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.
NAMEcompress — compress data
SYNOPSIScompress [−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)
