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

OPTIONS         top

       The 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 application, the bits argument shall be:

                     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 overwritten.

       −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.1‐2008, Section 8.2, Internationalization
                 Variables for the precedence of internationalization
                 variables used to determine the values of locale

       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 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.

       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 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.

EXAMPLES         top


RATIONALE         top




SEE ALSO         top

       uncompress(1p), zcat(1p)

       The Base Definitions volume of POSIX.1‐2008, 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, 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 .

       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                 2013                        COMPRESS(1P)