NAME
compress, uncompress.real - compress and expand data (version 4.2)
SYNOPSIS
compress [ -f ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ name ... ]
uncompress.real [ -f ] [ -v ] [ -c ] [ -V ] [ name ... ]
DESCRIPTION
Note that the program that would normally be installed as uncompress is
installed for Debian as uncompress.real. This has been done to avoid
conflicting with the more-commonly-used program with the same name that
is part of the gzip package.
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. Compress will only attempt to
compress regular files. In particular, it will ignore symbolic links.
If a file has multiple hard links, compress will refuse to compress it
unless the -f flag is given.
If -f is not given and compress is run in the foreground, the user is
prompted as to whether an existing file should be overwritten.
Compressed files can be restored to their original form using
uncompress.real.
uncompress.real takes a list of files on its command line and replaces
each file whose name ends with .Z and which begins with the correct
magic number with an uncompressed file without the .Z. The
uncompressed file will have the mode, ownership and timestamps of the
compressed file.
The -c option makes compress/uncompress.real write to the standard
output; no files are changed.
If the -r flag is specified, compress will operate recursively. If any
of the file names specified on the command line are directories,
compress will descend into the directory and compress all the files it
finds there.
The -V flag tells each of these programs to print its version and
patchlevel, along with any preprocessor flags specified during
compilation, on stderr before doing any compression or uncompression.
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), 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.real, 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 pack), or adaptive Huffman coding (compact), and
takes less time to compute.
Under the -v option, a message is printed yielding the percentage of
reduction for each file compressed.
SEE ALSO
pack(1), compact(1)
DIAGNOSTICS
Exit status is normally 0; if the last file is larger after (attempted)
compression, the status is 2; if an error occurs, exit status is 1.
Usage: compress [-dfvcVr] [-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.real 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 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 has been corrupted.
Compression: xx.xx%
Percentage of the input saved by compression. (Relevant only
for -v.)
-- not a regular file or directory: ignored
When the input file is not a regular file or directory, (e.g. a
symbolic link, socket, FIFO, device file), it is left
unaltered.
-- has xx other links: unchanged
The input file has links; it is left unchanged. See ln(1) for
more information. Use the -f flag to force compression of
multiply-linked files.
-- file unchanged
No savings is achieved by compression. The input remains
virgin.
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.)
Invoking compress with a -r flag will occasionally cause it to produce
spurious error warnings of the form
"<filename>.Z already has .Z suffix - ignored"
These warnings can be ignored. See the comments in
compress42.c:compdir() in the source distribution for an explanation.
local COMPRESS(1)