compress(1)

NAME

compress, uncompress, zcat, compressdir, uncompressdir — compress and expand data

SYNOPSIS

Compress Files

compress [-d] [-f|-z] [-z] [-v] [-c] [-V] [-b maxbits] [file ...]

uncompress [-f] [-v] [-c] [-V] [file ...]

zcat [-V] [file ...]

Compress Entire Directory Subtrees

compressdir [options] [directory ...]

uncompressdir [options] [directory ...]

DESCRIPTION

The following commands compress and uncompress files and directory subtrees as indicated:

compress

Reduce the size of the named files using adaptive Lempel-Ziv coding. If reduction is possible, each file is replaced by a new file of the same name with the suffix .Z added to indicate that it is a compressed file. Original ownership, modes, access, and modification times are preserved. If no file is specified, or if - is specified, standard input is compressed to the standard output.

uncompress

Restore the compressed files to original form. Resulting files have the original filename, ownership, and permissions, and the .Z filename suffix is removed. If no file is specified, or if - is specified, standard input is uncompressed to the standard output.

zcat

Restore the compressed files to original form and send the result to standard output. If no file is specified, or if - is specified, standard input is uncompressed to the standard output.

compressdir

Front-end processor. Recursively descend each specified directory subtree and use compress to compress each file in directory. Existing files are replaced by a compressed file having the same name plus the suffix .Z, provided the resulting file is smaller than the original. If no directories are specified, compression is applied to all files starting with the current directory.

options may include any valid compress command options (they are passed through to compress). To force compression of all files, even when the result is larger than the original file, use the -f option.

uncompressdir

Opposite of compressdir. Restore compressed files to their original form. options may include any valid uncompress command options (they are passed through to uncompress).

The amount of compression obtained depends on the size of the input, the maximum number of bits (maxbits) per code, and the distribution of common substrings. Typically, text such as source code or English is reduced by 50-60 percent. Compression is generally much better than that achieved by Huffman coding (as used in pack), or adaptive Huffman coding (compact), and takes less time to compute.

Options

These commands recognize the following options in the combinations shown above in SYNOPSIS:

-d: Decompress file. compress -d is equivalent to uncompress.
-f: Force compression of file. This is useful for compressing an entire directory, even if some of the files do not actually shrink. If -f is not given and compress is run in the foreground, the user is prompted as to whether an existing file should be overwritten.
-z: This is the same as the -f option except that it does not force compression when there is null compression.
-v: Print a message describing the percentage of reduction for each file compressed.
-c: Force compress and uncompress to write to the standard output; no files are changed. The nondestructive behavior of zcat is identical to that of uncompress -c.
-V: Print the current version and compile options onto the standard error.
-b maxbits: Specify the maximum number of bits the compress algorithm will use. The default is 16 and the range can be any integer between 9 and 16.

compress 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), pages 8-19. Common substrings in the file are first replaced by 9-bit codes 257 and up. When code 512 is reached, the algorithm switches to 10-bit codes and continues to use more bits until the limit specified by the -b flag is reached (default 16).

After the maxbits limit is attained, compress periodically checks the compression ratio. If it is increasing, compress continues to use the existing code dictionary. However, if the compression ratio is decreasing, compress discards the table of substrings and rebuilds it from scratch. This allows the algorithm to adapt to the next "block" of the file.

Note that the -b flag is omitted for uncompress since the maxbits parameter specified during compression is encoded within the output, along with a magic number to ensure that neither decompression of random data nor recompression of compressed data is attempted.

Access Control Lists

compress retains a file's access control list when compressing and expanding data.

EXTERNAL INFLUENCES

Environment Variables

LC_MESSAGES determines the language in which messages are displayed.

If LC_MESSAGES is not specified in the environment or is set to the empty string, the value of LANG is used as a default for each unspecified or empty variable. If LANG is not specified or is set to the empty string, a default of "C" (see lang(5)) is used instead of LANG.

If any internationalization variable contains an invalid setting, compress, uncompress, and zcat behave as if all internationalization variables are set to "C". See environ(5).

International Code Set Support

Single- and multi-byte character code sets are supported.

RETURN VALUE

These commands return the following values upon completion:

0: Completed successfully.
2: Last file is larger after (attempted) compression.
1: An error occurred.

DIAGNOSTICS

Usage: compress [-f|-z] [-dvcV] [-b maxbits] [file ...]: Invalid options were specified on the command line.
Missing maxbits: maxbits must follow -b.
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 a higher value of maxbits than the compress code on this machine. Recompress the file with a lower value of maxbits.
file: already has .Z suffix -- no change: The file is assumed to be already compressed. Rename the file and try again.
file: filename too long to tack on .Z: The output file name, which is the source file name with a .Z extension, is too long for the file system on which the source file resides. Make the source file name shorter and try again.
file already exists; do you wish to overwrite (y or n)?: Respond y if you want the output to replace the existing file; otherwise, respond n.
uncompress: corrupt input: A SIGSEGV violation was detected which usually means that the input file has been 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 (a directory for example), it is left unaltered.
-- has xxother links: unchanged: The input file has links which are not symbolic links and has been left unchanged. See ln(1) for more information.
-- has symbolic links: unchanged: The input file has symbolic links and has been left unchanged. See ln(1) for more information.
-- file unchanged: No savings is achieved by compression. The input remains unaltered.

EXAMPLES

Compress the file named zenith and print compression information to the terminal:

compress -v zenith 

The terminal display shows either a line resembling

zenith: Compression: 23.55% -- replaced with zenith.Z 

indicating that the compressed file is 23.55% smaller than the original, or a line resembling

zenith: Compression: -12.04% -- file unchanged 

indicating that an additional 12.04% space must be used to compress the file.

Undo the compression by typing either of the following commands:

uncompress zenith.Z 
compress -d zenith.Z 

This restores file zenith.Z to its original uncompressed form and name.

uncompress will perform on standard input if no files are specified. For example, to list a compressed tar file:

uncompress -c arch.tar.Z | tar -tvf -  

WARNINGS

Although compressed files are compatible between machines with large memory, -b12 should be used for file transfer to architectures with a small process data space (64K bytes or less).

NFS

Access control lists of networked files are summarized (as returned in st_mode by stat(), but not copied to the new file (see stat(2)).

AUTHOR

compress was developed by Joseph M. Orost, Kenneth E. Turkowski, Spencer W. Thomas, and James A. Woods.

FILES

*.Z: Compressed file created by compress and removed by uncompress.

STANDARDS CONFORMANCE

compress: XPG4