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
IEEE Std 1003.1-2001, 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.
-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
IEEE Std 1003.1-2001, 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_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).
LC_MESSAGES
Determine the locale that should be used to affect the format
and contents of diagnostic messages 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 IEEE Std 1003.1-2001.
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
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .