# compress

## 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.1‐2017, _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_**.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.1‐2017, _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.
    


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