compress, uncompress, zcat - compress, uncompress files or display expanded
files
compress [-fv/] [-b bits] [file]...
compress -c [-fv] [-b bits] [file]
uncompress [-fv] [-c | -/] [file]...
zcat [file]...
The compress utility attempts to reduce the size of the named files by
using adaptive Lempel-Ziv coding. Except when the output is to the standard
output, each file is replaced by one with the extension .Z, while
keeping the same ownership modes, change times and modification times, ACLs,
and extended attributes. The compress utility also attempts to set the owner
and group of file.Z to the owner and group of file, but does not
fail if this cannot be done. If appending the .Z to the file pathname
would make the pathname exceed 1023 bytes, the command fails. If no
files are specified, the standard input is compressed to the standard output.
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(1)) and it takes less time to
compute. The bits parameter specified during compression is encoded
within the compressed file, along with a magic number to ensure that neither
decompression of random data nor recompression of compressed data is
subsequently allowed.
The uncompress utility restores files to their original state after they
have been compressed using the compress utility. If no files are
specified, the standard input is uncompressed to the standard output.
This utility supports the uncompressing of any files produced by
compress. For files produced by compress on other systems,
uncompress supports 9- to 16-bit compression (see -b).
The zcat utility writes to standard output the uncompressed form of files
that have been compressed using compress. It is the equivalent of
uncompress -c. Input files are not affected.
The following options are supported:
-b bits
Sets the upper limit (in bits) for common substring
codes. bits must be between 9 and 16 (16 is the default). Lowering the
number of bits result in larger, less compressed files.
-c
Writes to the standard output; no files are changed and
no .Z files are created. The behavior of zcat is identical to
that of `uncompress -c'.
-f
When compressing, forces compression of
file, even
if it does not actually reduce the size of the file, or if the corresponding
file.Z file already exists.
If the -f option is not specified, and the process is not
running in the background, prompts to verify whether an existing file should
be overwritten. If the response is affirmative, the existing file is
overwritten. When uncompressing, does not prompt for overwriting files. If
the -f option is not specified, and the process is not running in the
background, prompts to verify whether an existing file should be
overwritten. If the standard input is not a terminal and -f is not
specified, writes a diagnostic message to standard error and exits with a
status greater than 0.
-v
Verbose. Writes to standard error messages concerning the
percentage reduction or expansion of each file.
-/
When compressing or decompressing, copies any extended
system attributes associated with the source file to the target file and
copies any extended system attributes associated with extended attributes of
the source file to the corresponding extended attributes associated with the
target file. If any extended system attributes cannot be copied, the original
file is retained, a diagnostic is written to stderr, and the final exit
status is non-zero.
The following operand is supported:
file
A path name of a file to be compressed by
compress, uncompressed by uncompress, or whose uncompressed form
is written to standard out by zcat. If file is −,
or if no file is specified, the standard input is used.
See largefile(5) for the description of the behavior of compress,
uncompress, and zcat when encountering files greater than or
equal to 2 Gbyte (2^31 bytes).
See environ(5) for descriptions of the following environment variables
that affect the execution of compress, uncompress, and
zcat: LANG, LC_ALL, LC_COLLATE, LC_CTYPE,
LC_MESSAGES, and NLSPATH.
Affirmative responses are processed using the extended regular
expression defined for the yesexpr keyword in the LC_MESSAGES
category of the user's locale. The locale specified in the LC_COLLATE
category defines the behavior of ranges, equivalence classes, and
multi-character collating elements used in the expression defined for
yesexpr. The locale specified in LC_CTYPE determines the
locale for interpretation of sequences of bytes of text data a characters,
the behavior of character classes used in the expression defined for the
yesexpr. See locale(5).
The following error values are returned:
0
Successful completion.
1
An error occurred.
2
One or more files were not compressed because they would
have increased in size (and the -f option was not specified).
>2
An error occurred.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE
TYPE |
ATTRIBUTE VALUE |
| CSI |
Enabled |
| Interface Stability |
Committed |
| Standard |
See standards(5). |
ln(1), pack(1), fgetattr(3C), fsetattr(3C),
attributes(5), environ(5), largefile(5),
locale(5), standards(5)
Usage: compress [-fv/] [-b maxbits] [file...
]
compress c [-fv] [-b maxbits] [file]
Invalid options were specified on the command line.
Usage: uncompress [-fv] [-c | -/]
[file]...
Invalid options were specified on the command line.
Missing maxbits
Maxbits must follow -b, or invalid maxbits, not a
numeric value.
file: not in compressed format
The file specified to uncompress has not been
compressed.
file: compressed with xxbits, can only handle
yybits
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 already exists; do you wish to overwrite (yes
or no)?
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, (such as a
directory), it is left unaltered.
- - has xx other links: unchanged
The input file has links; it is left unchanged. See
ln(1) for more information.
- - file unchanged
No savings are achieved by compression. The input remains
uncompressed.
- -filename too long to tack on .Z
The path name is too long to append the .Z
suffix.
- -cannot preserve extended attributes. file unchanged
Extended system attributes could not be copied.
Although compressed files are compatible between machines with large memory,
-b 12 should be used for file transfer to architectures with a small
process data space (64KB or less).
compress should be more flexible about the existence of the
.Z suffix.