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1995-03-09
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COMPRESS(1) USER COMMANDS COMPRESS(1)
NAME
compress, uncompress, zcat - compress and expand data
SYNOPSIS
compress [ -f ] [ -v ] [ -c ] [ -V ] [ -b _b_i_t_s ] [ _n_a_m_e ... ]
uncompress [ -f ] [ -v ] [ -c ] [ -V ] [ _n_a_m_e ... ]
zcat [ -V ] [ _n_a_m_e ... ]
DESCRIPTION
_C_o_m_p_r_e_s_s 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 owner-
ship 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 _u_n_c_o_m_p_r_e_s_s or _z_c_a_t.
The -f option will force compression of _n_a_m_e. This is useful
for compressing an entire directory, even if some of the
files do not actually shrink. If -f is not given and
_c_o_m_p_r_e_s_s is run in the foreground, the user is prompted as
to whether an existing file should be overwritten.
The -c option makes _c_o_m_p_r_e_s_s/_u_n_c_o_m_p_r_e_s_s write to the stan-
dard output; no files are changed. The nondestructive
behavior of _z_c_a_t is identical to that of _u_n_c_o_m_p_r_e_s_s -c.
_C_o_m_p_r_e_s_s uses the modified Lempel-Ziv algorithm popularized
in "A Technique for High Performance Data Compression",
Terry A. Welch, _I_E_E_E _C_o_m_p_u_t_e_r, 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). _B_i_t_s must be between 9 and 16. The default
can be changed in the source to allow _c_o_m_p_r_e_s_s to be run on
a smaller machine.
After the _b_i_t_s limit is attained, _c_o_m_p_r_e_s_s periodically
checks the compression ratio. If it is increasing, _c_o_m_p_r_e_s_s
continues to use the existing code dictionary. However, if
the compression ratio decreases, _c_o_m_p_r_e_s_s 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 _u_n_c_o_m_p_r_e_s_s, since the
_b_i_t_s 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.
Sun Release 4.0 Last change: local 1
COMPRESS(1) USER COMMANDS COMPRESS(1)
The amount of compression obtained depends on the size of
the input, the number of _b_i_t_s 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
_p_a_c_k), or adaptive Huffman coding (_c_o_m_p_a_c_t), and takes less
time to compute.
Under the -v option, a message is printed yielding the per-
centage of reduction for each file compressed.
If the -V option is specified, the current version and com-
pile options are printed on stderr.
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.
SEE ALSO
pack(1), compact(1)
DIAGNOSTICS
Usage: compress [-dfvcV] [-b maxbits] [file ...]
Invalid options were specified on the command line.
Missing maxbits
Maxbits must follow -b.
_f_i_l_e: not in compressed format
The file specified to _u_n_c_o_m_p_r_e_s_s has not been
compressed.
_f_i_l_e: compressed with _x_x bits, can only handle _y_y bits
_F_i_l_e was compressed by a program that could deal
with more _b_i_t_s than the compress code on this
machine. Recompress the file with smaller _b_i_t_s.
_f_i_l_e: already has .Z suffix -- no change
The file is assumed to be already compressed.
Rename the file and try again.
_f_i_l_e: 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.
_f_i_l_e 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: _x_x._x_x%
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.
Sun Release 4.0 Last change: local 2
COMPRESS(1) USER COMMANDS COMPRESS(1)
-- has _x_x other links: unchanged
The input file has links; it is left unchanged. See
_l_n(1) for more information.
-- 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.)
Sun Release 4.0 Last change: local 3