compress - phpMan

COMPRESS(P) COMPRESS(P)

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 Corpora-
tion.

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 owner-
ship, 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 com-
pressed 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 appli-
cation, 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 overwrit-
ten.

-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 â€â€™-â€â€™ .

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 interna-
tionalization 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 char-
acters 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 â€â€™-â€â€™ , 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 (includ-
ing 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 uncom-
pressed input file. Since the general operation of compress is to delete the uncom-
pressed file only after the .Z file has been successfully filled, an application
should always carefully check the exit status of compress before arbitrarily delet-
ing files that have like-named neighbors with .Z suffixes.

The limit of 14 on the bits option-argument is to achieve portability to all sys-
tems (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.