c

compress(1)

compress(1)

 

uncompress -c.

-V

Print the current version and compile options onto the standard error.

-bmaxbits

Specify the maximum number of bits the compress algorithm will use. The

 

default is 16 and the range can be any integer between 9 and 16.

compress uses the modi®ed Lempel-Ziv algorithm popularized in A Technique for High Performance Data Compression , Terry A. Welch, IEEE Computer, vol. 17, no. 6 (June 1984), pages 8-19. Common substrings in the ®le are ®rst replaced by 9-bit codes 257 and up. When code 512 is reached, the algorithm switches to 10-bit codes and continues to use more bits until the limit speci®ed by the -b¯ag is reached (default 16).

After the maxbits limit is attained, compress periodically checks the compression ratio. If it is increas- ing, compress continues to use the existing code dictionary. However, if the compression ratio is decreasing, compress discards the table of substrings and rebuilds it from scratch. This allows the algorithm to adapt to the next "block" of the ®le.

Note that the -b¯ag is omitted for uncompress since the maxbits parameter speci®ed during compression is encoded within the output, along with a magic number to ensure that neither decompression of random data nor recompression of compressed data is attempted.

Access Control Lists

compress retains a ®le's access control list when compressing and expanding data.

EXTERNAL INFLUENCES

Environment Variables

LC_MESSAGES determines the language in which messages are displayed.

If LC_MESSAGES is not speci®ed in the environment or is set to the empty string, the value of LANG is used as a default for each unspeci®ed or empty variable. If LANG is not speci®ed or is set to the empty string, a default of "C" (see lang(5)) is used instead of LANG.

If any internationalization variable contains an invalid setting, compress, uncompress, and zcat behave as if all internationalization variables are set to "C". See environ(5).

International Code Set Support

Single- and multi-byte character code sets are supported.

RETURN VALUE

These commands return the following values upon completion:

0Completed successfully.

2Last ®le is larger after (attempted) compression.

1An error occurred.

DIAGNOSTICS

Usage: compress [-f-z] [-dvcV] [-b maxbits] [file ...]

Invalid options were speci®ed on the command line.

Missing maxbits

maxbits must follow -b.

file: not in compressed format

The ®le speci®ed to uncompress has not been compressed.

file: compressed with xxbits, can only handle yybits

®le was compressed by a program that could deal with a higher value of maxbits than the compress code on this machine. Recompress the ®le with a lower value of maxbits.

file: already has .Z suffix -- no change

The ®le is assumed to be already compressed. Rename the ®le and try again.

file: filename too long to tack on .Z

The output ®le name, which is the source ®le name with a .Z extension, is too long for the ®le system on which the source ®le resides. Make the source ®le name shorter and try again.

file already exists; do you wish to overwrite (y or n)?

Respond y if you want the output to replace the existing ®le; otherwise, respond n.

Section 1−110

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HP-UX Release 11i: December 2000