The df utility shall write the amount of available space and file slots for file systems on which the invoking user has appropriate read access. File systems shall be specified by the file operands; when none are specified, information shall be written for all file systems. The format of the default output from df is unspecified, but all space figures are reported in 512-byte units, unless the -k option is specified. This output shall contain at least the file system names, amount of available space on each of these file systems, and the number of free file slots, or inodes, available; when -t is specified, the output shall contain the total allocated space as well.
The df utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
The following operand shall be supported:
The following environment variables shall affect the execution of df:
When both the -k and -P options are specified, the following header line shall be written (in the POSIX locale):
"Filesystem 1024-blocks Used Available Capacity Mounted on\n"
When the -P option is specified without the -k option, the following header line shall be written (in the POSIX locale):
"Filesystem 512-blocks Used Available Capacity Mounted on\n"
The implementation may adjust the spacing of the header line and the individual data lines so that the information is presented in orderly columns.
The remaining output with -P shall consist of one line of information for each specified file system. These lines shall be formatted as follows:
"%s %d %d %d %d%% %s\n", <file system name>, <total space>,
<space used>, <space free>, <percentage used>,
<file system 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 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 appears.
The output format is unspecified when -t is used.
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
Default.
The following sections are informative.
On most systems, the "name of the file system, in an implementation-defined format" is the special file on which the file system is mounted.
On large file systems, the calculation specified for percentage used can create huge rounding errors.
df -P /usr
df -P /usr/src
The behavior of df with the -P option is the default action of the 4.2 BSD df utility. The uppercase -P was selected to avoid collision with a known industry extension using -p.
Historical df implementations vary considerably in their default output. It was therefore necessary to describe the default output in a loose manner to accommodate all known historical implementations and to add a portable option ( -P) to provide information in a portable format.
The use of 512-byte units is historical practice and maintains compatibility with ls and other utilities in this volume of IEEE Std 1003.1-2001. 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.
It was suggested that df and the various related utilities be modified to access a BLOCKSIZE environment variable to achieve consistency and user acceptance. Since this is not historical practice on any system, it is left as a possible area for system extensions and will be re-evaluated in a future version if it is widely implemented.