COMPRESS(1) User Commands COMPRESS(1)

NAME

compress, uncompress, zcat - compress, uncompress files or display
expanded files

SYNOPSIS

compress [-fv/] [-b bits] [file]...


compress -c [-fv] [-b bits] [file]


uncompress [-fv] [-c | -/] [file]...


zcat [file]...

DESCRIPTION

compress
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.

uncompress
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).

zcat
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.

OPTIONS

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.

OPERANDS

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.

USAGE

See largefile(7) for the description of the behavior of compress,
uncompress, and zcat when encountering files greater than or equal to
2 Gbyte (2^31 bytes).

ENVIRONMENT VARIABLES

See environ(7) 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(7).

EXIT STATUS

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.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|CSI | Enabled |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|Standard | See standards(7). |
+--------------------+-------------------+

SEE ALSO

ln(1), pack(1), fgetattr(3C), fsetattr(3C), attributes(7),
environ(7), largefile(7), locale(7), standards(7)

DIAGNOSTICS

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.

NOTES

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.

February 5, 2020 COMPRESS(1)