сжимать данные (compress data)
Пролог (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.
Имя (Name)
compress — compress data
Синопсис (Synopsis)
compress [-fv] [-b bits] [file...]
compress [-cfv] [-b bits] [file]
Описание (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.
Параметры (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.
Операнды (Operands)
The following operand shall be supported:
file A pathname of a file to be compressed.
Стандартный ввод (Stdin)
The standard input shall be used only if no file operands are
specified, or if a file operand is '-'.
Входные файлы (Input files)
If file operands are specified, the input files contain the data
to be compressed.
Переменные окружения (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.
Асинхронные события (Asynchronous events)
Default.
Стандартный вывод (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.
Стандартный вывод сообщений (Stderr)
The standard error shall be used only for diagnostic and prompt
messages and the output from -v.
Выходные файлы (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.
Расширенное описание (Extended description)
None.
Статус выхода (Exit)
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.
Последствия ошибок (Consequences of errors)
The input file shall remain unmodified.
The following sections are informative.
Использование в приложениях (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.
Примеры (Examples)
None.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
uncompress(1p), zcat(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
