compress(1) — Commands
NAME
compress, uncompress, zcat − Compresses and expands data
SYNOPSIS
compress [-cCdfFnqvV] [-b bits] [file ...]
uncompress [-cCfFnqvV] [file ...]
zcat [-n] [file ...]
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
compress: XPG4, XPG4−UNIX
uncompress: XPG4, XPG4−UNIX
zcat: XPG4, XPG4−UNIX
Refer to the standards(5) reference page for more information about industry standards and associated tags.
FLAGS
-b bitsSpecifies the maximum number of bits to use to replace common substrings in the file. The default for bits is 16, with values of 9 through 16 acceptable. First, the algorithm uses 9-bit codes 257 through 512. Then it uses 10-bit codes, continuing until the bits limit is reached. (This flag applies to the compress command only.)
After the bits limit is attained, the compress command periodically checks the compression ratio. If it is increasing, compress continues to use the existing code dictionary. However, if the compression ratio decreases, compress discards the table of substrings and rebuilds it from the beginning. This lets the algorithm adapt to the next block of the file.
[Digital] The -b flag must be the last flag on the command line.
-cMakes compress and uncompress write to the standard output; no files are changed. The nondestructive behavior of zcat is identical to that of uncompress -c.
-C[Digital] Produces output compatible with compress 2.0.
-d[Digital] Uncompresses a file.
-f or -FWhen used with the compress command, forces the compression of file even if no reduction in the size of the file will occur.
With both the compress and uncompress commands, if you run the command in the background and -f is not specified, you are prompted as to whether an existing file should be overwritten.
-n[Digital] Specifies that no header has been added.
-q[Digital] Specifies a quiet operation. This is the default. Diagnostics messages, which display if you specify the -v flag, do not print. If the -q and -v flags are both specified, the last one on the command line is honored.
-vPrints the percentage reduction of each file when compressing the file. Prints messages to standard error concerning the expansion of each file when uncompressing the file. If the -q and -v flags are both specified, the last one on the command line is honored.
-V[Digital] Specifies a version.
PARAMETERS
fileThe path name of a file to be processed. If - (hyphen) is used, standard input will be read.
The compress command will create an output file name by appending a suffix of .Z to the input file name.
With the uncompress and zcat commands, if the file name has a .Z suffix, the command will look for a file of that name. If the suffix is not specified, the command will append the .Z suffix before searching for the file.
The uncompress command will store output in a file named with the .Z suffix removed if it was specified. If no .Z suffix is specified, the uncompress command will accept file as the name of the output file and append a .Z suffix before searching for the file.
DESCRIPTION
The compress command reduces the size of the named files using adaptive Lempel-Ziv coding.
Whenever possible, each file is replaced by one with the extension .Z while keeping the same ownership modes, access, and modification times. If no files are specified, standard input is compressed to the standard output.
Compressed files can be restored to their original form by using the uncompress or zcat command.
The uncompress command replaces the compressed .Z file with an uncompressed version of the file, identical to the file that was originally compressed with compress; the .Z suffix is removed. When issuing an uncompress command, you can refer to the compressed target file with or without the .Z suffix. If you do not specify the suffix, uncompress assumes it.
The zcat command writes the uncompressed version of a compressed file to standard output. The compressed (.Z) files remain intact. The zcat command is identical to uncompress -c. When issuing a zcat command, you can refer to the compressed target file with or without the .Z suffix; if you do not specify the suffix, zcat assumes it.
[Digital] The compress command uses the modified Lempel-Ziv algorithm popularized in "A Technique for High Performance Data Compression," Terry A. Welch, IEEE Computer, vol. 17, no. 6 (June 1984), pp. 8-19.
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, files containing source code or plain text are reduced by 50 to 60 percent. Compression is generally much better than that achieved by Huffman coding (as used in the pack command) or adaptive Huffman coding, and takes less time to compute.
ENVIRONMENT VARIABLES
The following environment variables affect the execution of compress, uncompress, and zcat:
LANGProvides a default value for the internationalization variables that are unset or null. If LANG is unset or null, the corresponding value from the default locale is used. If any of the internationalization variables contain an invalid setting, the utility behaves as if none of the variables had been defined.
LC_ALLIf set to a non-empty string value, overrides the values of all the other internationalization variables.
LC_CTYPEDetermines the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multibyte characters in arguments).
LC_MESSAGESDetermines the locale for the format and contents of diagnostic messages written to standard error.
NLSPATHDetermines the location of message catalogues for the processing of LC_MESSAGES.
NOTES
The pack, pcat, and unpack commands are marked to be withdrawn from the XPG4−UNIX standard. These compress, uncompress, and zcat commands should be used instead.
[Digital] The compress command may not preserve the extended file attributes (property list) of a file, including the access control list (ACL) if any. Verify that any ACLs have not been removed or modified after using compress or uncompress.
EXAMPLES
To compress folder and print the savings, enter:
compress -v folder
The system responds with a message like:
folder: Compression: 43.94% -- replaced with folder.Z
The following command displays the uncompressed version of the testlog.Z file:
zcat testlog.Z
DIAGNOSTICS
Usage: compress [-fvc] [-b maxbits] [file ...]
Invalid arguments were specified on the command line.
Missing maxbits
maxbits must follow -b.
file not in compressed format
The file cannot be uncompressed because it was never compressed.
file compressed with xx bits, can only handle yy bits.
The 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
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, or the locale’s equivalent of a y, if you want the output file to be replaced; n, or the locale’s equivalent of a n, if not. (The LC_MESSAGES variable determines the locale’s equivalent of y or n.)
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, (for example, a directory), it is left unaltered.
-- has xx other links: unchanged
The input file has links; it is left unchanged. (See the ln command for more information.)
-- file unchanged
No saving is achieved by compression. The input remains unchanged.
EXIT VALUES
The following exit values are returned:
compress command
0Successful completion
1An error occurred
2One or more files were not compressed because they became larger.
>2An error occurred
uncompress and zcat commands
0Successful completion
>0An error occurred