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du - estimate file space usage
du [-a | -s][-kx][-H | -L][file ...]
By default, the du utility shall write to standard output the size of the file space allocated to, and the size of the file space allocated to each subdirectory of, the file hierarchy rooted in each of the specified files. By default, when a symbolic link is encountered on the command line or in the file hierarchy, du shall count the size of the symbolic link (rather than the file referenced by the link), and shall not follow the link to another portion of the file hierarchy. The size of the file space allocated to a file of type directory shall be defined as the sum total of space allocated to all files in the file hierarchy rooted in the directory plus the space allocated to the directory itself.
When du cannot stat() files or stat() or read directories, it shall report an error condition and the final exit status is affected. Files with multiple links shall be counted and written for only one entry. The directory entry that is selected in the report is unspecified. By default, file sizes shall be written in 512-byte units, rounded up to the next 512-byte unit.
The du 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:
Specifying more than one of the mutually-exclusive options -H and -L shall not be considered an error. The last option specified shall determine the behavior of the utility.
The following operand shall be supported:
The following environment variables shall affect the execution of du:
The output from du shall consist of the amount of space allocated to a file and the name of the file, in the following format:
"%d %s\n", <size>, <pathname>
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
Default.
The following sections are informative.
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 1024-byte quantity can easily alias du to du -k without breaking the many historical scripts relying on the 512-byte units.
The -b option was added to an early proposal to provide a resolution to the situation where System V and BSD systems give figures for file sizes in blocks, which is an implementation-defined concept. (In common usage, the block size is 512 bytes for System V and 1024 bytes for BSD systems.) However, -b was later deleted, since the default was eventually decided as 512-byte units.
Historical file systems provided no way to obtain exact figures for the space allocation given to files. There are two known areas of inaccuracies in historical file systems: cases of indirect blocks being used by the file system or sparse files yielding incorrectly high values. An indirect block is space used by the file system in the storage of the file, but that need not be counted in the space allocated to the file. A sparse file is one in which an lseek() call has been made to a position beyond the end of the file and data has subsequently been written at that point. A file system need not allocate all the intervening zero-filled blocks to such a file. It is up to the implementation to define exactly how accurate its methods are.
The -a and -s options were mutually-exclusive in the original version of du. The POSIX Shell and Utilities description is implied by the language in the SVID where -s is described as causing "only the grand total" to be reported. Some systems may produce output for -sa, but a Strictly Conforming POSIX Shell and Utilities Application cannot use that combination.
The -a and -s options were adopted from the SVID except that the System V behavior of not listing non-directories explicitly given as operands, unless the -a option is specified, was considered a bug; the BSD-based behavior (report for all operands) is mandated. The default behavior of du in the SVID with regard to reporting the failure to read files (it produces no messages) was considered counter-intuitive, and thus it was specified that the POSIX Shell and Utilities default behavior shall be to produce such messages. These messages can be turned off with shell redirection to achieve the System V behavior.
The -x option is historical practice on recent BSD systems. It has been adopted by this volume of IEEE Std 1003.1-2001 because there was no other historical method of limiting the du search to a single file hierarchy. This limitation of the search is necessary to make it possible to obtain file space usage information about a file system on which other file systems are mounted, without having to resort to a lengthy find and awk script.
ls , the System Interfaces volume of IEEE Std 1003.1-2001, stat()
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