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compress(1) General Commands Manual compress(1) 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 stan- dards 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 in- dustry standards and associated tags. OPTIONS Specifies the maximum number of bits to use to replace common sub- strings 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 option applies to the compress command only.) After the bits limit is attained, the compress command periodi- cally checks the compression ratio. If it is increasing, com- press 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 option must be the last option on the command line. Makes compress and uncompress write to the standard out- put; no files are changed. The nondestructive behavior of zcat is identical to that of uncompress -c. [DIGITAL] Produces out- put compatible with compress 2.0. [DIGITAL] Uncompresses a file. When used with the compress command, forces the compres- sion 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. [DIGITAL] Specifies that no header has been added. [DIGI- TAL] Specifies a quiet operation. This is the default. Diag- nostics messages, which display if you specify the -v option, do not print. If the -q and -v options are both specified, the last one on the command line is honored. Prints the percentage re- duction of each file when compressing the file. Prints messages to standard error concerning the expansion of each file when un- compressing the file. If the -q and -v options are both speci- fied, the last one on the command line is honored. [DIGI- TAL] Specifies a version. OPERANDS The path name of a file to be processed. If - (hyphen) is used, stan- dard input will be read. The compress command will create an output file name by append- ing a suffix of to the input file name. With the uncompress and zcat commands, if the file name has a suffix, the command will look for a file of that name. If the suffix is not specified, the command will append the suffix be- fore searching for the file. The uncompress command will store output in a file named with the suffix removed if it was specified. If no suffix is speci- fied, the uncompress command will accept file as the name of the output file and append a 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 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 file with an uncom- pressed version of the file, identical to the file that was originally compressed with compress; the suffix is removed. When issuing an un- compress command, you can refer to the compressed target file with or without the suffix. If you do not specify the suffix, uncompress as- sumes 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 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. 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 at- tributes (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. EXIT STATUS The following exit values are returned: compress command Successful completion. An error occurred. One or more files were not compressed because they became larger. An error occurred. uncompress and zcat commands Successful completion. An error occurred. DIAGNOSTICS Invalid arguments were specified on the command line. maxbits must follow -b. The file cannot be uncompressed because it was never com- pressed. 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 is assumed to be already compressed. Rename the file and try again. Respond y, or the locale's equivalent of a y, if you want the output file to be replaced; n, or the locale's equiva- lent of a n, if not. (The LC_MESSAGES variable determines the locale's equivalent of y or n.) 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). When the input file is not a regular file, (for example, a directory), it is left unaltered. The input file has links; it is left unchanged. (See the ln command for more information.) No saving is achieved by compression. The input remains unchanged. 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 ENVIRONMENT VARIABLES The following environment variables affect the execution of compress, uncompress, and zcat: Provides a default value for the international- ization 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 in- ternationalization variables contain an invalid setting, the utility behaves as if none of the variables had been defined. If set to a non- empty string value, overrides the values of all the other internation- alization variables. Determines 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). Determines the lo- cale for the format and contents of diagnostic messages written to standard error. Determines the location of message catalogues for the processing of LC_MESSAGES. SEE ALSO Commands: pack(1), pcat(1), unpack(1) Files: acl(4) Standards: standards(5) compress(1)