Df (Unix)

df (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. df is typically implemented using the statfs or statvfs system calls.

df first appeared in Version 1 AT&T UNIX.

The Single UNIX Specification specifications for df are:

Most Unix and Unix-like operating systems add extra options. The BSD and GNU coreutils versions include -h, where free space is listed in human readable format, adding units with the appropriate SI prefix (e.g. 10MB), -i, listing inode usage, and -l, restricting display to only local filesystems. GNU df includes -T as well, listing filesystem type information, but the GNU df shows the sizes in 1K blocks by default.

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 ls and other utilities. This does not mandate that the file system itself be based on 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 easily alias df to df -k without breaking many historical scripts relying on the 512-byte units.

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