COMPRESS
Section: User Commands (1)
Updated: local
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NAME
compress, uncompress, zcat - compress and expand data
SYNOPSIS
compress
[
-f
] [
-v
] [
-c
] [
-V
] [
-r
] [
-b
bits
] [
name ...
]
uncompress
[
-f
] [
-v
] [
-c
] [
-V
] [
name ...
]
zcat
[
-V
] [
name ...
]
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.
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
or
zcat.
uncompress
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
write to the standard output; no files are changed.
zcat
is identical to
uncompress
-c.
zcat
uncompresses either a list of files on the command line or its
standard input and writes the uncompressed data on standard output.
zcat
will uncompress files that have the correct magic number whether
they have a
.Z
suffix or not.
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,
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.
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
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.
SEE ALSO
pack(1),
compact(1)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- DIAGNOSTICS
-
- BUGS
-
- SEE ALSO
-
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