compress, uncompress, zcat(1,C) compress, uncompress, zcat(1,C)
-------------------------------------------------------------------------------
compress, uncompress, zcat
PURPOSE
Reduces the size of named files.
SYNTAX
+--------------+ +------------+
compress ---| +----------+ |---| |---|
+-| -f -v -c |-+ +--- name ---+
^| -b bits || ^ |
|+----------+| +--------+
+------------+
+--------+ +------------+
uncompress ---| +----+ |---| |---|
+-| -v |-+ +--- name ---+
^| -c || ^ |
|+----+| +--------+
+------+
+------------+
zcat ---| |---|
+--- name ---+
^ |
+--------+
DESCRIPTION
The compress command 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.
The compress command 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.
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.
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compress, uncompress, zcat(1,C) compress, uncompress, zcat(1,C)
If the program finishes normally, the exit status is 0. The exit status is 1
if an error occurs. If the last file was not compressed because it became
larger, the exit status is 2.
FLAGS
-f Forces compression of name even if it does not actually shrink or
the corresponding name 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 name file should be
overwritten.
-c Makes compress/uncompress write to the standard output; no files
are changed. The nondestructive behavior of zcat is identical to
that of uncompress -c.
-b bits Specifies the maximum number of bits to use to replace common
substrings in the file. The default for bits is 16, with values
of 9 through 16 acceptable. First, the algorithm uses 9-bit
codes 257 through 512. Then it uses 10-bit codes, continuing
until -b is reached. (Not permitted with uncompress.)
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.
-v Causes the percentage reduction of each file to print.
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.
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compress, uncompress, zcat(1,C) compress, uncompress, zcat(1,C)
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, (for example, a directory),
it is left unaltered.
-- has xx other links: unchanged
The input file has links; it is left unchanged. See ln for more
information.
-- file unchanged
No savings is achieved by compression. The input remains unchanged.
NOTE
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).
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