Specification
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:
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
, , 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:
- expressed as a percentage. This percentage may be greater than 100 if
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
Read more about this topic: Df (Unix)