gzip

gzip, gunzip, zcat - compress or expand files

gzip [ -acdfhlLnNrtvV19 ] [-S\suffix] [ name ... ]
gunzip [ -acfhlLnNrtvV ] [-S\suffix] [ name ... ]
zcat [ -fhLV ] [ name ... ]

Gzip reduces the size of the named files using Lempel-Ziv coding (LZ77). Whenever possible, each file is replaced by one with the extension .gz, while keeping the same ownership modes, access and modification times. (The default extension is -gz for VMS, z for MSDOS, OS/2 FAT, Windows NT FAT and Atari.) If no files are specified, or if a file name is "-", the standard input is compressed to the standard output. Gzip will only attempt to compress regular files. In particular, it will ignore symbolic links.

If the compressed file name is too long for its file system, gzip truncates it. Gzip attempts to truncate only the parts of the file name longer than 3 characters. (A part is delimited by dots.) If the name consists of small parts only, the longest parts are truncated. For example, if file names are limited to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz. Names are not truncated on systems which do not have a limit on file name length.

By default, gzip keeps the original file name and timestamp in the compressed file. These are used when decompressing the file with the -N option. This is useful when the compressed file name was truncated or when the time stamp was not preserved after a file transfer.

Compressed files can be restored to their original form using gzip -d or gunzip or zcat. If the original name saved in the compressed file is not suitable for its file system, a new name is constructed from the original one to make it legal.

gunzip takes a list of files on its command line and replaces each file whose name ends with .gz, -gz, .z, -z, _z or .Z and which begins with the correct magic number with an uncompressed file without the original extension. gunzip also recognizes the special extensions .tgz and .taz as shorthands for .tar.gz and .tar.Z respectively. When compressing, gzip uses the .tgz extension if necessary instead of truncating a file with a .tar extension.

gunzip can currently decompress files created by gzip, zip, compress, compress -H or pack. The detection of the input format is automatic. When using the first two formats, gunzip checks a 32 bit CRC. For pack, gunzip checks the uncompressed length. The standard compress format was not designed to allow consistency checks. However gunzip is sometimes able to detect a bad .Z file. If you get an error when uncompressing a .Z file, do not assume that the .Z file is correct simply because the standard uncompress does not complain. This generally means that the standard uncompress does not check its input, and happily generates garbage output. The SCO compress -H format (lzh compression method) does not include a CRC but also allows some consistency checks.

Files created by zip can be uncompressed by gzip only if they have a single member compressed with the 'deflation' method. This feature is only intended to help conversion of tar.zip files to the tar.gz format. To extract a zip file with a single member, use a command like gunzip <foo.zip or R gunzip -S .zip foo.zip . To extract zip files with several members, use unzip instead of gunzip.

zcat is identical to gunzip -c. (On some systems, zcat may be installed as gzcat to preserve the original link to compress.) zcat uncompresses either a list of files on the command line or its standard input and writes the uncompressed data on standard output. zcat will uncompress files that have the correct magic number whether they have a .gz suffix or not.

Gzip uses the Lempel-Ziv algorithm used in zip and PKZIP. The amount of compression obtained depends on the size of the input and the distribution of common substrings. Typically, text such as source code or English is reduced by 60-70%. Compression is generally much better than that achieved by LZW (as used in R compress ), Huffman coding (as used in R pack ), or adaptive Huffman coding (compact).

Compression is always performed, even if the compressed file is slightly larger than the original. The worst case expansion is a few bytes for the gzip file header, plus 5 bytes every 32K block, or an expansion ratio of 0.015% for large files. Note that the actual number of used disk blocks almost never increases. gzip preserves the mode, ownership and timestamps of files when compressing or decompressing.

The gzip file format is specified in P. Deutsch, \s-1GZIP\s0 file format specification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Internet RFC 1952 (May 1996). The zip deflation format is specified in P. Deutsch, \s-1DEFLATE\s0 Compressed Data Format Specification version 1.3, <ftp://ftp.isi.edu/in-notes/rfc1951.txt>, Internet RFC 1951 (May 1996).

-a --ascii

-c --stdout --to-stdout

-d --decompress --uncompress

-f --force

-h --help

-l --list

-L --license

-n --no-name

-N --name

-q --quiet

-r --recursive

--rsyncable

-S .suf --suffix .suf

-t --test

-v --verbose

-V --version

-# --fast --best

Multiple compressed files can be concatenated. In this case, gunzip will extract all members at once. For example: gzip -c file1 > foo.gz gzip -c file2 >> foo.gz Then gunzip -c foo is equivalent to cat file1 file2 In case of damage to one member of a .gz file, other members can still be recovered (if the damaged member is removed). However, you can get better compression by compressing all members at once: cat file1 file2 | gzip > foo.gz compresses better than gzip -c file1 file2 > foo.gz If you want to recompress concatenated files to get better compression, do: gzip -cd old.gz | gzip > new.gz If a compressed file consists of several members, the uncompressed size and CRC reported by the --list option applies to the last member only. If you need the uncompressed size for all members, you can use: gzip -cd file.gz | wc -c If you wish to create a single archive file with multiple members so that members can later be extracted independently, use an archiver such as tar or zip. GNU tar supports the -z option to invoke gzip transparently. gzip is designed as a complement to tar, not as a replacement.

The environment variable GZIP can hold a set of default options for gzip. These options are interpreted first and can be overwritten by explicit command line parameters. For example: for sh: GZIP="-8v --name"; export GZIP for csh: setenv GZIP "-8v --name" for MSDOS: set GZIP=-8v --name On Vax/VMS, the name of the environment variable is GZIP_OPT, to avoid a conflict with the symbol set for invocation of the program.

znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1), pack(1), compact(1)

The gzip file format is specified in P. Deutsch, \s-1GZIP\s0 file format specification version 4.3, R <ftp://ftp.isi.edu/in-notes/rfc1952.txt> , Internet RFC 1952 (May 1996). The zip deflation format is specified in P. Deutsch, \s-1DEFLATE\s0 Compressed Data Format Specification version 1.3, R <ftp://ftp.isi.edu/in-notes/rfc1951.txt> , Internet RFC 1951 (May 1996).

Exit status is normally 0; if an error occurs, exit status is 1. If a warning occurs, exit status is 2.

Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]

file\^: not in gzip format

file\^: Corrupt input. Use zcat to recover some data.

file\^: compressed with xx bits, can only handle yy bits

file\^: already has .gz suffix -- no change

file already exists; do you wish to overwrite (y or n)?

gunzip: corrupt input

xx.x% Percentage of the input saved by compression.

-- not a regular file or directory: ignored

-- has xx other links: unchanged

When writing compressed data to a tape, it is generally necessary to pad the output with zeroes up to a block boundary. When the data is read and the whole block is passed to gunzip for decompression, gunzip detects that there is extra trailing garbage after the compressed data and emits a warning by default. You have to use the --quiet option to suppress the warning. This option can be set in the GZIP environment variable as in: for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0 for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0 In the above example, gzip is invoked implicitly by the -z option of GNU tar. Make sure that the same block size (-b option of tar) is used for reading and writing compressed data on tapes. (This example assumes you are using the GNU version of tar.)

The gzip format represents the input size modulo 2^32, so the --list option reports incorrect uncompressed sizes and compression ratios for uncompressed files 4 GB and larger. To work around this problem, you can use the following command to discover a large uncompressed file's true size: zcat file.gz | wc -c The --list option reports sizes as -1 and crc as ffffffff if the compressed file is on a non seekable media. In some rare cases, the --best option gives worse compression than the default compression level (-6). On some highly redundant files, compress compresses better than gzip.

Copyright 1998, 1999, 2001, 2002 Free Software Foundation, Inc.
Copyright 1992, 1993 Jean-loup Gailly

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