COMPRESS(1) User Commands COMPRESS(1)


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

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.

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
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.

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:

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(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).


See environ(7) for descriptions of the following environment variables
that affect the execution of compress, uncompress, and zcat: LANG,

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).


The following error values are returned:

Successful completion.

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(7) for descriptions of the following attributes:

|CSI | Enabled |
|Interface Stability | Committed |
|Standard | See standards(7). |


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


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

file: already has .Z suffix -- no change

The file is assumed to be already compressed. Rename the file and try

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.


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

- - file unchanged

No savings are achieved by compression. The input remains

- -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.

February 5, 2020 COMPRESS(1)