compress, uncompress, zcat - compress, uncompress files or display
compress [-fv/] [-b bits] [file]...
compress -c [-fv] [-b bits] [file]
uncompress [-fv] [-c | -/] [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 attempt
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 cod‐
ing (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
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 com‐
press. For files produced by compress on other systems, uncompress sup‐
ports 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:
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.
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'.
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 uncom‐
pressing, 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 termi‐
nal and -f is not specified, writes a diagnostic message to
standard error and exits with a status greater than 0.
Verbose. Writes to standard error messages concerning the
percentage reduction or expansion of each file.
When compressing or decompressing, copies any extended sys‐
tem 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 corre‐
sponding 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:
A path name of a file to be compressed by compress, uncom‐
pressed 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 expres‐
sion 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 col‐
lating 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:
An error occurred.
One or more files were not compressed because they would have
increased in size (and the -f option was not specified).
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). │
SEE ALSOln(1), pack(1), fgetattr(3C), fsetattr(3C), attributes(5), environ(5),
largefile(5), locale(5), standards(5)DIAGNOSTICS
Usage: compress [-fv/] [-b bits] [file... ]
compress c [-fv] [-b bits] [file... ]
Invalid options were specified on the command line.
Usage: uncompress [-fv] [-c | -/] [file]...
Invalid options were specified on the command line.
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
file: already has .Z suffix -- no change
The file is assumed to be already compressed. Rename the file and
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.
Percentage of the input saved by compression. (Relevant only for
- - 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
- - file unchanged
No savings are achieved by compression. The input remains uncom‐
- -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.
Mar 13, 2008 COMPRESS(1)