte units, when writing space figures. Use a standard, portable, output format Display in more easily human readable units such as KB, MB, GB or TB. Write the amount of free space of the file system containing the specified file Most implementations of in Unix and Unix-like operating systems include extra options. The BSD and GNU coreutils versions include , which lists free space in human readable format displaying units with the appropriate SI prefix (e.g. 10 MB), , which lists inode usage, and , restricting display to only local filesystems. GNU includes as well, listing filesystem type information, but the GNU shows the sizes in 1K blocks by default. Specification The Single Unix Specification (SUS) specifies by original space is reported in blocks of 512 bytes, and that at a minimum, the file system names and the amount of free space. The use of 512-byte units is historical practice and maintains compatibility with and other utilities. This does not mandate that the file system itself be based on 512-byte blocks. The 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 option to switch to 1024-byte units was a good compromise. Users who prefer the more logical 1024-byte quantity can easily to without breaking many historical scripts relying on the 512-byte units. The output with consists of one line of information for each specified file system.