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COMPRESS(1P)               POSIX Programmer's Manual              COMPRESS(1P)
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  speci-
       fied, 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  com-
       press:
       LANG      Provide  a  default  value for the internationalization vari-
                 ables that are unset or null. (See the Base Definitions  vol-
                 ume  of POSIX.1-2008, Section 8.2, Internationalization Vari-
                 ables 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  mes-
       sages 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 mecha-
       nisms) 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 cod-
       ing  or  adaptive Huffman coding (compact), and takes less time to com-
       pute.
       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 Electri-
       cal  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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                         COMPRESS(1P)