COMPRESS(1) 386BSD Reference Manual COMPRESS(1)
NAME
compress, uncompress, zcat - compress and expand data
SYNOPSIS
compress [-f] [-v] [-c] [-b bits] [file ...]
uncompress [-f] [-v] [-c] file ...
zcat [file ...]
DESCRIPTION
Compress reduces the size of the named files using adaptive Lempel-Ziv
coding. Whenever possible, each file is replaced by one with the
extension .Z, while keeping the same ownership modes, access and
modification times. If no files are specified, the standard input is
compressed to the standard output. Compressed files can be restored to
their original form using uncompress or zcat
-f Force compression of file, even if it does not actually shrink or
the corresponding file.Z file already exists. Except when run in
the background under /bin/sh, if -f is not given the user is
prompted as to whether an existing file.Z file should be
overwritten.
-c (``cat''). Compress/uncompress writes to the standard output; no
files are changed. The nondestructive behavior of zcat is
identical to that of uncompress -c.
-b Specify bits code limit (see below).
-v Print the percentage reduction of each file.
Compress uses the modified Lempel-Ziv algorithm popularized in "A
Technique for High Performance Data Compression", Terry A. Welch, IEEE
Computer, vol. 17, (June 1984), pp. 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). Bits
must be between 9 and 16. The default can be changed in the source to
allow compress to be run on a smaller machine.
After the bits limit is attained, compress periodically checks the
compression ratio. If it is increasing, compress continues to use the
existing code dictionary. However, if the compression ratio decreases,
compress discards the table of substrings and rebuilds it from scratch.
This allows the algorithm to adapt to the next "block" of the file.
Note that the -b flag is omitted for uncompress since the bits parameter
specified 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.
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
(as used in the historical command pack), or adaptive Huffman coding (as
used in the historical command compact), and takes less time to compute.
If an error occurs, exit status is 1; if the last file was not compressed
because it became larger, the status is 2; otherwise the status is 0.
DIAGNOSTICS
Usage: compress [-fvc] [-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 xx bits, can only handle yy bits
File was compressed by a program that could deal with more bits
than the compress code on this machine. Recompress the file with
smaller bits.
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 file cannot be compressed because its name is longer than 12
characters. Rename and try again. This message does not occur on
BSD UNIX systems.
file already exists; do you wish to overwrite (y or n)?
Respond "y" if you want the output file to be replaced; "n" if not.
uncompress: corrupt input
A SIGSEGV violation was detected which usually means that the input
file is corrupted.
Compression: xx.xx%
Percentage of the input saved by compression. (Relevant only for
-v.)
-- not a regular file: unchanged
When the input file is not a regular file, (e.g. a directory), it
is left unaltered.
-- has xx other links: unchanged
The input file has links; it is left unchanged. See ln(1) for more
information.
-- file unchanged
No savings is achieved by compression. The input remains virgin.
FILES
file.Z compressed file is file.Z
BUGS
Although compressed files are compatible between machines with large
memory, -b12 should be used for file transfer to architectures with a
small process data space (64KB or less, as exhibited by the DEC PDP
series, the Intel 80286, etc.)
Compress should be more flexible about the existence of the `.Z' suffix.
HISTORY
The compress command appeared in 4.3BSD.
4.3 Berkeley Distribution July 30, 1991 2