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COMPRESS
Section: POSIX Programmer's Manual (1P)
Updated: 2003
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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 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 .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- OPERANDS
-
- STDIN
-
- INPUT FILES
-
- ENVIRONMENT VARIABLES
-
- ASYNCHRONOUS EVENTS
-
- STDOUT
-
- STDERR
-
- OUTPUT FILES
-
- EXTENDED DESCRIPTION
-
- EXIT STATUS
-
- CONSEQUENCES OF ERRORS
-
- APPLICATION USAGE
-
- EXAMPLES
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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