PROLOG

       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

       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 .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

       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 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.1‐2008, 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.1‐2008, 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, 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
       http://www.unix.org/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 .