compress, uncompress, zcat - Compresses and expands data
compress [-cCdfFnqvV] [-b bits] [file...]
uncompress [-cCfFnqvV] [file...]
zcat [-n] [file...]
Interfaces documented on this reference page conform to
industry standards as follows:
compress: XCU5.0
uncompress: XCU5.0
zcat: XCU5.0
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
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 the bits limit is reached.
(This option applies to the compress command only.)
After the bits limit is attained, the compress command
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 the beginning.
This lets the algorithm adapt to the next block of
the file.
[Tru64 UNIX] The -b option must be the last option
on the command line. Makes compress and uncompress
write to the standard output; no files are changed.
The nondestructive behavior of zcat is identical to
that of uncompress -c. [Tru64 UNIX] Produces output
compatible with compress 2.0. [Tru64
UNIX] Uncompresses a file. When used with the
compress command, forces the compression of file
even if no reduction in the size of the file will
occur.
With both the compress and uncompress commands, if
you run the command in the background and -f is not
specified, you are prompted as to whether an existing
file should be overwritten. [Tru64
UNIX] Specifies that no header has been added.
[Tru64 UNIX] Specifies a quiet operation. This is
the default. Diagnostics messages, which display
if you specify the -v option, do not print. If the
-q and -v options are both specified, the last one
on the command line is honored. Prints the
percentage reduction of each file when compressing
the file. Prints messages to standard error concerning
the expansion of each file when uncompressing
the file. If the -q and -v options are both
specified, the last one on the command line is honored.
[Tru64 UNIX] Specifies a version.
The path name of a file to be processed. If - (hyphen) is
used, standard input will be read.
The compress command will create an output file
name by appending a suffix of to the input file
name.
With the uncompress and zcat commands, if the file
name has a suffix, the command will look for a file
of that name. If the suffix is not specified, the
command will append the suffix before searching for
the file.
The uncompress command will store output in a file
named with the suffix removed if it was specified.
If no suffix is specified, the uncompress command
will accept file as the name of the output file and
append a suffix before searching for the file.
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 while keeping the same ownership modes, access,
and modification times. If no files are specified, standard
input is compressed to the standard output.
Compressed files can be restored to their original form by
using the uncompress or zcat command.
The uncompress command replaces the compressed file with
an uncompressed version of the file, identical to the file
that was originally compressed with compress; the suffix
is removed. When issuing an uncompress command, you can
refer to the compressed target file with or without the
suffix. If you do not specify the suffix, uncompress
assumes it.
The zcat command writes the uncompressed version of a compressed
file to standard output. The compressed (.Z)
files remain intact. The zcat command is identical to
uncompress -c. When issuing a zcat command, you can refer
to the compressed target file with or without the suffix;
if you do not specify the suffix, zcat assumes it.
[Tru64 UNIX] 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, files containing
source code or plain text are reduced by 50 to 60 percent.
Compression is generally much better than that achieved by
Huffman coding (as used in the pack command) or adaptive
Huffman coding, and takes less time to compute.
[Tru64 UNIX] The compress command may not preserve the
extended file attributes (property list) of a file,
including the access control list (ACL) if any. Verify
that any ACLs have not been removed or modified after
using compress or uncompress.
The following exit values are returned:
compress command
Successful completion. An error occurred. One or more
files were not compressed because they became larger. An
error occurred.
uncompress and zcat commands
Successful completion. An error occurred.
Invalid arguments were specified on the command line.
maxbits must follow -b. The file cannot be uncompressed
because it was never compressed. The 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 is assumed to be already compressed.
Rename the file and try again. Respond y, or the locale's
equivalent of a y, if you want the output file to be
replaced; n, or the locale's equivalent of a n, if not.
(The LC_MESSAGES variable determines the locale's equivalent
of y or n.) A SIGSEGV violation was detected, which
usually means that the input file is corrupted. Percentage
of the input saved by compression (relevant only for
-v). When the input file is not a regular file, (for
example, a directory), it is left unaltered. The input
file has links; it is left unchanged. (See the ln command
for more information.) No saving is achieved by compression.
The input remains unchanged.
To compress folder and print the savings, enter: compress
-v folder
The system responds with a message like: folder:
Compression: 43.94% -- replaced with folder.Z The
following command displays the uncompressed version
of the testlog.Z file: zcat testlog.Z
ENVIRONMENT VARIABLES [Toc] [Back] The following environment variables affect the execution
of compress, uncompress, and zcat: Provides a default
value for the internationalization variables that are
unset or null. If LANG is unset or null, the corresponding
value from the default locale is used. If any of the
internationalization variables contain an invalid setting,
the utility behaves as if none of the variables had been
defined. If set to a non-empty string value, overrides
the values of all the other internationalization variables.
Determines the locale for the interpretation of
sequences of bytes of text data as characters (for example,
single-byte as opposed to multibyte characters in
arguments). Determines the locale for the format and contents
of diagnostic messages written to standard error.
Determines the location of message catalogues for the processing
of LC_MESSAGES.
Commands: pack(1), pcat(1), unpack(1)
Files: acl(4)
Standards: standards(5)
compress(1)
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