compress(1p) — Linux manual page


COMPRESS(1P)            POSIX Programmer's Manual           COMPRESS(1P)

PROLOG         top

       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.

NAME         top

       compress — compress data

SYNOPSIS         top

       compress [-fv] [-b bits] [file...]

       compress [-cfv] [-b bits] [file]

DESCRIPTION         top

       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.

OPTIONS         top

       The compress utility shall conform to the Base Definitions volume
       of POSIX.1‐2017, 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

                     9 <= bits <= 14

                 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. If the
                 response is affirmative, the existing file will be

       -v        Write the percentage reduction of each file to standard

OPERANDS         top

       The following operand shall be supported:

       file      A pathname of a file to be compressed.

STDIN         top

       The standard input shall be used only if no file operands are
       specified, or if a file operand is '-'.

INPUT FILES         top

       If file operands are specified, the input files contain the data
       to be compressed.


       The following environment variables shall affect the execution of

       LANG      Provide a default value for the internationalization
                 variables that are unset or null. (See the Base
                 Definitions volume of POSIX.1‐2017, 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.

                 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.

       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), the behavior of character
                 classes used in the extended regular expression defined
                 for the yesexpr locale keyword in the LC_MESSAGES

                 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.

       NLSPATH   Determine the location of message catalogs for the
                 processing of LC_MESSAGES.



STDOUT         top

       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.

STDERR         top

       The standard error shall be used only for diagnostic and prompt
       messages and the output from -v.

OUTPUT FILES         top

       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 POSIX.1‐2008.



EXIT STATUS         top

       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

       >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.

EXAMPLES         top


RATIONALE         top




SEE ALSO         top

       uncompress(1p), zcat(1p)

       The Base Definitions volume of POSIX.1‐2017, Chapter 8,
       Environment Variables, Section 12.2, Utility Syntax Guidelines

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see .

IEEE/The Open Group               2017                      COMPRESS(1P)

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