compress, uncompress, zcat - compress, uncompress files or
display expanded files
compress [-fv] [-b bits] [file...]
compress [-cfv] [-b bits] [file]
uncompress [-cfv] [file...]
The compress utility will attempt 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 will be
replaced by one with the extension .Z, while keeping the
same ownership modes, change times and modification times.
If appending the .Z to the file pathname would make the
pathname exceed 1023 bytes, the command will fail. If no
files are specified, the standard input will be 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 parame-
ter 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 will restore files to their original
state after they have been compressed using the compress
utility. If no files are specified, the standard input will
be uncompressed to the standard output.
This utility supports the uncompressing of any files pro-
duced by compress. For files produced by compress on other
systems, uncompress supports 9- to 16-bit compression (see
The zcat utility will write 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:
-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 given,
and the process is not running in the background,
prompts to verify whether an existing file.Z file
should be overwritten. When uncompressing, does
not prompt for overwriting files. If the -f
option is not given, and the process is not run-
ning 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
given, writes a diagnostic message to standard
error and exits with a status greater than 0.
-v Verbose. Writes to standard error messages con-
cerning the percentage reduction or expansion of
Sets the upper limit (in bits) for common sub-
string codes. bits must be between 9 and 16 (16
is the default). Lowering the number of bits will
result in larger, less compressed files.
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 will be 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_CTYPE, LC_MESSAGES, and
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
>2 An error occurred.
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Availability | SUNWesu |
| CSI | Enabled |
| Interface Stability | Standard |
ln(1), pack(1), attributes(5), environ(5), largefile(5),
Usage: compress [-fvc] [-b maxbits] [file... ]
Invalid options were specified on the command line.
Maxbits must follow -b, or invalid maxbits, not a
file: not in compressed format
The file specified to uncompress has not been
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 (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 -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
filename too long to tack on .Z
The path name is too long to append the .Z suffix.
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
compress should be more flexible about the existence of the
Man(1) output converted with