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PROGRAM:

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.1‐2008, 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 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.1‐2008, 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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