HP-UX Reference (11i v3 07/02) - 1 User Commands A-M (vol 1)

c
compress(1) compress(1)
uncompress -c.
-V Print the current version and compile options onto the standard error.
-b maxbits 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 modified 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 file are first 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 specified by the
-b flag 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 algo-
rithm to adapt to the next "block" of the file.
Note that the -b flag is omitted for
uncompress since the maxbits parameter specified during compres-
sion is encoded within the output, along with a magic number to ensure that neither decompression of ran-
dom data nor recompression of compressed data is attempted.
Access Control Lists
compress retains a file’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 specified in the environment or is set to the empty string, the value of
LANG is
used as a default for each unspecified or empty variable. If
LANG is not specified 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:
0 Completed successfully.
2 Last file is larger after (attempted) compression.
1 An error occurred.
DIAGNOSTICS
Usage: compress [-f|-z] [-dvcV] [-b maxbits] [file ...]
Invalid options were specified on the command line.
Missing maxbits
maxbits must follow -b.
file: not in compressed format
The file specified to uncompress has not been compressed.
file: compressed with xxbits, can only handle yybits
file was compressed by a program that could deal with a higher value of maxbits than the compress
code on this machine. Recompress the file with a lower value of maxbits.
file: already has .Z suffix -- no change
The file is assumed to be already compressed. Rename the file and try again.
file: filename too long to tack on .Z
The output file name, which is the source file name with a .Z extension, is too long for the file system
on which the source file resides. Make the source file 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 file; otherwise, respond n.
154 Hewlett-Packard Company − 2 − HP-UX 11i Version 3: February 2007