Originální popis anglicky:
compress - compress data
Návod, kniha: POSIX Programmer's Manual
compress [-fv][-b
bits][file ...]
compress
[-cfv][-b
bits][file]
The
compress utility shall attempt to reduce the size of the named files
by using adaptive Lempel-Ziv coding algorithm.
- Note:
- Lempel-Ziv is US Patent 4464650, issued to William Eastman,
Abraham Lempel, Jacob Ziv, Martin Cohn on August 7th, 1984, and assigned
to Sperry Corporation.
Lempel-Ziv-Welch compression is covered by US Patent 4558302, issued to Terry A.
Welch on December 10th, 1985, and assigned to Sperry Corporation.
On systems not supporting adaptive Lempel-Ziv coding algorithm, the input files
shall not be changed and an error value greater than two shall be returned.
Except when the output is to the standard output, each file shall be replaced
by one with the extension
.Z. If the invoking process has appropriate
privileges, the ownership, modes, access time, and modification time of the
original file are preserved. If appending the
.Z to the filename would
make the name exceed {NAME_MAX} bytes, the command shall fail. If no files are
specified, the standard input shall be compressed to the standard output.
The
compress 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:
- -b bits
- Specify the maximum number of bits to use in a code. For a
conforming application, the bits argument shall be:
The implementation may allow
bits values of greater than 14. The default
is 14, 15, or 16.
- -c
- Cause compress to write to the standard output; the
input file is not changed, and no .Z files are created.
- -f
- Force compression of file, even if it does not
actually reduce the size of the file, or if the corresponding file
.Z file already exists. If the -f option is not given, and
the process is not running in the background, the user is prompted as to
whether an existing file .Z file should be overwritten.
- -v
- Write the percentage reduction of each file to standard
error.
The following operand shall be supported:
- file
- A pathname of a file to be compressed.
The standard input shall be used only if no
file operands are specified,
or if a
file operand is
'-' .
If
file operands are specified, the input files contain the data to be
compressed.
The following environment variables shall affect the execution of
compress:
- LANG
- Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization
Variables 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).
- LC_MESSAGES
- Determine the locale that should be used to affect the
format and contents of diagnostic messages written to standard error.
- NLSPATH
- Determine the location of message catalogs for the
processing of LC_MESSAGES .
Default.
If no
file operands are specified, or if a
file operand is
'-' , or if the
-c option is specified, the standard output
contains the compressed output.
The standard error shall be used only for diagnostic and prompt messages and the
output from
-v.
The output files shall contain the compressed output. The format of compressed
files is unspecified and interchange of such files between implementations
(including access via unspecified file sharing mechanisms) is not required by
IEEE Std 1003.1-2001.
None.
The following exit values shall be returned:
- 0
- Successful completion.
- 1
- An error occurred.
- 2
- One or more files were not compressed because they would
have increased in size (and the -f option was not specified).
- >2
- An error occurred.
The input file shall remain unmodified.
The following sections are informative.
The amount of compression obtained depends on the size of the input, the number
of
bits per code, and the distribution of common substrings. Typically,
text such as source code or English is reduced by 50-60%. Compression is
generally much better than that achieved by Huffman coding or adaptive Huffman
coding (
compact), and takes less time to compute.
Although
compress strictly follows the default actions upon receipt of a
signal or when an error occurs, some unexpected results may occur. In some
implementations it is likely that a partially compressed file is left in
place, alongside its uncompressed input file. Since the general operation of
compress is to delete the uncompressed file only after the
.Z
file has been successfully filled, an application should always carefully
check the exit status of
compress before arbitrarily deleting files
that have like-named neighbors with
.Z suffixes.
The limit of 14 on the
bits option-argument is to achieve portability to
all systems (within the restrictions imposed by the lack of an explicit
published file format). Some implementations based on 16-bit architectures
cannot support 15 or 16-bit uncompression.
None.
None.
None.
uncompress ,
zcat
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html
.
