man/cat1/gzip.1.txt
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GZIP(1) GZIP(1)
NAME gzip, gunzip, zcat - compress or expand files SYNOPSIS gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ] gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ] zcat [ -fhLV ] [ name ... ] DESCRIPTION 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 speci- fied, 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 charac- ters. (A part is delimited by dots.) If the name con- sists of small parts only, the longest parts are trun- cated. 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 times- tamp in the compressed file. These are used when decom- pressing 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 exten- sions .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 gener- ally 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 'defla- tion' method. This feature is only intended to help con- version 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 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 uncom- press 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 com- press), Huffman coding (as used in 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 com- pressing or decompressing. The gzip file format is specified in P. Deutsch, GZIP 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, DEFLATE Compressed Data Format Specifica- tion version 1.3, <ftp://ftp.isi.edu/in- notes/rfc1951.txt>, Internet RFC 1951 (May 1996). OPTIONS -a --ascii Ascii text mode: convert end-of-lines using local conventions. This option is supported only on some non-Unix systems. For MSDOS, CR LF is con- verted to LF when compressing, and LF is con- verted to CR LF when decompressing. -c --stdout --to-stdout Write output on standard output; keep original files unchanged. If there are several input files, the output consists of a sequence of inde- pendently compressed members. To obtain better compression, concatenate all input files before compressing them. -d --decompress --uncompress Decompress. -f --force Force compression or decompression even if the file has multiple links or the corresponding file already exists, or if the compressed data is read from or written to a terminal. If the input data is not in a format recognized by gzip, and if the option --stdout is also given, copy the input data without change to the standard output: let zcat behave as cat. If -f is not given, and when not running in the background, gzip prompts to verify whether an existing file should be over- written. -h --help Display a help screen and quit. -l --list For each compressed file, list the following fields: compressed size: size of the compressed file uncompressed size: size of the uncompressed file ratio: compression ratio (0.0% if unknown) uncompressed_name: name of the uncompressed file The uncompressed size is given as -1 for files not in gzip format, such as compressed .Z files. To get the uncompressed size for such a file, you can use: zcat file.Z | wc -c In combination with the --verbose option, the following fields are also displayed: method: compression method crc: the 32-bit CRC of the uncompressed data date & time: time stamp for the uncompressed file The compression methods currently supported are deflate, compress, lzh (SCO compress -H) and pack. The crc is given as ffffffff for a file not in gzip format. With --name, the uncompressed name, date and time are those stored within the compress file if present. With --verbose, the size totals and compression ratio for all files is also displayed, unless some sizes are unknown. With --quiet, the title and totals lines are not displayed. -L --license Display the gzip license and quit. -n --no-name When compressing, do not save the original file name and time stamp by default. (The original name is always saved if the name had to be trun- cated.) When decompressing, do not restore the original file name if present (remove only the gzip suffix from the compressed file name) and do not restore the original time stamp if present (copy it from the compressed file). This option is the default when decompressing. -N --name When compressing, always save the original file name and time stamp; this is the default. When decompressing, restore the original file name and time stamp if present. This option is useful on systems which have a limit on file name length or when the time stamp has been lost after a file transfer. -q --quiet Suppress all warnings. -r --recursive Travel the directory structure recursively. If any of the file names specified on the command line are directories, gzip will descend into the directory and compress all the files it finds there (or decompress them in the case of gunzip ). --rsyncable While compressing, synchronize the output occa- sionally based on the input. This increases size by less than 1 percent most cases, but means that the rsync(1) program can much more efficiently synchronize files compressed with this flag. gunzip cannot tell the difference between a com- pressed file created with this option, and one created without it. -S .suf --suffix .suf Use suffix .suf instead of .gz. Any suffix can be given, but suffixes other than .z and .gz should be avoided to avoid confusion when files are transferred to other systems. A null suffix forces gunzip to try decompression on all given files regardless of suffix, as in: gunzip -S "" * (*.* for MSDOS) Previous versions of gzip used the .z suffix. This was changed to avoid a conflict with pack(1). -t --test Test. Check the compressed file integrity. -v --verbose Verbose. Display the name and percentage reduc- tion for each file compressed or decompressed. -V --version Version. Display the version number and compila- tion options then quit. -# --fast --best Regulate the speed of compression using the spec- ified digit #, where -1 or --fast indicates the fastest compression method (less compression) and -9 or --best indicates the slowest compression method (best compression). The default compres- sion level is -6 (that is, biased towards high compression at expense of speed). ADVANCED USAGE Multiple compressed files can be concatenated. In this case, gunzip will extract all members at once. For exam- ple: 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 bet- ter 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 uncom- pressed size for all members, you can use: gzip -cd file.gz | wc -c If you wish to create a single archive file with multi- ple members so that members can later be extracted inde- pendently, 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. ENVIRONMENT 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 parame- ters. 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. SEE ALSO 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, GZIP 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, DEFLATE Compressed Data Format Specifica- tion version 1.3, <ftp://ftp.isi.edu/in- notes/rfc1951.txt>, Internet RFC 1951 (May 1996). DIAGNOSTICS Exit status is normally 0; if an error occurs, exit sta- tus is 1. If a warning occurs, exit status is 2. Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...] Invalid options were specified on the command line. file: not in gzip format The file specified to gunzip has not been com- pressed. file: Corrupt input. Use zcat to recover some data. The compressed file has been damaged. The data up to the point of failure can be recovered using zcat file > recover file: compressed with xx bits, can only handle yy bits File was compressed (using LZW) by a program that could deal with more bits than the decompress code on this machine. Recompress the file with gzip, which compresses better and uses less mem- ory. file: already has .gz suffix -- no change The 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" if you want the output file to be replaced; "n" if not. gunzip: corrupt input A SIGSEGV violation was detected which usually means that the input file has been corrupted. xx.x% Percentage of the input saved by compression. (Relevant only for -v and -l.) -- not a regular file or directory: ignored When the input file is not a regular file or directory, (e.g. a symbolic link, socket, FIFO, device file), it is left unaltered. -- has xx other links: unchanged The input file has links; it is left unchanged. See ln(1) for more information. Use the -f flag to force compression of multiply-linked files. CAVEATS 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 com- pressed data on tapes. (This example assumes you are using the GNU version of tar.) BUGS 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 com- pression than the default compression level (-6). On some highly redundant files, compress compresses better than gzip. COPYRIGHT NOTICE Copyright (C) 1998, 1999, 2001, 2002 Free Software Foun- dation, Inc. Copyright (C) 1992, 1993 Jean-loup Gailly Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verba- tim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute transla- tions of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. local GZIP(1) |