||This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (December 2011)|
df (abbreviation for disk free) is a standard Unix computer program used to display the amount of available disk space for filesystems on which the invoking user has appropriate read access. df is usually implemented by reading the mtab file or using statfs.
df first appeared in Version 1 AT&T UNIX.
The Single UNIX Specification specifications for df 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 KB, MB, or GB
- Write the amount of free space of the file system containing the specified file
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 default 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.
The output with -P shall consist of one line of information for each specified file system. These lines shall be formatted as follows:
<fs name>, <total space>, <space used>, <space free>, <percentage used>, <fs root>
In the following list, all quantities expressed in 512-byte units (1024-byte when -k is specified) shall be rounded up to the next higher unit. The fields are:
- The name of the file system, in an implementation-defined format.
- The total size of the file system in 512-byte units. The exact meaning of this figure is implementation-defined, but should include <space used>, <space free>, plus any space reserved by the system not normally available to a user.
- The total amount of space allocated to existing files in the file system, in 512-byte units.
- The total amount of space available within the file system for the creation of new files by unprivileged users, in 512-byte units. When this figure is less than or equal to zero, it shall not be possible to create any new files on the file system without first deleting others, unless the process has appropriate privileges. The figure written may be less than zero.
- The percentage of the normally available space that is currently allocated to all files on the file system. This shall be calculated using the fraction:
<space used> / (<space used>+ <space free>)
- expressed as a percentage. This percentage may be greater than 100 if <space free> is less than zero. The percentage value shall be expressed as a positive integer, with any fractional result causing it to be rounded to the next highest integer.
- The directory below which the file system hierarchy appear
$ df -k Filesystem 1024-blocks Free %Used Iused %Iused Mounted on /dev/hd4 32768 16016 52% 2271 14% / /dev/hd2 4587520 1889420 59% 37791 4% /usr /dev/hd9var 65536 12032 82% 518 4% /var /dev/hd3 819200 637832 23% 1829 1% /tmp /dev/hd1 524288 395848 25% 421 1% /home /proc - - - - - /proc /dev/hd10opt 65536 26004 61% 654 4% /opt
See also 
- The Single UNIX® Specification, Issue 7 from The Open Group : report free disk space – Commands & Utilities Reference,
Manual pages 
- df — manual page from GNU coreutils
- OpenBSD General Commands Manual : display free disk space –
- The df Command - by The Linux Information Project (LINFO)