DF(1P) POSIX Programmer's Manual DF(1P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
df — report free disk space
df [−k] [−P|−t] [file...]
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 fig‐
ures 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, if no options other
than −t are specified, the number of free file slots, or inodes, avail‐
able; when −t is specified, the output shall contain the total allo‐
cated space as well.
The df utility shall conform to the Base Definitions volume of
POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
−k Use 1024-byte units, instead of the default 512-byte units,
when writing space figures.
−P Produce output in the format described in the STDOUT section.
−t Include total allocated-space figures in the output.
The following operand shall be supported:
file A pathname of a file within the hierarchy of the desired file
system. If a file other than a FIFO, a regular file, a
directory, or a special file representing the device contain‐
ing the file system (for example, /dev/dsk/0s1) is specified,
the results are unspecified. If the file operand names a file
other than a special file containing a file system, df shall
write the amount of free space in the file system containing
the specified file operand. Otherwise, df shall write the
amount of free space in that file system.
The following environment variables shall affect the execution of df:
LANG Provide a default value for the internationalization vari‐
ables that are unset or null. (See the Base Definitions vol‐
ume of POSIX.1‐2008, Section 8.2, Internationalization Vari‐
ables for the precedence of internationalization variables
used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of
all the other internationalization variables.
LC_CTYPE Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments).
Determine the locale that should be used to affect the format
and contents of diagnostic messages written to standard error
and informative messages written to standard output.
NLSPATH Determine the location of message catalogs for the processing
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
The remaining output with −P shall consist of one line of information
for each specified file system. These lines shall be formatted as fol‐
"%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:
<file system name>
The name of the file system, in an implementation-defined
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
The percentage of the normally available space that is cur‐
rently 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 high‐
<file system root>
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:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
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
On large file systems, the calculation specified for percentage used
can create huge rounding errors.
1. The following example writes portable information about the /usr
df −P /usr
2. Assuming that /usr/src is part of the /usr file system, the follow‐
ing produces the same output as the previous example:
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 out‐
put. 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 compati‐
bility with ls and other utilities in this volume of POSIX.1‐2008. 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.
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
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IEEE/The Open Group 2013 DF(1P)