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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       compress -- compress data

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

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

DESCRIPTION
       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 ..ZZ.  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
       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 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..ZZ
                 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..ZZ 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
                 error.

OPERANDS
       The following operand shall be supported:

       file      A pathname of a file to be compressed.

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

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

ENVIRONMENT VARIABLES
       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 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.

       LC__COLLATE
                 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.

       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.

ASYNCHRONOUS EVENTS
       Default.

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

OUTPUT FILES
       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.

EXTENDED DESCRIPTION
       None.

EXIT STATUS
       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.

CONSEQUENCES OF ERRORS
       The input file shall remain unmodified.

       The following sections are informative.

APPLICATION USAGE
       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
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       uncompress, zcat

       The Base Definitions volume of POSIX.1-2017, Chapter _8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

COPYRIGHT
       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 http://www.opengroup.org/unix/online.html .

       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                          COMPRESS(1)
