PROLOGThis 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.
NAMEcompress --- compress data
compress [-fv] [-b bits] [file...] compress [-cfv] [-b bits] [file]
DESCRIPTIONThe compress utility shall attempt to reduce the size of the named files by using adaptive Lempel-Ziv coding algorithm.
Lempel-Ziv is US Patent 4464650, issued to William Eastman, Abraham
Lempel, Jacob Ziv, Martin Cohn on August 7th, 1984, and assigned to
Lempel-Ziv-Welch compression is covered by US Patent 4558302, issued to Terry A. Welch on December 10th, 1985, and assigned to Sperry Corporation.
OPTIONSThe compress utility shall conform to the Base Definitions volume of POSIX.1-2008, 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
argument shall be:
9 <= bits <= 14
- Cause compress to write to the standard output; the input file is not changed, and no .Z files are created.
- 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. If the response is affirmative, the existing file will be overwritten.
- Write the percentage reduction of each file to standard error.
OPERANDSThe following operand shall be supported:
- A pathname of a file to be compressed.
STDINThe standard input shall be used only if no file operands are specified, or if a file operand is '-'.
INPUT FILESIf file operands are specified, the input files contain the data to be compressed.
ENVIRONMENT VARIABLESThe following environment variables shall affect the execution of compress:
- Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of POSIX.1-2008, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
- If set to a non-empty string value, override the values of all the other internationalization variables.
Determine the locale for the behavior of ranges, equivalence classes, and multi-character collating elements used in the extended regular expression defined for the yesexpr locale keyword in the LC_MESSAGES category.
- 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), the behavior of character classes used in the extended regular expression defined for the yesexpr locale keyword in the LC_MESSAGES category.
Determine the locale used to process affirmative responses, and the locale used to affect the format and contents of diagnostic messages, prompts, and the output from the -v option written to standard error.
- Determine the location of message catalogs for the processing of LC_MESSAGES.
STDOUTIf 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.
STDERRThe standard error shall be used only for diagnostic and prompt messages and the output from -v.
OUTPUT FILESThe 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 POSIX.1-2008.
EXIT STATUSThe following exit values shall be returned:
- Successful completion.
- An error occurred.
- One or more files were not compressed because they would have increased in size (and the -f option was not specified).
- An error occurred.
CONSEQUENCES OF ERRORSThe input file shall remain unmodified.
The following sections are informative.
APPLICATION USAGEThe 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.
SEE ALSOuncompress, zcat The Base Definitions volume of POSIX.1-2008, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines
COPYRIGHTPortions 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 Electrical 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 www.unix.org/online.html .
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